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Saint Hilaire

see History of evolutionism - pre-Darwinist period

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Saltationist evolution

- The punctualist model of evolution has sometimes erroneously been confused with the models of saltationist evolution, i.e. models that assume the existence of typostrophic saltation – sudden instantaneous phenotype changes of major extent. According to saltationists, all important evolutionary changes occur in jumps, basically from one generation to the next. The mechanism of the relevant changes leading to the particular evolutionary jump mostly includes macromutation, a genetic change with major phenotype impact. For example, some mutations in the genes controlling the progress of the early stages of ontogenesis could be considered to be macromutations. This type of mutation, leading to the instantaneous formation of hopeful monsters, could be feasible in the emergence of biological diversity and especially disparities and could thus occur in the evolution of some large taxa that substantially differ in the basic organization of their body structure and thus in the formation of the individual strains. However, it could not be very important for the evolution of adaptive traits, as it is highly improbable that a random change of major extent could increase the functionality of a complicated organism. The vast majority of evolutionary changes occurred through the gradual accumulation of mutations with minor impact.
            However, the punctualist model of evolution, similar to the gradualist model, does not require the participation of sudden phenotype changes, saltations. The time during which evolution occurs according to punctualists is of the order of tens of thousands of years; this is a very short time on the scale employed by paleontologists; however, from the viewpoint of the usual rate of microevolution, this is quite adequate for the gradual accumulation of normal mutations to collect a sufficient number of anagenetic changes that would be quite sufficient for the formation of a new species. Thus, it is erroneous to confuse the punctualist model with the saltationist model.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Samesense mutations

see Point mutations in the protein-encoding DNA

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Schindelwolf

see History of evolutionism – classical Darwinist period

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Scientific nomenclature

The creation of names for the individual taxa (or animals only from the level of a subspecies to the level of a superfamily) has fixed formalized rules. Some of them are binding, while others have only the character of recommendations. The international rules of zoological and biological nomenclature used in the systematics of fungi, bacteria and viruses differ in certain details.
Each species has a two-word, generally Latin or Latinized name consisting of the name of the genus to which it belongs and the name of the species. The names of higher taxa consist of a single word, while the names of species in a genus that contains subgenera have three words, where the name of the subgenus is placed in round brackets between the name of the genus and the name of the species. The names of the genus, subgenus and species are written in text in italics; the names of the genus and subgenus and also the names of higher taxa start with a capital letter and the second part of the name of the species starts with a small letter. The name of the author and the year in which the species was described should, and in taxonomic articles must, be written after the name. If the particular author originally described the species under a different genus name, the name of this author and the year of description are enclosed in round brackets. If a species was described and named independently by several authors, the name under which the species was first described takes precedence; however, this priority principle does not apply to descriptions published prior to a certain date; for example for the vast majority of animals, it does not apply to descriptions published in the pre-Linnean period, i.e. prior to 1758. In especially justified cases, the continuity principle can be given preference; if a younger synonym is generally known and broadly used and return to the formally more correct name would lead to chaos in the professional literature, the international nomenclature commission may approve the use of a younger name.
The type principle continues to be employed in taxonomy. Where possible, the species names are connected with a certain type specimen. If it is discovered that the type specimen belongs to some other species than the author of the description originally thought, or if the original species is divided into several separate species, the original name is applied to the species to which this type specimen belongs. The names of higher taxa have a single word and are again bound to the name of an internal taxon at a higher level – a type taxon. Thus, a type taxon of a certain genus is a certain species and a type taxon of a certain family is a certain genus. The names of taxa at the individual levels (for example only at the superfamily level in a zoological system) have specific suffixes according to which it is possible to determine the level to which the taxon belongs; however, these suffixes differ in the botanical and zoological literature. The system of laws in international nomenclature is, of course, far more extensive and complicated and attempts to cover all the situations that could occur and that could endanger the unambiguity and continuity of the scientific names employed in the taxonomic system.

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Segregation of chromosomes during meiosis

The zygote, i.e. the cell formed by fusion of two parent gametes, bears two sets of chromosomes from the two parents. Random separation of chromosomes into the daughter cells occurs during nuclear division, mitosis and meiosis, so that each of these cells contains a unique genotype formed by mixing the chromosomes derived from the two parents. Simultaneously, the mechanism of nuclear division ensures that each of the cells obtains a complete set of chromosomes, i.e. one chromosome of each type of chromosome.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection artificial

see Selection – the relationships between natural andsexual

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection background

see Genetic draft

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection centrifugal

see Selectiondisruptive, stabilizing and directional

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection centripetal

see Selection disruptive, stabilizing and directional

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection cyclic

Cyclic selectionis also frequently mentioned as a possible source of polymorphism in the population. This occurs when populations of a certain species are exposed to several opposing selection pressures.In dry summers, individuals with a certain phenotype can have greater fitness, while individuals with a different phenotype have greater fitness in wet summers. Individuals with a certain phenotype can survive better in the winter, while others survive better in the summer. As the individual periods alternate in time, selection pressures also alternate and their action increases and decreases cyclically, as do the frequencies of the individual alleles. A classical example, on which the phenomenon of cyclic selection was studied, corresponds to the coexistence of the red and black forms of two-spotted lady beetles (Adalia bipunctata). It is stated that the dark forms are at a disadvantage in the winter months, in damp and cold conditions, when their frequency decreases from the original 55-70% to 30-45%, while the red form is at a disadvantage in the warm and dry months (Timofeeff-Ressovsky 1940).

A similar effect of two opposing selection pressures on the preservation of polymorphism was also described in a system encompassing three species: the pea aphid Acyrthosiphon pisum, its parasitic wasp Aphidius ervi and the lady beetle predator (Coccinella septempunctata). In this case, the red and green forms of the aphid exist over long periods in the population, where the green form is more resistant to the predator and the red form is more resistant to parasites (Losey et al. 1997).

It is certainly not easy to decide whether polymorphism can be maintained in the long term through the action of cyclic selection alone or whether it is also necessary that frequency-dependent selection simultaneously act on the population or at least that allele recessivity effect also play a role. Mathematical models show that, in most cases, cyclic selection alone is not sufficient in the long term (Kimura 1955). However, they simultaneously show that a number of factors can affect the long-term stability of the system, including factors that are as wide-spread as sexual dimorphism or the existence of dormant stages in the particular species (Reinhold 2000). This aspect merits more detailed analysis, which would, however, substantially exceed the scope of this text.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection directional

see Selectiondisruptive, stabilizing and directional

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection disruptive, stabilizing and directional

If we monitor the values of a certain quantitative trait, e.g. body length, in a large population of organisms, we usually find that this trait had normal distribution in the population. There are very few very large and very small individuals in the population, while there are the greatest numbers of individuals of medium size. This is a result of the fact that a quantitative trait is mostly determined by a large number of relatively independent and mutually replaceable genes. It follows from the rules of combinatorics that only a very small number of individuals inherit from all their genes those alleles that have an identical effect, e.g. cause a larger body size. Most individuals inherit part of the alleles causing larger size and part causing smaller size, so that their phenotype will approach the average. If we compare the distribution of a given quantitative trait (number of individuals in individual size classes) prior to commencing the process of natural selection and after its termination, then we frequently find substantial differences. Three types of natural selection can be distinguished on the basis of the character of these differences (Fig. IV.7).

Disruptive (diversifying, centrifugal)selection is the opposite of stabilizing selection (Fig. IV.7). In this case, individuals with an average value of the trait are affected most and individuals with values far from the average are affected least (however, this need not necessarily be the most extreme groups, e.g. the largest and smallest organisms). This situation occurs, e.g., when the members of a single species exhibit two different life strategies. For example, small individuals are capable of hiding from predators, while large individuals cannot fit in the available hiding places but can try to fight with predators, with greater or lesser success. Medium-sized individuals are at a disadvantage – they cannot fit in hiding places and they are not strong enough to fight predators.

A similar situation can occur in species using mimicry. If the forest contains dark-coloured spruce trees and light-coloured birch trees, it is advantageous for a butterfly to be either dark or light in colour, to optically merge with the bark of spruce or birch trees. Butterflies with medium-coloured wings are easily visible on both spruce and birch trees.

Disruptive selectionis disadvantageous from the standpoint of the population and of a typical individual because it has the greatest effect on the most numerous frequency class. Thus, it is probable that this kind of selection pressure will sooner or later lead to the development of genetic, ethological or other mechanisms that reduce the frequency of individuals with an average value of the given trait. For example, it can increase the importance of one of the genes determining the value of the trait, so that value of the trait will finally be determined predominantly (or exclusively) by a pair of alleles, one of which will be dominant and the other recessive. Preferential mating between individuals with the same phenotype (positive assortative pairing) is an example of an ethological mechanism. This mechanism could possibly lead to speciation, in which two new species can be formed from one original polymorphous species through disruptive selection.

We most often encounter a situation where the distribution of the frequency of individual phenotypes prior to selection and after selection have the same mean (same position of the maximum frequency); however, the distribution following selection is much narrower, as particularly individuals with extreme values of the monitored trait (the smallest and the largest) were removed from the population. This type of selection is called stabilizing or normalizing or centripetal (Fig. IV.7)..If the population is present under unchanging conditions, there is usually an optimal value of each quantitative trait, for example optimal body length. During evolution, the action of natural selection generally establishes a frequency of the alleles of the individual genes affecting the particular quantitative trait, so that most of the progeny formed through genetic recombination exhibit the optimal or almost optimal phenotype and are thus least affected by natural selection. Šmalgauzen and Waddington (Waddington 1953a)used the term stabilizing selection in a somewhat different sense (selection of alleles reducing the ability of aberrant genes to affect the phenotype).

The third type of natural selection of quantitative traits consists in directional selection (Fig. IV.7). In contrast to the two previous types of selection, in this case selection leads to a shift in the frequency maximum towards the left or the right. Directional selection leads to a change, not only in the average value of a particular trait, but also a change (decrease or increase in size) in the variability of the given trait in the population.

A shift in the frequency maximum occurs when natural selection preferentially eliminates individuals with a certain extreme value of a trait (largest or smallest). Through the action of directional selection, a species gradually changes, for example organisms become either larger or smaller. It is clear that this must be a temporary situation from the standpoint of evolution (although it sometimes lasts a very long time). This can most frequently be a reaction to a change in living conditions, a change in a biotic or abiotic factor. In this case, over time, the individuals attain a new optimum value of the particular trait and will remain in the vicinity of this value through stabilizing natural selection.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection environmental

see Selection – the relationships between natural andsexual

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection for heterozygotes

It followed from Equations (8) and (9) in Chapter IV.6.1 that an equilibrium frequency of the two alleles is established in dependence on the ratio of the selection coefficients of individuals with the particular genotypes. Polymorphism maintained by selection for heterozygotes is sometimes also termed balanced polymorphism. This mechanism is valid under conditions where the fitness of the heteozygote is greater than the fitness of any of the homozygotes. This case is apparently quite frequent (Fig. VIII.3) Geneticists sometimes connect this phenomenon with the heterosis effect, the greater viability of individuals with a high degree of heterozygosity (Hawkins & Day 1999). However, it is always necessary to strictly differentiate when the heterozygote actually has greater fitness and when it only has greater weight, e.g. as a consequence of poor balancing of ontogenetic processes. It is possible that a great many cases of the heterosis effect utilized in agriculture fall in this category. However, comparative studies simultaneously indicate highly significant correlation between the average heterozygosity in populations and the average fitness of their members (Reed & Frankham 2003). It is, however, apparent that, within a single population, individuals that are heterozygote in a large number of genes frequently actually do have greater fitness, exhibit mutually lower individual variability and their ontogenesis is more regular and more resistant against the action of external interfering effects (Fig. VIII.4). The latter fact is manifested, e.g., in lower fluctuation asymmetry in the body structures of species with bilateral symmetry, i.e. that component of morphological asymmetry that is manifested in some individuals in the population in a greater size of a certain structure on the right-hand side of the body and in some individuals on the left-hand side.

The cause of the greater viability of heterozygotes is not currently completely clear. According to some theories, this lies in the relatively greater number of recessive harmful (negative) mutations in the homozygous state in the genotype of homozygous individuals. An individual that is a homozygote in a great many genes is apparently the progeny of two mutually related individuals so that an elevated probability of the occurrence of rare harmful mutations in the homozygous state can be expected (Fig. VIII.5). According to some authors, the importance of this mechanism is reflected, e.g., in the fact that haplodiploid species, in which recessive lethal and highly harmful mutations are eliminated in haploid males, exhibit relatively lower polymorphism (Edwards & Hoy 1993). According to this model, the elevated viability of heterozygotes tends to be a manifestation of the reduced viability of homozygotes occurring through inbreeding, i.e. reproduction amongst relatives. If this were actually true, then the phenomenon of elevated viability of heterozygotes and the long-term survival of polymorphism in the population would not be functionally interconnected. A high degree of heterozygosity would only be an indication of a low level of inbreeding and thus low probability of the occurrence of rare harmful recessive mutations in the homozygous state in the given individual.

Another possible explanation of the greater fitness of heterozygotes is based on the assumption that the products of the individual alleles of a single gene fulfill a different function in the cell to at least some degree.  A heterozygous individual with two different alleles of a given gene is thus necessarily at an advantage over any homozygous individual. For example, different alleles exist in the population for a large percentage of enzymes, differing in their isoelectric point and thus mobility in an electric field. The occurrence of these alleles forms the basis for alozyme analysis (see XXIV.3.6). It is quite possible that the cell both accelerates and regulates its physiological processes by forming a pH-gradient and simultaneously an electric field in its interior and thus both concentrates and, as required, relocates the molecules of the individual enzymes to various areas of its inner space through intracellular isoelectric focusing (see also XII.5) (Flegr 1990; Flegr 1996a). The presence of two forms of enzymes differing in their isoelectric points can substantially assist heterozygotes to increase the effectiveness of some cellular processes, as it facilitates the simultaneous presence of the same enzyme activity of a monomeric enzyme at two places and of a multimeric enzyme at several places in the inner space of the cell (Fig. VIII.6).

The correlation between the polymorphism in the population and the intensity of environmental stress to which the populations or species are exposed in their environment is an indirect proof for maintenance of polymorphism through selection for heterozygotes. Thus, populations occurring at places with extreme and very variable natural conditions, for example in warm areas on the side of a valley exposed to the sun (FIG. VIII.7) exhibit substantially greater polymorphism. On the other hand, species occurring in an unvarying, stable environment, such as underground species, exhibit low polymorphism (Nevo, Filippucci, & Beiles 1994; Nevo 2001). This general trend indicates that polymorphism apparently has functional importance and increases the resistance of the population and evidently also of individuals against the action of various stress factors occurring in the environment.

The main objection to the importance of selection for heterozygotes for maintenance of the more important part of polymorphism in the natural population is that, in this case, the fitness of inbred individuals would have to be unrealistically low compared with outbred individuals. The selection coefficients of the individual alleles in the homozygous state would have to have a certain minimal value for any of the polymorphic genes in order to maintain the relevant content of the two alleles in the population. However, for inbred individuals, all the genes would have to be in the homozygous state, so that the overall fitness of these individuals would basically have to equal zero.

The relatively high polymorphism of a number of haploid species, including bacterial species, provides further evidence for the lower importance of selection for heterozygotes(Kimura 1985). This mechanism can, of course, not be operative for haploid species.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection frequency dependent

The biological fitness of an individual is frequently determined, not only by his phenotype, but also by the phenotypes of the other members of the population. For example, in any form of soft selection, the chances of survival of an individual depend not on the absolute value of his traits, but on the degree to which and the direction in which his traits differ from the traits of an average member of the given population.

However, more complicated cases also frequently occur, where the fitness of an individual changes stepwise in dependence on the frequency of his allele in the remaining population, even under the conditions of hard selection.  An example is the situation in which a population of prey, exposed to the activities of a certain predator, finds itself. It is known that a predator will frequently select the commonest type of prey as a target in a particular environment (Amalraj & Das 1996; Allen 1988; Gotmark & Olsson 1997). If the prey occurs in two different colour forms, determined, e.g., by a pair of alleles, then the predator will always concentrate on the bearer of the more common allele. Thus, the frequency of this allele will decrease in the population as a consequence of “apostatic selection” (Allen, Raison, & Weale 1998)until the bearers of the alternative allele predominate in the population. Then, the predator will concentrate on the bearers of the alternative allele, so that the fitness of the individuals with the originally frequent allele (now rare) will suddenly increase.

Frequency-dependent selectionoccurs, e.g., in some types of sexual selection. In some species of organisms, the rare-male advantage phenomenon  is active. Here females mate preferentially with the bearers of rare traits, i.e. the bearers of rare alleles (Dernoncourt-Sterpin, Leichien, & Elens 1991; Depiereux et al. 1990). Because of the preference for these males, the frequency of the rare alleles increases and thus other alleles become rare, i.e. advantageous. On the other hand, in some species, females can prefer the bearers of the most frequent alleles; in this case, we once again speak of frequency-dependent selection; however the less frequent alleles then rapidly disappear from the population.

Frequency-dependent selection acting in favour of less frequent alleles is probably one of the most important mechanisms for long-term maintenance of polymorphism in the population (Antonovics & Ellstrand 1984; Elena & Lenski 1997; Benkman 1996). As this type of selection can occur not only within populations and within species, but is also a matter of interspecies competition (a predator can select the members of the commonest species), frequency-dependent selection can create the preconditions for the long-term co-existence of two various species at a single location.

Frequency-dependent selection occurs if the selection coefficient for an allele is not constant, but changes in dependence on the frequency of this allele in the population. If this type of allele is to maintain polymorphism in the population, it is necessary that the selection value of this allele increase with decreasing frequency of this allele in the population. Then, if this allele is rare in the population, the fitness of its bearers is high; if it is frequent, the fitness of its bearers is low. A typical example is the situation occurring in a species that is capable of using two different resources in the environment. Only one of the two alleles of a certain gene is advantageous for using each of the two resources. If there is a greater frequency of one of the alleles in the population, most individuals will preferentially use the given resource, this will be rapidly exhausted, the bearers of the allele will begin to starve and will reproduce more slowly and their frequency, i.e. the frequency of the relevant allele in the population, will decrease.

For example, this phenomenon has been described for predaceous cichlids (Perissodus microlepis) (Hori 1993). Fish of this species feed on the scales off the bodies of other fish. They are adapted to this means of obtaining nutrition, amongst other things, in that their jaws are asymmetrical. Without regard to the external conditions in the individual water reservoirs, they all contain the same numbers of cichlids with left-handed and right-handed asymmetry. Research has shown that right-handed asymmetric cichlids can effectively bite off the scales on the left-hand side of fish and vice-versa. If right-handed asymmetric individuals multiply excessively in the reservoir, other fish will be wary of danger coming from the left, so left-handed asymmetric cichlids will be more successful and will, as a result, multiply more rapidly.

The right-handed and left-handed curvature of the bills of crossbills (Loxiacurvirostra) is a somewhat less exotic example. Once again, the ratio of the two forms in a flock is frequently just 1:1 and here it is also assumed that frequency-dependent selection is responsible for maintaining this ratio.In this case, the curvature of the bill is related to the effectiveness of removing seeds from tree cones; one curvature can utilize only half the seeds from poorly accessible cones, while the beak with the opposite curvature can reach the other seeds (Benkman 1996). The crossbills with the less common form of beak will thus be able to obtain more food than crossbills with the more common form and will thus multiply faster.As a consequence, the ratio of the two forms reaches an equilibrium value of 1:1 in every population.

Another, in principle, similar case occurs when the size of the population of a certain organism is limited by the activities of a predator that is capable of differentiating between the individuals of the two phenotypes.It has been repeatedly demonstrated in experiments with various model organisms that some kinds of predators regularly select the more common type as their prey (Brockmann 2001). If there are two very different forms in the population of prey, e.g. two coloured forms of grove snails (Cepaea nemoralis), these kinds of predators (in this case, thrushes (Turdus), concentrate on the type that is momentarily more common (Brockmann 2001). The less frequent type is attacked less and its frequency in the population can increase. As soon as it predominates in the population, it attracts the attention of thrushes and the size of its population again decreases.

Similar phenomena of preference for individuals with a less common phenotype is frequently important in sexual selection, where the females of some species of birds, mammals and even insects mate preferentially with males of the less common phenotype (Singh & Sisodia 2000). However, this rare-male phenomenon occurs only in some species; in others, to the contrary, males with the more common phenotype are preferred (see XV.4.3).

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Selection group

Group selectionis encountered wherever the species forms a large number of more or less independent social groups, i.e. herds, flocks or bands, and where the survival or reproduction success of the individual is closely connected with the survival and success of its social group. If group selection exists in the given species, then its action can lead to preference for those properties of organisms that are advantageous for the group as a whole, but need not provide any advantage or can even be harmful for the bearer. The pattern of altruistic behaviour is a typical example of the second category of traits, i.e. behaviour that is useful for the group as a whole but is harmful for its bearer.

For example, if a predator appears in the vicinity of a flock of jackdaws, the first jackdaw that notices its presence lets out a warning cry and the whole flock tries to escape or defend itself. From the standpoint of the individual, the issuing of the warning signal and participation in protection of the flock is highly irrational and disadvantageous behaviour. The individual would have a much better chance of survival if it were to selfishly use the information about the presence of the predator for itself alone and, according to the circumstances, either crouch down or inconspicuously move to the other side of the flock and leave some other individual, perhaps its potential competitor, to be eaten. But, instead of this, it warns the rest, gives up its advantage, as there is, at the very least, the same probability that the predator will attack it as against any other member of the flock.

However, from the standpoint of the group, this altruistic behaviour is useful, because it reduces the probability that the predator will be successful in attacking the flock. Attacking the centre of a scattering flock, amongst a great many moving targets, is difficult and frequently unsuccessful. A flock that contains altruistic individuals thus has a better chance than a flock of the same size that does not contain altruists. At the end of a certain period of time, for example a season, it will thus be more numerous and it is thus more probable that it will split off a greater number of daughter flocks.

However, in this case, individual andgroup selection act in the opposite direction and there is substantial selection within the flock against altruistic individuals. It is disadvantageous for the individual to be an altruist, but much more advantageous to utilize the advantages provided by altruistic individuals, to behave selfishly and not warn the others about the predator. Whether altruists or selfish individuals predominate in a particular species is determined primarily by the population structure of the particular species, the manner of forming and disbanding of social groups, the degree of advantageousness and disadvantageousness of altruistic behaviour for the individual and the group and other properties of the particular biological system (Fig. IV.8).

In most cases, individual selection is much stronger than group selection. Consequently, until the 1980’s, biologists were mostly of the opinion that group selection is almost never an important factor in nature. However, new results of analysis of theoretical models clearly indicate that, under conditions where individual subpopulations regularly emerge and disappear in the framework of the population as a whole, group selection can be an important factor and, within a certain range of population parameters, can even predominate over individual selection (Alexander & Borgia 1978; Shanahan 1998).

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection hard and soft

We speak of hard selection if the selection removes from the population all the individuals whose critical biological parameter, i.e. property that is a measure of the success of the individual under the given conditions, does not attain certain limiting values. For example, all individuals whose body weight is less than 30 kg could be eliminated. In contrast, soft selection ignores the absolute values of a critical trait and eliminates from the population individuals that do not achieve a certain relative value of the given trait, e.g. it eliminates 25% of the smallest individuals in the population. Elimination of the slowest individuals in a herd of ungulates by predators could be an example of soft selection.

From the standpoint of microevolution, soft selection is apparently a more effective evolutionary factor than hard selection (Fig. IV.1). While a species can extricate itself from the effects of hard selection (for example, by increasing its bodily dimensions above a certain critical value), it cannot escape from soft selection; a constant percentage of individuals will be eliminated in every generation without regard to an increase in the mean value of the critical parameter. However, hard selection can be effective over prolonged period of time in a changing environment or in an environment whose properties oscillate in the medium-term. Similarly, from the standpoint of macroevolution, hard selection which, under certain circumstances, can lead to the extinction of a particular species, can apparently be of substantial importance.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Selection individual

see Selection group

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Selection intraspecific and interspecific

The term natural selection is sometimes erroneously extended to include two completely different phenomena, intraspecies andinterspecies selection, also denoted(especially by ecologists) as interspecies competition. Another evolutionary mechanism – species selection – is based not on mutual ecological competition between the members of various species, but rather on the competition between entire evolutionary lines, and follows from the existence of differences in the rates of speciation and extinction of various taxons (Stanley 1975). This mechanism was first described relatively recently and will be discussed in part IV.8.4 and also in the part concerned with macroevolution (XXVI.3). All the phenomena described to date were related to intraspecies selection and Darwinism as a whole can, with certain simplifications, be understood as the theory of emergence and gradual development of modern organisms through the mechanism of intraspecies selection.

Intraspecies selection andinterspecies competition are two phenomena that are incomparable in their biological importance. While intraspecies selection is capable of gradually forming and improving various useful biological structures, organs, macromolecules and patterns of behaviour, interspecies competition functions only as a one-step process capable, in the final analysis, of  deciding which of the mutually competing species is better at the given time and place. Species with worse parameters mostly do not have a chance to evolutionarily adapt to the competition and are usually immediately eliminated on an evolutionary time scale.  If two competing species have only partly overlapping niches and only partly overlapping areas of occurrence, the weaker species need not be completely eliminated; however, there can be a drastic change in its niche and, because of interspecies competition, it can successfully survive only in certain, strictly limited types of biotopes or only in those parts of its original area of occurrence in which the other species is not present. Thus, it can gain time for the relevant evolutionary changes and could, in time, eventually expand back into its original biotope or to other parts of its original area of occurrence. However, in this case, the relevant evolutionary changes accumulate through the classical mechanism of intraspecies selection.

While intraspecies selection is apparently the most important factor in biological evolution, interspecies competition, similar, e.g., to genetic drift and sexual selection, is only an important factor in evolution, affecting some of its properties and determining some properties of the organisms. Its main importance apparently lies in “niche reduction”. The fact that each kind of organism is limited to only a relatively narrow niche forces it to specialize, to select only a specialized life strategy and to improve this strategy as much as possible through intraspecies selection If there were no interspecies competition, e.g. if only one kind of organism were to live on the Earth, this would probably be an unspecialized species, capable of living under various conditions and utilizing various resources. Its individual organs and life functions would probably not be as well adapted to the environment as those of contemporary, mostly highly specialized species. If the survival of a member of a certain species is dependent on how fast it can run, the evolution of the motor system will occur much more rapidly for this species (and will advance much further) than if its survival were determined by a number of various factors or even by chance.

Natural selectioncould theoretically occur even at higher levels than the population or species. Consequently, it is possible that entire flora and fauna communities could compete together or, on a cosmic scale, entire biospheres of various planets. Ecological data indicate that competition between communities does actually occur and sequences of succession stages have been described, which regularly alternate in a certain biotope. However, from the standpoint of evolutionary biology, competition at a higher level than intraspecies is a rare phenomenon. The main factor responsible for minimal effectiveness of selection at a community level consists in low heritability of the properties of communities. If selection actually occurs at the level of ecological communities, then everything that was said of group selection is also true here (to an elevated degree).

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Selection kin

A great many biologists doubt that group selection could be sufficiently effective to enable the formation of altruistic behaviour. They base their considerations on the assumption that the structure and dynamics of the population in most species of organisms substantially favour individual selection overgroup selection. Thus, altruistic individuals should be rapidly eliminated in every case in competition with selfish individuals (Williams 1966).

At the present time, biologists mostly assume that a major part of altruistic behaviour that we encounter in nature was formed by kin selection (selection amongst related clans) and is thus primarily intended to assist the close relatives of the altruistic individual. As discussed in the part devoted to inclusive fitness (I.10.2), an organism can increase its evolutionary success in two ways. It can attempt to produce the greatest possible number of its own progeny or it can assist in producing the greatest number of the progeny of its relatives, i.e. individuals with which it has a great many common genes. Thus, if a certain pattern of behaviour improves the chances of survival of one’s own young, siblings, progeny of siblings or other close relatives, this is a pattern of behaviour that is almost as selectionally advantageous as the pattern that increases the chances of survival of the organism itself. Basically, individuals do not compete together, but rather individual families (genuses, related clans) of mutually more or less related individuals. However, the effectiveness of kin selection depends not only on the ratio of the costs and benefits (from the standpoint of biological fitness) of altruistic behaviour and on the relatedness of mutually assisting individuals, but on the relative importance of competition for resources between relatives. If sufficiently effective dispersion of the related individuals does not occur in the given species, then the main competitors for all the resources will be related individuals and any advantage of altruistic behaviour in relation to related individuals will be lost (West, Pen, & Griffin 2002).

A number of models have been published since the beginning of the 1980’s (Wilson 1983; van Baalen & Rand 1998; Day & Taylor 1998), indicating that fixation of altruistic behaviour can occur in a great many situations even by classical group selection in which mutually unrelated individuals assist one another, and groups with these altruists prosper better than groups with selfish individuals. The degree to which group selection, now frequently denoted interdemic selection or kin selection, predominates in the formation of a certain pattern of altruistic behaviour, probably depends in each particular case on the structure and dynamics of the population of the particular species (Shanahan 1998). If, for example, a flock is formed by a group of mutually related individuals in each case, e.g. the progeny of a single nesting pair, then a substantial effect of kin selection can be expected. If a flock is formed at the beginning of each season by mutually unrelated individuals and again disintegrates at the end of each season, a substantial effect of classical group selection can be expected. Various types of selection can, of course, act simultaneously in real situations.

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Selection natural

Natural selection is defined as the process of uneven transfer of alleles derived from particular individuals to the gene pool of the following generations through their progeny.This process can occur in a number of quite different ways, and thus it is possible to differentiate several basic types of natural selection and also their combinations.The individual types of selection can be studied from the standpoint of their impact on the course of evolution, i.e. on the speed and direction of changes that they cause in the gene pool of the population, and from the standpoint of the level at which the selection acts (alleles, individuals, populations, etc.).

One of basic preconditions for the functioning of natural selection is the existence of heritability of the properties of organisms (I.9). Over time, organisms can develop complicated adaptive structures and patterns of behaviour only if randomly formed mutations and phenotype manifestations of these mutations and their consequences for the biological fitness of the individual are transferred from the parent organisms to their progeny. This precondition is fulfilled for organisms reproducing asexually – an individual with a certain mutation produces progeny whose genome contains copies of the same mutation and, if further mutation does not occur in the progeny that would somehow change the manifestations of the original mutation, the phenotype manifestations of this mutation and their impact on the biological fitness of the individuals will be the same as for the parent organism. However, a very different situation occurs in organisms with sexual reproduction. In these organisms, the progeny do not receive a copy of the genome of their parents, but rather their zygote is formed with a unique genome through combination of the genes derived half from the mother and half from the father. Although the newly formed mutations are also transferred (with a probability of 50%) from the parents to the progeny, their impact on the phenotype and thus on the biological fitness of the individual is usually fundamentally different than for the parent organism. Compared to asexually reproducing organisms, sexually reproducing organisms have substantially limited heredity of phenotype properties as, because of epistatic interactions between the individual genes, the same allele in the context of various genomes can cause the formation of completely different phenotype traits. Similarly, they have substantially limited heritability of biological fitness as, in the context of certain phenotype traits, a single trait can increase the biological fitness of its bearers, while it can reduce it in the context of other traits. This means that a great many mutations cannot become fixed in the population because, while they can contribute to increasing the biological fitness in the genomes of some individuals, and are thus preferred by natural selection here, in the genomes of the progeny of these individuals they can, on the other hand, reduce their fitness and their frequency is then reduced by natural selection. The degree to which the heritability of properties in sexually reproducing organisms only reduces the effectiveness of the functioning of Darwinist evolution and the degree to which it prevents its functioning is a question that has not yet been resolved.

Attempts to come to terms with the problem of the apparent existence of biological evolution under the conditions of low heritability of traits and biological fitness in sexually reproducing organisms are exemplified in the theory of the selfish gene (Dawkins 1976)and thetheory of frozen plasticity (Flegr 1998).

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Selection negative (or purifying selection)

denotes  removing from the population mutations that reduce the fitness of their bearers. The vast majority of all mutations fall in this category in a sufficiently large population. Some genes are capable of tolerating a large number of changes without greatly affecting the functioning of the relevant protein. These proteins change very rapidly in evolution. On the other hand, other genes are very conservative and any change in their sequence is greatly manifested in the functioning of the protein and thus in the fitness of the particular individual (Fig. IX.3).

            If the intensity and character of selection pressures, to which its representatives are exposed, change during the evolutionary history of a certain taxon, then the intensity of selection acting on the individual genes also changes. If a gene is exposed to more intense selection in a certain period, this is generally manifested by a reduction in the substitution rate for selectively significant mutations. In this case, at a molecular level, a reduction in the overall substitution rate and also a reduction in the ratio of the number of nonsynonymous mutations to the number of synonymous mutations are observed. In both cases, we must relate the numbers of synonymous and nonsynomous mutations to the numbers of positions in which synonymous or nonsynonymous mutations can occur in the given gene. In highly conservative genes exposed to intense negative selection, synonymous mutations substantially predominate and this ratio is thus much lower than 1. From an evolutionary standpoint, the genes for proteins that have a great many functions and that, e.g., interact physically with a large number of other proteins, are very conservative (Fraser et al. 2002). This ratio approaches a value of 1 in DNA sections that usually do not have any function, e.g. in pseudogenes, i.e. in unused and usually incomplete or otherwise damaged copies of genes. In contrast, in genes that are, or were in the past, exposed to intense positive selection, i.e. selection for evolutionary change, nonsynonymous mutations can predominate and this ratio can substantially exceed a value of 1. Genes participating in some way in the co-evolutionary battle amongst parasites and hosts, for example the genes for the components of the immune system, change especially rapidly (Endo, Ikeo, & Gojobori 1996) (Fig. IX.4). Genes that participate in interactions between members of the same species during reproduction, for example receptors on the surface of gametes, proteins expressed in the somatic tissues of the reproductive organs, etc. also develop rapidly (Lee, Ota, & Vacquier 1995; Vacquier 1998; Singh & Kulathinal 2000). This trend is especially important in taxa with polyandrous species in which both intense competition amongst sperms and also stronger genetic conflict between males and females occurs (Panhuis et al. 2001). The ratio of fixed nonsynonymous and synonymous mutations is usually greater than one in these proteins. This situation is encountered in approximately 0.5% of all the proteins that have so far been sequenced.

 

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Selection of chemostat type an turbidostat type

Although the death rate and the rate of reproduction of organisms in time vary very irregularly and very strongly, from the long-term perspective, the sizes of the populations of the individual species remain constant. This long-term stability can be ensured only by the existence of some kind of negative feedback regulating the size of the population and compensating random effects of the varying intensities of reproduction and death. In principle, there can be only two types of this feedback and technical laboratory models exist for both types (Flegr 1997)(Fig. IV.3). The first of these, “top-down regulation”, can be modeled in the laboratory in continuous cultivation systems of the turbidostattype. In this system a sensor (mostly optical) monitors the size of the population and, when it increases, the instrument increases the flow of nutrient medium through the cultivation vessel and thus increases the rate of flushing organisms out of the vessel. Thus, an increase in the population increases the rate at which individuals are flushed out of the vessel, subsequently leading to reduction in the size of the population to the original value. In nature, negative feedback of the turbidostat type is functional for systems in which an increase in the population leads to an increase in the death rate of its members. This occurs, e.g., in populations in which the size of the population of prey is regulated by the activity of predators. An increase in the number of prey leads to an increase in the number of predators, leading to increased predation and thus to a decrease in the size of the population of prey (and subsequently to a decrease in the size of the population of predators). Similar negative feedback exists in systems in which the size of the population is regulated by the action of an infectious agent (a contagious disease, a parasite). In this case, the effectiveness of spreading of an infectious agent is frequently directly proportional to the number of contacts between members of the population and the frequency of these contacts is directly proportional to the density of the population of hosts or, to be more precise, to the square of the population density.

The second type of negative feedback, “bottom-up-regulation” is modelled in continuous cultivation systems of the chemostat type. In these systems, nutrient medium flows into the cultivation vessel at a constant rate. If the population increases, nutrients are consumed more rapidly from the medium, their concentration decreases, the organisms begin to suffer from a lack of nutrition and the rate of reproduction is reduced. Thus, the natural death rate predominates over the rate of reproduction and the size of the population begins to decrease, the consumption of nutrients also decreases and the rate of reproduction of the population increases again. In nature, this type of negative feedback occurs everywhere where the size of the population is limited (and regulated) by the amount of some resource. For example, the population of predators in the previous case is limited by the size of the population of prey; however, a population can be similarly limited by any scarce resource, such as the number of available hiding places. In case of regulation of the population by lack of hiding places, the “superfluous individuals” are finally eliminated by predators, but the primary reason for their superfluousness is that lack of a resource (hiding places) and thus the growth of the population are regulated by negative feedback of the chemostat type.

The type of negative feedback determines which of the parameters of the organism will decide on the success of the individual and thus which will be the subject of natural selection. It follows from theoretical analysis (Flegr 1997) that the maximum rate of reproduction is the critical parameter in systems of the turbidostat type, e.g., the number of glucose molecules that the organism is capable of converting to biomass per time unit in the presence of an excess of all resources. In contrast, in systems of the chemostat type, the critical parameter is the effectiveness of utilization of a limiting resource, thus, e.g., the number of ATP molecules that a given individual is capable of forming from one glucose molecule (Fig. IV.4). Recalling the properties of r-strategists and K-strategists in the previous part, it can be seen that the properties of r-strategists, i.e. greater rate of reproduction, shorter life cycle, greater number of not very fit progeny, poor ability to compete with other species in stabilized biotopes, can be interpreted as the result of selection for the maximum rate of reproduction under turbidostatic conditions, while the opposite properties of K-strategists can be interpreted as being a result of selection for maximum effectiveness utilizing a limiting resource. This means that the long-known existence of two distinct ecological strategies could be related to the existence of two, and only two, types of negative feedback capable of maintaining a constant size of the population.

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Selection parental

see Selection – the relationships between natural andsexual

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Selection positive

 see selection negative

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Selection purifying

see selection negative.

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Selection r- and K

The model of r- and K-selection, also presented as the model of r- and K-strategy, was a popular concept of field ecology in their time (Pianka 1970). In nature, organisms with two fundamentally different life strategies can be encountered. One group of organisms prefers rapid reproduction. Its members are called r-strategists and the natural selection that they undergo is called r-selection. The second group tends to prefer the ability to compete with other organisms. The members of this group are called K-strategists and the relevant selection is called K-selection.

The ecology of r-strategists differs from that of K-strategists in a number of respects. Compared to K-strategists, they have shorter life cycles, greater maximum rate of reproduction, reproduce sooner, are usually smaller in size, frequently reproduce only once in their lifetime, and usually have a large number of progeny that, however, are not very viable and most of which do not even reach adulthood. The size of the populations of r-strategists tends to fluctuate in time and is mostly much smaller than would be permitted by the capacity of their environment, and thus within-species competition is very low. Death is usually caused by factors that do not differentiate amongst individuals according to genotype (random elimination, see below).r-Strategists occur particularly in a variable and unpredictable environment that is typical for the habitat in the early stages of succession. K-Strategists behave in the opposite way in all these respects.

The names of the two ecological strategies were chosen on the basis of the traditional designation of the constants in the logistic equation(Fig. IV.2), i.e. the equation describing the growth of a population in an environment capable of sustaining only a limited number of individuals:

 

 

where N denotes the number of individuals in the population, r is the rate of growth and K is the capacity of the environment, corresponding to the maximum number of individuals that the given environment is capable of sustaining. The logistic equation is fundamentally only a modification of the polynomial

 

 

where a = r, b = – r/K,i.e. an equation that successfully describes the shape of the growth curve (rapid, almost exponential growth at the beginning and slowing down or even stopping after a high population density is achieved); however, in fact, it has very little in common with the actual mechanisms of regulation in populations limited by the availability of nutrition.

The naming of the two types of natural selection according to the designation of the constants in the logistic equation is a very illustrative approach, but is rather unfortunate. To begin with, the logistic equation describes only a single, rather specific model of population growth, the specific case where the rate of reproduction is directly proportional to the size of the population and the death rate is directly proportional to the square of the size of the population – i.e., for example, population growth limited by a parasite transferred by direct contact. In addition, some authors noticed in the past that the existence of two types of natural selection does not follow from the logistic equation (Ginzburg 1992). We would expect that constant K would not play a role in a population of r-strategists and that the fitness of the organisms would be decided only by constant r, while the situation should be just the opposite in a population of K-strategists. However, in a population whose growth is described by the logistic equation, the fitness of an organism is affected to the same degree by constants r andK, so that there is only one strategy here, increasing r and/or K. Thus, although the concept of r- andK-strategies is one of the best known concepts in general ecology, the theoretical background of the relevant ecological phenomenon has so far been studied quite inadequately.

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Selection random

The concepts of random and nonrandom elimination(random and nonrandom selection) related to a certain degree to the concepts of r- and K-strategies. Individuals can be removed from the population either at random, and then the probability of the death of a certain individual will not depend in any way on his genotype, or differentially, in dependence on the genotype of the individual. A typical example ofrandom elimination consists in the reduction of the population of plankton fauna by a filtrator.

It is apparent that nonrandom elimination is always accompanied by natural selection. It is less apparent that random elimination is also accompanied by natural selection.While nonrandom elimination can lead to selection in favour of basically any trait, random elimination always selects in favour of rapid reproduction. Here, a certain connection can be seen with the model of r- and K-strategies, as most traits characteristic for r-strategists are in some way connected with a tendency to attain the maximum rate of reproduction. Consequently, some biologists are of the opinion that the existence of two different strategies, r and K, is based on differences between random andnonrandom elimination of superfluous individuals in the population.

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Selection sexual

see Selection – the relationships between natural andsexual

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Selection shadow theory of aging

The selection shadow theory or the theory of reduction of the effectiveness of selection during the life of an individual assumes that ageing does not occur in evolution as a mechanism ensuring, in advance, the programmed death of the individual, but rather as a consequence of the existence of an evolutionary barrier consisting in gradual reduction of the effectiveness of natural selection in dependence on the chronological age of the individual. A mutation that acts in the early stages of the life cycle of the individual and that increases the viability of its carrier, for example, by increasing his ability to regenerate damaged tissue, is extremely selectionally advantageous. If this same mutation were to function similarly at a later stage in the life cycle would be substantially less advantageous. This is because all the individuals in the population pass through the early stages of the life cycle, while only those individuals who live long enough and are not, for example, caught by a predator survive to a later stage. While a young individual has all its reproduction in the future, older individuals have already used up part of their reproduction potential. As a consequence of the reduced effectiveness of selection pressure at later stages in the life cycle, mutations that are negatively manifested in these later stages can accumulate in the population through genetic drift while the chance of fixation of mutations that would be manifested positively in these later stages is relatively reduced (Gavrilova et al. 1998).

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Selection soft

see Selection hard and soft

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Selection species

Species selection is currently considered to be an important evolutionary mechanism, which could be responsible for the existence of some macro-evolutionary trends. Consequently, it will be further discussed in the chapter devoted to macro-evolution (XXVI). In species selection, competition occurs between species and entire developmental lines as to which of them will most probably undergo speciation (split off daughter species) and will be less likely to suffer extinction. Phylogenetic lines containing species that will most probably split off daughter species or that will not rapidly suffer extinction, will be evolutionarily successful in the long term even if the properties that are the cause of more frequent speciation, e.g. low mobility of its members, are disadvantageous from the standpoint of survival of individuals within the species.

It is quite possible that a number of very important traits occurring in modern organisms, e.g. sexuality (Stanley 1979), emerged just because of species selection.  On the other hand, only individual selection could be responsible for the formation of all complex adaptive traits (inter-allele selection in sexually reproducing organisms – see below), which is the only known evolutionary mechanism that is capable of forming a complicated adaptive biological structure or function through gradual accumulation of minor changes leading to optimization of the relevant structure or function. The main handicap of species selection compared with individual selection lies in the small number of units that can compete together and the small space in time for the multistage evolution of more complicated traits. While, in individual selection, an enormous number of individuals compete together, the number of species that exist in a given territory at a given instant is substantially more limited. Simultaneously, the lifetime of an individual is much shorter than the period of existence of the species, so that intraspecies individual selection has sufficient time to accumulate a number of suitable changes, gradually leading to the formation of a certain complex adaptive trait, even if the species were capable of evolutionarily responding to selective pressure only for a certain time after its formation and not for the entire time of its existence (see XXVI.5). In contrast, the average time of existence of a species is approximately several million years, so that the entire period of existence of life on Earth (3.5 – 3.8 billion years) or even just the entire period of existence of macroscopic multicellular organisms (700 – 800 million years) could not easily encompass a sufficient number of “generations” of subsequent species.

In contrast, the advantage of species selection over individual selection lies in the fact that it has, so to speak, the final word in relation to fixation of a certain trait. A trait that can be in any way advantageous from the standpoint of the individual will finally disappear from nature if its existence leads to the extinction of the species whose members bear this trait. Human intelligence is certain advantageous from the standpoint of individual selection. However, if human beings kill themselves off in an atomic war, at the end of the day, the last laugh will be had, for example, by far less intelligent moles.

Compared to group selection, species selection has two great advantages. The first of them is resistance to invasion of an alternative form of the trait. If a certain trait is advantageous from the standpoint of the group and disadvantageous from the standpoint of the individual, then this trait will disappear from nature in the majority of cases as, sooner or later, individuals without this trait from another population will enter a prospering population of its bearers and will finally predominate through individual selection. In contrast, traits advantageous from the standpoint of the species and disadvantageous from the standpoint of individuals cannot disappear from nature in this way as reproduction isolation of the individual species will prevent invasion of the bearers of a trait of one species into the population of a second species.  

A further advantage of species selection compared to group selection lies in the fact that the genetic variability of species need not necessarily be less than the genetic variability of the individuals of these species. For populations, the situation is far less favourable from the standpoint of the effectiveness of selection.  The individual populations are formed by large groups of individuals and thus, quite necessarily, any property of various populations, i.e. the average properties of its members, varies less between populations than this property varies amongst individuals in the framework of the entire species. Simultaneously, the effectiveness of selection depends on the amount of variability amongst the individuals that are the subject of selection.

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Selection stabilizing

see Selectiondisruptive, stabilizing and directional

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Selection subconscious performed by humans

see Selection – the relationships between natural andsexual.

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Selection – the relationships between natural and sexual

Darwin introduced the expression natural selection as an analogy or rather as an antithesis of the term artificial selection,i.e. selection performed by humans. Later, it was found that natural selection consists of at least two components, of selection performed by the environment, i.e. environmental selection, and of selection that occurs through the competition of members of the same sex for partners for reproduction, i.e. sexual selection (Darwin 1909). However, Darwin did not explicitly introduce the term environmental selection as supplementary to the term sexual selection and used the expression natural selection in both a broader and a narrower sense.

This inadequacy in the professional terminology leads, for example, to frequent misunderstanding in relation to the position of sexual selection. One faction of biologists considers this to be part of natural selection, while another faction considers it, as Darwin did, to be the biological process itself, operating independently of environmental selection and acting in the same population, frequently in the opposite direction to environmental selection.  It is apparent that the proponents of the former concept understand natural selection in the broader sense of selection that is natural, not artificial, while the proponents of the latter concept understand it in a narrower sense, i.e. in the sense of selection performed by the environment (Flegr 1996b).

The category of natural selection can also include parental selection, i.e. selection performed by parents amongst their progeny. For example, it is assumed for altricial birds that the brightly coloured lining of their beaks and mouths emerged because parents preferentially fed young with the most obvious, i.e. most brightly coloured beak linings (Lyon, Eadie, & Hamilton 1994). With certain reservations the category of natural selection can also be considered to include subconscious selection performed by humans. It is quite possible, for example, that the phenotype of house cats (appearance, behaviour) was created through just this mechanism, through a form of unconscious domestication. People preferentially kept in their houses individuals that exhibited traits reminiscent of human young and that were also clean and affectionate.

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Selectionists

See  Evolutionary constraints.

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Selective sweep

see Genetic draft.

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Self-organized criticality

seeSpontaneous disturbances theory of mass extinctions

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Selfish DNA

- The term selfish DNA, i.e. designation for DNA segments that proliferate in the gene pool through molecular drive, must not be confused with the similar, but totally unrelated, term selfish gene or the somewhat related term ultraselfish gene.The selfish gene is the central concept of Dawkin’s model of biological evolution (Dawkins 1976)This model, based on the theoretical work of W.D. Hamilton (Hamilton 1964a; Hamilton 1964b; Hamilton 1967), assumes that the objects of selection in evolution are not individuals, and certainly not populations or species, but only the alleles of the individual genes. From the viewpoint of this hypothesis, all genes are selfish or, to be more exact, all the alleles of all genes are selfish. Selfishness here means that every allele is “out for itself”. Only an allele that affects the properties of an organism so that it increases the probability that it will be replicated and transferred down to future generations more frequently that other alleles of the same gene can be successful in evolution. In most cases, a successful allele somehow increases the biological fitness of the individual in whose gene it is contained. Consequently, the selfishness of genes is not fully apparent at first glance. It might seem that alleles that bring an advantage to individuals are most successful in evolution.

An ultraselfish gene is a gene or, to be more exact, an allele that, in order to increase the probability of its proliferation in the gene pool of the species, reduces the biological fitness of its bearers (see, for example, the bluebeard model in Section IV.9.1).

Molecular drive is a process through which mutations can proliferate within gene families (in process of homogenetization) and within the population (in process of fixation of mutations) through a number of mechanisms of nonreciprocal transfer of genetic information occurring on the chromosome or between different chromosomes (Dover 1986). Molecular drive differs from genetic drift in that changes in the frequencies of the individual alleles that occur through its action are not random in their direction. If a certain population of genetically identical organisms is divided into several smaller populations, then genetic drift will lead to fixation of different alleles in each population. In contrast, the effect of molecular drive should lead to fixation of the same alleles in all populations. Molecular drive differs from selection in that the alleles that are fixed through its action need not favourably affect the phenotype of the organism and can thus have a zero or even negative impact on the biological fitness of the individual. 

In molecular drive, one allele is replaced by another not because this is more advantageous for its bearer, but because, at the level of the DNA, it multiplies more effectively, either through a mechanism related to replication or through a mechanism related to gene conversion (see below).

Molecular drive differs from mutation bias and reparation drive mainly in that it is responsible for the proliferation of certain mutations in the genome or in the gene pool of the population, but not for their repeated formation.

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Selfish gene theory

In 1964, W.D. Hamilton published the results of his doctoral thesis concerned with some consequences of the existence of sexual reproduction for the progress of microevolution (Hamilton 1964a; Hamilton 1964b). His two articles, together with the ideas of G.C. Williams (Williams 1966), established the basis for a fundamentally new approach to biological evolution in sexually reproducing organisms. This model of biological evolution was popularized in the 1970’s and 1980’s by R. Dawkins as the selfish gene theory. In his best-known book “The Selfish Gene” (Dawkins 1976), he demonstrated, in contrast to Hamilton without using any mathematical models, that the subject of natural selection and thus the actual object of biological evolution cannot be individuals amongst sexually reproducing organisms, whose genome and thus biological properties are not inherited from one generation to the next, and certainly not families, populations or species, but only the various alleles of the individual genes, which are practically always passed down from one generation to the next in unaltered form. Thus, according to Dawkins, biological evolution must be understood as a race between the various alleles of a certain locus for the greatest frequency in the gene pool of the population. The individual alleles of the various genes can variously cooperate together, can conclude various coalitions but, in actual fact, all biological processes are based on a battle amongst the individual selfish genes or, to be more exact, selfish alleles, for the most effective and most frequent replication.

As an excellent popularizer, Dawkins called his book “The Selfish Gene” and not “The Selfish Allele” and almost always speaks of selfish genes and not alleles, as most lay people have an idea of what a gene is (although mostly erroneously, see II.3.1), while the definition of an allele is not part of the general consciousness. In this conception, organisms are understood to be sort of vehicles, instruments that the genes have created so that they can replicate as fast as possible under the conditions in our biosphere. When an evolutionary biologist studies a certain biological phenomenon, a certain property of living organisms, he should not ask which advantage that property brings its bearer (regardless of whether at the level of the individual, population or species), but how it is advantageous for the allele, the DNA section that codes the given property, how it helps to spread it in the gene pool at the expense of the other alleles of the same gene.

The selfish gene theory turned out to be an effective instrument for understanding and describing various evolutionary processes. It permits integration of our view of natural selection at all levels. It easily manages to explain the mechanism of the formation of altruistic behaviour and evolutionary processes at the molecular and chromosomal levels. Simultaneously, some biologists do not consider it to be an independent model of evolution, but only an alternative way of describing or viewing nature. Dawkins himself adopted this approach in the first edition of the “Selfish Gene” book. However, an increasing number of evolutionary biologists are coming to the opinion that the selfish gene theory or, to be more exact, the model of intralocus interallele selection, is substantially different from the Neodarwinist model of evolution of adaptive traits. While, according to the classical theory, only an allele that, compared to the other alleles, increases the relative biological fitness of its bearer can spread, it follows from the selfish gene theory that an allele that reduces the relative biological fitness of its bearer can spread in the population, of course, only under the assumption that it will simultaneously increase the probability of its own transfer to the gene pool of the next generation.

An illustrative, although originally purely theoretical example of the spreading of such an allele was described by Maynard Smith (Maynard Smith & Price 1973). This model can be described as the bluebeard modeland its principle is illustrated in Fig. IV.10. Imagine a gene on the Y-chromosome of a male, whose allele M causes that the male will kill all its daughters and feed its sons with their meat. As a consequence, a male with allele M will have almost half-lower biological fitness than a normal male with allele m. It will have only half as many progeny; however, the sons will not have to compete with daughters and will thus be stronger. It would follow from the Neodarwinist model that this bluebeard allele would rapidly disappear from the population. However, it follows from the selfish gene theory that the bluebeard allele will be highly successful and will spread in the population. The success of the allele is caused by the fact that it is transferred to the next generation on the Y-chromosome only to males and thus the fact that the bluebeard does not leave behind and daughters is insignificant. To the contrary, the fact that the sons will have no competition from daughters and will be better fed and thus stronger will contribute favourably to spreading of this gene in the population.

So far, no case of bluebeard alleles that would spread in the population through the above-described ethological mechanism is known in real nature, but some known ethological mechanisms are quite close to it (Foster, Wenseleers, & Ratnieks 2001). In addition, however, we know a number of alleles that achieve the same effect through other molecular or physiological mechanisms. Probably the best-studied systems consist in the SD-alleles of drosophila and the t-alleles of house mice (Carvalho & Vaz 1999; Ardlie 1998; Vanboven et al. 1996). In both cases, the relevant “bluebeard allele” manages during meiosis to reprogram a gene on the homologous chromosome so that, during spermatogenesis, in which normally the genes of the developing germ cell do not intervene, it actively destroys the developing germinal cell. This means that a heterozygote bearing the bluebeard allele produces substantially fewer sperm than a normal individual (and thus has lower biological fitness), but all its sperm (or most of them) bear the “bluebeard” allele (Fig. IV.11).

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Sensory drive of evolution of female preference

An individual receives signals from the environment through its sensory organs. This also applies to signals allowing the organism to obtain information on the presence or quality of a potential sexual partner. Any sensory organ has different sensitivity for various types of stimulation. For example, vision better differentiates certain colors or certain shapes and recognizes other colors and shapes less well. This kind of selective sensitivity of the sensory organs and related brain centers can be manifested in evolution as sensory drive (sensory bias, sensory exploitation) (Enquist & Arak 1993; Enquist & Arak 1994). Sensory drive can decide which traits will finally be fixed by sexual selection. If, for example, the vision of a pheasant is better capable of differentiating the horizontal than the vertical dimensions of an object, females will consider a male with long tail feathers to be a larger male than one that is actually larger, but has shorter tail feathers.
            In a number of species, individuals with symmetric patterns are preferred (Ridley 1992; Swaddle & Cuthill 1994) (Fig. XV.5). It has been found that even artificial neuron networks capable of learning are able to more readily identify symmetrical shapes than asymmetrical ones (Enquist & Arak 1994; Johnstone 1994). Females preferring males with symmetrical patterns are probably capable of identifying the presence of a male of their species at a much greater distance or under conditions of much worse visibility than females preferring asymmetric patterns.
            Even a very weak sensory drive can start a cycle of positive feedbacks that can lead, for example with contributions from the Fisher mechanism, to fixation of genes for preference for a certain trait and simultaneously fixation of the genes for the trait.

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Sex ratio distorters

Sex ratio distorters– see Meoitic drive

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Sexual promiscuity

In species in which uniparental care for progeny is possible, r-strategy, mostly of males, the greatest possible promiscuity, copulation with the greatest possible number of females, is evolutionarily advantageous. In contrast, careful choice of the biological fathers of their offspring, i.e. copulation with the best male in the population, is more advantageous for K-strategists, mostly for females. However, this theoretical conclusion has been repeatedly thrown into doubt by the results of the observations of the behavior of organisms in captivity and in nature. It has been found in a number of species that, not only males, but also females copulate with a great many partners.

            The simplest explanation of this, at first glance illogical, behavior of females is that organisms do not subject their reproductive behavior to attempts to achieve the greatest possible fitness, but to attempts to gain a reward in the form of satisfying feelings of physiological pleasure. However, this explanation is apparently not sufficient. An individual can subject and probably does subject his behavior to this aspect, to what his/her nervous system feels to be pleasant; however, evolution makes the final decision. With a certain degree of exaggeration, we can say that we do not eat an apple because it is sweet but that we experience an apple as being sweet because it is evolutionarily advantageous to eat it. Like almost everything in biology, this principle is not one hundred percent valid. A smoker doesn’t enjoy a cigarette because it is evolutionarily advantageous to smoke, not to mention other types of addictive drugs.

            The above-mentioned arena hypothesis is a frequently mentioned hypothesis explaining the function of female promiscuity. This hypothesis assumes that a female copulates with a greater number of males to obtain sperm from various individuals in the population to create conditions for intergamete competition. Through copulation with a greater number of males, the female can insure herself against infertile males or against males with reduced fertility.

            There are a number of other hypotheses that attempt to explain the phenomenon of female promiscuity. In some species, it can be assumed that the sperm or other components of the ejaculate constitute not only a source of genetic material for the female, but also a source of energy for nourishment of the developing embryos or even for herself (Wedell 1994). In other species, it is assumed that the females ensure greater genetic diversity of their offspring in this way, reducing their mutual competition for resources (see the elbow room hypothesis, XIII.3.2.2.1) and also increasing the chance that an individual ideally adapted to the conditions of any particular micro-habitat will be present amongst the offspring (see the lottery model, XIII.3.2.2.2).

            Other models assume that, in species living in groups, it is advantageous for females if they prevent the males from finding out with certainty who is the biological father of the offspring. If the female copulated at least once with any male in the group, then no male can exclude that he is actually the biological father of any offspring and will thus not behave in an unfriendly manner towards the offspring. The phenomenon of concealed ovulation, which is encountered in many species of animals, and the ability of the female to manipulate the ejaculate inside her reproductive organs simultaneously allows the female to affect which of the copulating males finally becomes the biological father of the offspring (Pizzari & Birkhead 2000; Pizzari, Froman, & Birkhead 2002). In human beings, similar to other animals, these processes frequently occur to a major degree outside of the consciousness of the woman. However, experimental studies have shown that, in the fertile phase of the menstrual cycle, women prefer different types of short-term sexual partners (more dominant and masculine types) than in phases where conception is less probable (Penton-Voak & Perrett 2000; Penton-Voak et al. 1999). The results of studies performed on the students of the Prague Faculty of Science of Charles University also demonstrated that the smell of dominant males was preferred in the fertile phase only by women who had a long-term partner at the time of the experiment and who were thus probably unconsciously more interested in “good genes” than in a “good care-giver” for their offspring {12514}.

The greater probability of conception in cases of rape than during normal sexual intercourse could also be connected with concealed female choice at subconscious level (Starks & Blackie 2000). A study in the U.S.A. showed that, in a single year, 32,000 pregnancies resulted from rape, of which 38% of the women finally gave birth to their babies. The data indicated that the probability of conception following rape was 5%, compared to approx. 1.2% in normal sexual intercourse. Only 11.8% of the pregnancies ended in spontaneous abortion, compared to 13.8% of normal pregnancies. Because of the size of the data set, all the observed differences were statistically highly significant. 

            Female (and male) promiscuity is, however, sometimes explained in an entirely different way. This need not in any way be behavior advantageous for its bearer, the promiscuous female, but may consist in behavior controlled by a parasite and advantageous only for this parasite.

            Control of the behavior of a host by a parasite is apparently a very common phenomenon (see XIX.6.5). There are a great many parasites whose transmission from one host to another occurs at the moment of copulation or during behavior directly or indirectly connected with reproduction of the host species. Promiscuity could provide certain advantages for the host species; however, for the sexually transmitted parasite, it is the most important parameter influencing the effectiveness of its spreading in the population, thus a question of life and death. If a mutation occurs in a parasite that allows it to affect the nervous system of the host organism in a way so that it will behave in a more promiscuous manner, this will greatly increase the chance for the parasite that it will spread in the population horizontally from one individual to another. It is understandable that such a mutant very rapidly predominates in a population of parasites. A parasite can achieve the required result in various ways, either by direct intervention in the nervous system of the host or through its endocrine system, through production of suitable hormones or their analogues, or very indirectly, for example in that it causes itchiness of the sexual organs of its host (Dawkins 1976).

            The parasite need not be transmitted during the actual copulation. It is, at the very least, suspicious how frequently the mouths come into contact as part of the precopulation behavior of various species (mutual feeding, licking, kissing). In general, it can be stated that we kiss one another because it is pleasant. This undoubted fact is, however, uninteresting from an evolutionary standpoint. What requires an explanation is who and why programmed our nervous system so that kissing is pleasant. One of the surprising answers could be – a parasite.

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Sexual selection

The emergence of sexual reproduction and differentiation of individuals of one species into males and females led to the appearance of a new factor, sexual selection. For the individual, it is not important to simply survive to reproductive age, but it is also necessary to find a sexual partner (or the optimal sexual partner) for reproduction. Competition occurs amongst members of the same sex for a suitable sexual partner. This competition is generally accompanied by very intense selection, which is then termed sexual selection. The direction and intensity of sexual selection acting on both sexes can differ substantially. This leads, amongst other things, to different evolution of morphological traits in the two sexes, to the formation of secondary sexual traits (epigamic traits) and thus frequently to very marked sexual dimorphism. The existence of striking secondary sexual traits and the impossibility of explaining their emergence through the action of environmental selection led Darwin to differentiation of a second type of natural selection – sexual selection.
            The action of sexual selection can be extremely intense. It is not rare that, in a population consisting of half males and half females, a single male is the father of all the offspring. Thus, the other males have zero exclusive fitness. Selection pressures following from this type of selection can thus be stronger than the selection pressures of environmental selection and, as will be shown in this chapter, can lead to quite interesting phenomena.
; In most cases, sexual selection does not act with the same intensity on the members of the two sexes. Females are usually more selective and males less selective in choice of a sexual partner. Consequently, males are usually subjected to more intense sexual selection and thus more obvious secondary sexual traits tend to emerge amongst them. The cause of this asymmetry lies primarily in the different cost of production of microgametes and macrogametes and secondarily in the different cost of the two parent roles. Because a female invests more energy in the production of macrogametes from the very beginning of reproduction than a male into the production of microgametes, she must generally accept the future thankless role of the parent that invests more into feeding and bringing up the offspring than her partner. In organisms with internal fertilization, this tendency is further reinforced by the fact that the greatest part of the cost of reproduction, nutrition of the embryo, is, in its biological nature, not transferrable to the male. Asymmetry in expenditure of energy thus attains extreme values here. The fitness of the individual depends both on the number of offspring produced in a lifetime and also on their quality. Males can increase their fitness through greater sexual activity, while females basically do not have any reserves in this area and the numbers of their offspring do not increase with an increasing number of sexual partners (Fig. XV.1). Thus, males need not be as discriminating in the quality of their sexual partners and can even reproduce with females that are of inferior quality phenotypically or genotypically. In contrast, females must carefully differentiate the quality of the males with whom they will reproduce. The quality of their future offspring depends to the same degree on the quality of their own genes as on the quality of the genes of the male.
            Thus, the population contains a constant excess supply of males that are willing to reproduce over the supply of females that are willing to reproduce. This leads to strong intrasexual competition and thus more intense sexual selection amongst males.

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Sexy son hypothesis

Several key moments are important in the game for the laziest parent, which takes place between males and females in a great many species. The female attempts to delay the actual act of copulation for as long as possible, so that the male has already invested as much as possible at the beginning of reproduction. Simultaneously, she cannot postpone copulation disproportionately long, because the male could lose patience and could try to obtain a more compliant (and more permissive) female. Simultaneously, intrasexual competition takes place amongst the males in the population – the evolutionary game of “who’s dumbest”, i.e. a competition for the most patient suitor, “if not this one, then another one”, i.e. a competition for the most successful searcher for a permissive female, “for the dude”, i.e. a competition in willingness to risk invested precopulation efforts and, after achieving copulation, go on to the next house, and a great many more, at least as interesting games.

            A number of evolutionary games also take place within the population of females; I will leave their designation to the fantasy of the reader. Simultaneously, choice of suitable strategy is frequently even more complicated for females in that the choice of a suitable sexual partner must be subject to other criteria to a far greater degree than only the probability of immediate gain from the game as to who is the laziest. While the optimal (although not always evolutionarily stable) strategy of males lies in maximum quantity, i.e. in lack of selectivity in reproduction, females must rather favor the qualitative aspect. They can influence their probable reproductive success primarily through selection of the best sexual partner, i.e. the male with the greatest fitness. Thus, for a female it is not a simple matter to determine the criteria according to which she should chose a suitable male. The fitness of a male need not be in any way correlated with his willingness to contribute to care for his progeny. In most cases, we can probably expect negative correlation, following from the existence of evolutionarily advantageous conditional strategy. If you are beautiful  and strong, then play the game of “if not this one then another one”; if you are low on sex appeal, than you still have a chance in the game of “who’s dumbest” {11547}. Observations of swallows has shown that females prefer to copulate with individuals with long tail feathers, while these individuals provide the least paternal care for their offspring (Reynolds 1996).

             The sexy-son hypothesis refers to the situation where it is more advantageous for females to choose a sexual partner that does not care much for his offspring, but which is most attractive for other females on the basis of different criteria. The disadvantage that choice of such a male represents for bringing up one’s own young is compensated here by the fact that the sons inherit attractiveness from their fathers and have a greater chance of reproducing in the next generation.

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Shaw-Mohler principle

see Evolution of secondary sex ratio

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Shifting balance theory

Relatively strong barriers to adaptive evolution exist in unstructured populations. This is because, in order to modify the phenotype of organisms – a prerequisite for a change in the environmental niche – the population must often pass through intermediate stages during which the organisms are already less adapted to utilize the old niche but not yet perfectly adapted to utilize the new niche. Another major obstacle to a population’s adaptive response to selection pressure lies in the genetic architecture of the phenotype traits, namely in the fact that alleles that, in their dominant status or in combination with particular alleles in other loci, help to create a particular trait very frequently in their heterozygotous state or, in combination with other alleles in other loci, participate in the creation of a completely different trait. The shifting balance theory formulated by Sewal Wright (Wright 1931; Wright 1982) in the 1930’s assumes that adaptive traits that cannot be fixed in a large unstructured population can be more easily fixed in a large structured population, i.e. in a population consisting of a large number of genetically and environmentally partly isolated small populations (Fig. VII.5). Dividing the population into a series of relatively isolated partial populations will, amongst other things, change non-additive genetic variability into additive genetic variability (Wade & Goodnight 1998)(see also II.7) which can, indeed, serve as the material for directional selection. Wright believed that evolution of an adaptive trait in a structured population has, in a certain sense, three stages. During the first stage, genetic drift in some small populations causes a shift in allele frequencies in favour of alleles that, in a particular combination, can determine a useful phenotype. During the second stage, the relevant alleles, and thus the relevant phenotype, are fixed in some of these populations through natural selection. In the third stage, group selection through migrants infecting other populations or establishing new populations causes the relevant alleles to spread newly throughout the metapopulation until they finally become generally fixed. Gene flow plays a crucial role in all three stages. In the first and second stage, high-intensity gene flow reduces the likelihood of useful trait fixation; in the third stage, on the contrary, it increases this probability. The validity of this model has repeatedly been confirmed by a number of theoretical (Barton & Rouhani 1993, 12322)and empirical studies (Katz & Young 1975; Wade & Goodnight 1991). However, some studies (Schamber & Muir 2001)did not confirm the conclusions of Wright’s shifting balance theory quite as clearly and the theory is therefore still a subject of discussion (Coyne, Barton, & Turelli 1997; Coyne, Barton, & Turelli 2000). Its opponents mostly argue that the theory is too complex and that the rise of adaptive traits through directional selection can be explained by even the simplest model of Darwinian selection in a large unstructured population. Its advocates, on the other hand, respond by arguing that the simplicity or complexity of a theory cannot be the final criterion of its accuracy (Peck, Ellner, & Gould 2000)and that Wright’s theory can explain even the fixation of alleles that, according to the simple neo-Darwinian model of selection, should not become fixed in large unstructured populations. 

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Sickle-cell anemia

The importance of selection for heterozygotes for maintaining polymorphism is not currently apparent. However, it is almost certain that at least some alleles are maintained in the population in this way. The best known example of selection for heterozygotes is selection for persons with sickle-cell anemia in areas affected by the occurrence of malaria, a disease caused by parasitic protozoa of the Plasmodium genus.  Sickle-cell anemia is a hereditary disease that appears in individuals with allele s, i.e. with an allele coding an abnormal β-chain of haemoglobulin. While the normal β-chain of haemoglobulin, coded by allele S, has glutamic acid in position 6, allele s codes valine in this position. This single substitution means that the blood cells containing abnormal haemoglobulin are deformed into a sickle shape at sites with lower oxygen partial pressure, i.e. in the capillaries, so that they are used up more rapidly and removed from circulation.  This effect is manifested drastically in s/s homozygotes, so that these individuals do not generally survive to reproductive age. s/S heterozygotes are also somewhat handicapped, but the reduction in their fitness compared to S/S homozygotes is not so significant.

The frequency of the occurrence of s alleles is highly positively correlated with the occurrence of malaria. It has been found that this is not a random correlation. Heterozygous s/S individuals are much more resistant against malaria than S/S homozygous individuals. The mechanism of this resistance is not exactly defined. As the protozoa develop in the red blood cells, it can be assumed that blood cells attacked by the parasite are deformed more readily than cells that are not attacked. Thus, they are more rapidly removed from circulation in the spleen, together with the parasite. Because malaria is a very serious disease that currently affects about 500 million people and that kills 1-2 million people every year, resistance against malaria provides a substantial selection advantage to heterozygotes. Thus, individuals with the s/S heterozygous genotype have the greatest fitness in areas with increased occurrence of malaria and selection for heterozygotes permanently maintains the presence of both alleles in the population. Malaria similarly maintains polymorphism in some other genes (Ruwende et al. 1995).See also Origin of Rh-blood group polymorphism.

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Signor-Lipps effect

see Fossils age of

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Simpson

see History of evolutionism - neo-Darwinist period

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Simultaneous selection model of advantage of sexuality

Only a sort of sequential evolution can occur in asexually reproducing organisms, i.e. gradual fixation of advantageous mutations in a single genealogical line (Fig. XIII.6). If several advantageous mutations occur at once in the population, the carriers of various mutations will compete. Only the most successful of them will, in the end, survive and its progeny will have to again “wait” for another mutation to occur, even though it already existed in the population in the past. In contrast, in sexually reproducing species, the evolution of various traits can occur in parallel and the individual mutations are selected simultaneously in various individuals (Fisher 1958; Muller 1932). Recombinants are formed sooner or later during sexual reproduction and at least some of them carry the advantageous mutation from both parents. If the positive effect of the mutations is additive, then these recombinants and their progeny are positively selected with greater effectiveness than the original parent line (Muller 1964). Mathematical models have shown that acceleration of evolution through this effect can be significant, especially in populations of medium size {12233}. However, in normal-size populations, evolution tends to be limited by the number of newly formed mutations rather than by their mutual competition. Thus, the advantage of parallel selection is much smaller here (Crow & Kimura 1965; Crow 1994). Another reduction in the effectiveness of this process (even in large populations) follows from the Hill-Robertson effect (Hill & Robertson 1966), i.e. a reduction in the effectiveness of selection occurring in favour of several alleles at the same time (see IV.2.1).

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Sir Sebright effect

Selection-determine differentiation of populations or individuals living under various conditions could be the mechanism responsible for the Sir Sebright effect (Flegr 2002). Darwin apparently first described this effect at the level of whole populations (Darwin 1868, Part II, pp. 115 – 117, 143) and Lysenkoists claimed to have observed it at the level of individual plants in their experiments in the 1930’s. Darwin described the experience of domestic animal breeders, according to which it is necessary to occasionally rejuvenate the breeding by crossing animals of the same race that have been bred for a long time under very different conditions. For this purpose, breeders kept one herd of domestic animals in the lowlands and one in the mountains and occasionally performed crossing between the animals in the two herds. This empirically tested procedure, for which a theoretical basis was long sought without success, continues to be used by breeders to the present day in some cases.  The most probable explanation of this phenomenon assumes that certain alleles are selected in a herd that is maintained under the same conditions for a long time, which gradually reduces the genetic variability of the population by elimination of other alleles (Flegr 2002). In two herd kept under different conditions (in lowlands and in mountains), different alleles are selected each time. If the animals from the two herds are occasionally mixed together and crossed, the original genetic variability of the population is renewed, so that some advantageous heterozygote genotypes with high viability begin again to be produced. Renewal of the original genetic variability can also reverse the consequences of gradual micro-evolution, during which the action of natural selection under the particular conditions can lead to a gradual increase in the fitness of members of the population at the expense of their production efficiency.

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Sir Sebright effect at the individual level

The Sir Sebright effect can also apply at the individual level to organisms without a Weismann barrier. Lysenkoists described experiments in which two plants cultivated by cloning from a single rhizome were crossed together. They stated that the progeny obtained are more viable when the two plants were grown under different conditions, e.g. one in dry and one in damp soil, than if they are grown under the same conditions (Turbin 1952, p. 138). It is quite possible that the Lysenkoists thought up this result, similar to a great many others. However, it is also possible that the Sir Sebright effect was manifested in their experiments, i.e. that intra-organism cell-line selection selected genetically different cell lines of the parent plants under different conditions and that genetically different germinal cells gradually formed from them. The combination of genetically different germinal cells subsequently led to the formation of more viable progeny than would have been formed by combination of genetically more similar germinal cells derived from plant clones kept under the same conditions. In addition to classical somatic mutations, the source of variability for intra-organism selection could, of course, also have been epigenetic changes (see II.8).

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Sister killers

A very interesting, although certainly not generally accepted hypothesis – see e.g. {11160}, which explains the absence of a mechanism  for the formation of haploid sex cells based on simple division of diploid cells into two haploid cells, assumes that the purpose of the present complicated mechanisms of nuclear division is to prevent the formation and spreading of hypothetical genes termed sister killers (Butcher & Deng 1994; Hurst 1993; Haig 1993a). If reduction of ploidy were to occur through simple division of diploid cells into two daughter cells, ideal conditions would be created for the formation and spreading of alleles that, following division of the diploid cells into two haploid cells, would program the haploid cell, in whose nuclei they would be present, to kill its sister haploid cell. The sister-killer allele would, at least initially, spread very rapidly in the population, as heterozygote diploid cells would produce only haploid sex cells with this allele. From the long-term viewpoint, such a system would be unstable as homozygotes with two copies of sister-killer alleles would not produce viable progeny. The allele would have to learn to recognize whether its copy is present in the sister cell and, on this basis, trigger or not trigger killing. Following the creation of such a mechanism, it would become fixed in the given species and would simultaneously cease to be manifested externally in the phenotype. If such a mechanism were not created, the action of sister-killers could even lead to the extinction of the particular species. The third possibility is apparently most probable – through the drastic selection pressure of sister-killer alleles, alleles would become fixed at some locus that would provide their carriers with resistance to sister-killer alleles. Species exposed to constant waves of fixation of sister-killer alleles would, of course, be at a disadvantage compared to species in which a mechanism would exist to prevent the formation of these alleles in advance, so that this mechanism could be fixed in the biosphere by species selection (IV.8.4). Meiosis and other currently known means of creation of haploid sex cells could be the mechanism for preventing the spreading of sister-killer alleles.

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Sisyphean genotypes model of advantage of sexuality

The spreading of phenotype variability in sexually reproducing species allows generation amongst progeny of Sisyphean genotypes (genetic elite), i.e. individuals that are so well adapted to current conditions that they produce far more progeny during their lives than other individuals in the population. G.C. Williams introduced the term Sisyphean genotypes for them; the original name, genetic elite, was introduced by T. Dobzhansky. Williams’ name is probably more appropriate, as it primarily emphasizes the fact that the emerging genotype of these individuals appeared only because of segregation and recombination and thus has minimal heritability and must be formed anew in subsequent generations (Williams 1975). Both authors assumed that extraordinarily biologically fit individuals, whose direct or indirect descendants will subsequently constitute most of the individuals in the population, occur with very low frequency in very large populations of sexually reproducing species. s

            It is very probable that this mechanism can work only for species with extremely high reproduction potential, i.e. where one individual produces so many embryos that its progeny could, in the ideal case, i.e. if all the embryos lived to maturity, maintain the existing population size even if the other members of the population did not reproduce at all. Williams introduced the term non-Markovian species for such species. While, in Markovian species, the number of individuals in the population depends on the number in the given population in the previous generation, this is not true in non-Markovian species, as the occurrence of only a single individual with optimum genotype (or with a good portion of luck) can supplement the size of the population from any random value to the maximum size, limited from above only by the relevant ecological regulation mechanisms (see IV.4.1). At the present time, only some species of wild fauna could fall in this category, possibly including fish, and also some endoparasites; however, in the past, the common ancestors of most of present-day Markovian species could have passed through this stage. Sexuality could be a trait that was advantageous in the past for non-Markovian species, as it allowed them to create Sisyphean genotypes; however this need not be advantageous from this viewpoint for modern species and could be preserved, for example, through the evolutionary trap mechanism (see XIII.3.3).

 

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Slightly negative mutations

– It followed from equation (5) that the number of mutations fixed per time unit should not depend on the size of the population. However, it is apparent from empirical data obtained from experiments and study of natural populations that more molecular polymorphism is frequently maintained or even fixed over the same time in small populations than in large populations (Nevo et al. 1997). This discrepancy between the theoretical conclusions and fact was explained by the Japanese geneticist  T. Ohta (Ohta & Gillespie 1996; Ohta 1998). The core of her explanation consisted in the quite reasonable assumption that most mutations that occur in organisms fall in the category of slightly negative (slightly deleterious) mutations (Fig. V.11). On the basis of study of the mathematical models, W.H. Li derived that mutations that positively or negatively affect the biological fitness of their bearer act as effectively neutral when the absolute value of their relative selection coefficient Thus, in small populations, the product of the selection coefficient for slightly negative mutations and the effective size of the relevant population (Nes) is less than 1, so that these mutations act as effectively neutral, i.e. their fate is determined by genetic drift and not natural selection. The probability of fixation of these slightly negative mutations is thus approximately equal to their frequency in the population. In contrast, in large populations, the product of the selection coefficient and the size of the population is relatively larger. In a great many cases, it exceeds a value of 1, so that natural selection decides on the fate (elimination) of this mutation. As a consequence, slightly negative mutations are fixed less in a large population than in a small population over the same period of time. In contrast, slightly positive mutations are fixed more in a large population, as natural selection effectively assists in their proliferation here. As there are more negative mutations than positive mutations, it is apparent that overall more mutations are fixed in a small population than in a large population over the same period of time. Thus, the results of theoretical studies are not in any way contradictory to the empirical data: over a given period of time, neutral mutations are fixed with the same effectiveness in large and small populations; however, a greater fraction of all mutations represent effectively neutral mutations in a small population.

see also Mutations.

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Slipped strand mispairing

Slipped‑strand mispairing leads first to local short-term denaturing of the double-helix DNA molecule and subsequently to renaturation of the segment of the strand, not with the original complementary part of the opposing strand, but with some other area containing the complementary base. If this event is followed by reparation and further replication of the given DNA segment, this is followed by either multiplication or, on the other hand, deletion of a certain sequential motif (Levinson & Gutman 1987)(Fig. VI.7).

Slippagecan also occur during the replication itself by slippage of the template for the DNA-polymerase enzyme (Fig. VI.8). In the site where repetition occurs of a certain nucleotide or short oligonucleotide, the DNA-polymerase can “slip” backwards or forwards along the template at a certain moment and synthesize a new DNA strand twice in a row according to the same template section, or leave out a certain section of the template. The probability of this event is inversely proportional to the length of the oligonucleotide of which the given repetition consists and directly proportional to the number of oligonucleotides in the repetition. As a consequence, although slippage can be followed by insertion or deletion of the particular motif, there is clear asymmetry in the probability of the two events. Insertion of a new copy increases the probability of further slippage, while deletion reduces it. For example, it is known that the frequency of spontaneous insertion and deletion of A in pentanucleotide AAAAA is more than one order of magnitude greater than their frequency in tetranucleotide AAAA. This means that the familiar principle of positive feedback is important in multiplication of repetitive motifs and the entire process of multiplication of certain motifs has a marked accelerating, avalanche character.

Avalanche multiplication of motifs in the genome of the organism is responsible for a number of serious diseases. For example, multiplication of the (CAG)n motif in the gene for androgen causes Kennedy’s disease, multiplication of the same motif on a different gene is responsible for Huntington’s disease, and multiplication of (CCG)n is observed in all cases of the fragile X chromosome syndrome (Lubjuhn, Schwaiger, & Epplen 1994).

Multiplication and the general mutability of simple repetition segments form the basis for the microsatellite analysis technique (see XXIV.3.7).This technique, which is based on amplification and characterization of the individual loci containing simple repetitions, is broadly employed in various population studies. The mutability of these segments allows microsatellite analysis to monitor a large amount of genetic polymorphism even in mutually quite related individuals. This can be employed, for example, in determining paternity or in determining the internal structure of a population.

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Small-world network

Small-world network – see Metapopulation

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SMRS

see Species recognition concept of species

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Sneaking through of an antigenic agent

see Infrapopulation of parasites

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Snowball Earth hypothesis

see Mass extionction causes

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Social Darwinism

see Presumed negative impact of evolution theory on people’s ethical attitudes

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Social learning

see Cultural traits transmission of

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Sociobiology

The behavior of organisms very soon became a subject of interest for evolutionary biologists. Ethology itself became concerned with studying patterns of behavior and how they became fixed (i.e. through natural selection) during evolution. Soon, patterns of behavior controlling the relationship between individuals within a social group came to the forefront of the interest of evolutionary biologists. It was found that knowledge of the mechanisms of evolution allow successful prediction of which patterns of behavior have a chance of becoming fixed in a particular species and which would, on the other hand, disappear, even though their fixation might be advantageous for a social group or species. Study of these patterns of behavior and the mechanisms of their evolutionary formation has become the subject of sociobiology. The author of the book “Sociobiology: The new synthesis” (Wilson 1975b), Edward Osborne Wilson (*1929), is mostly considered to be the most important author in this field.

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Somatic mutations hypothesis

- The hypothesis of somatic mutations (Gorshkov & Makarieva 1999) is based on the suggestion that one of the important functions of diploidy could consist in protection against somatic mutations. Somatic mutations occur during the ontogenesis of every multicellular organism. In large organisms, the number of cell divisions between the zygote and the adult organism is rather large. Consequently, during ontogenesis, a great many loss and thus frequently recessive lethal mutations accumulate in their cells. Their presence does not matter in diploidal organisms, as they do not enter the germinal line and are generally not manifested in the somatic cells, because there is only a small probability that similar mutations would occur in both copies of a single gene. However, sex chromosomes are present in the cells of members of the heterogametic sex in the haploid state which is quite fundamental for the viability of the organism in the case of the X-chromosome, which generally contains a large number of genes. While, in homogametic females, the X-chromosomes act as autosomes, so that loss somatic mutations of their genes are recessive and are not externally manifested, in members of the heterogametic sex with a single copy of the X-chromosome, the presence of these mutations is manifested in reduced functioning of the cells and tissues and thus reduced viability of the organism. Similar to the recessive gene hypothesis, the hypothesis of somatic mutations also explains the strong manifestations of this effect in interspecific hybrids by an exponential dependence between the number of mutations and the decrease in the fitness of the individual. In contrast to the recessive gene hypothesis, the reduced average frequency of recessive negative mutations on X-chromosomes, which is necessary consequence of the greater effectiveness of selection acting on the representatives of the heterogametic sex, in which the harmful effect of mutations is not masked by the presence of a functional copy on the second X-chromosome, does not represent a complication here.. The frequency of these mutations in the gene pool is totally irrelevant; the reduced viability of hybrids is a result of somatic mutations occurring during the life of the individual.
The hypothesis of somatic mutations also explains the results of experiments with hybrid drosophila with an “unbalanced” genome (see XXI.4.3.1). It is apparent that, in males, which bear two X-chromosomes from the same species, the effect of recessive somatic mutations on the X-chromosome must be less than in hybrid males with a single X-chromosome, i.e. just that demonstrated by experiments with drosophila, i.e. contrary to expectations following from the dominance hypothesis.
The hypothesis of somatic mutations also offers a simple explanation of why the Haldane rule for viability is basically not valid for mammals. In this taxon, compensation of the genetic dose, i.e. inactivation of one of the X-chromosomes, occurs in the somatic cells. Because of this inactivation, each somatic cell of a female, similar to males, contains only one active copy of the X-chromosome, and the viability of hybrid females is thus reduced here similarly to the viability of hybrid males. However, this effect can analogously also be explained by the dominance hypothesis.
The hypothesis of somatic mutations also explains other phenomena encountered in nature and not directly related to the Haldane rule. It explains why all large fauna are diploid, while small fauna, whose somatic cells undergo only a small number of divisions during ontogenesis and accumulate only a small number of mutations, are sometimes haploid. It also offers an explanation of why male haplodiploid insects are much more sensitive to irradiation in the early stages of ontogenesis, when their cells are haploid, where the differences between the sexes disappear in adulthood, when diploidization or tetraploidization occurs in most of the cells in the somatic tissues of males. This explains why large species have small X-chromosomes, while small species, for example drosophila, can have X-chromosomes that bear up to 40% of all the genes. This also provides an explanation for the fact that, in mammals (with heterogametic males), the males mostly exhibit higher mortality and a shorter average lifetime, while the opposite is true of birds (with heterogametic females) (Promislow 2003).
            The hypothesis of somatic mutations, in itself, cannot explain the existence of all the phenomena described by the Haldane rule. The sterility of the members of the heterogametic sex is a result, to a major degree, of defects in the germinal cells themselves, where the number of cell divisions that occur in the germinal line is low for large organisms. It is thus apparent that a number of mechanisms are responsible for the phenomena described by the Haldane rule; however, they most probably also include the accumulation of somatic mutations in the sex chromosomes.

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Sorting from the standpoint of stability

Sorting from the standpoint of stability is an important mechanism of any evolution, including biological evolution. If structures that differ in their degree of stability are formed in the system, there will be a gradual increase in the content of those that will exhibit greater stability – lifetime. These structures will also be accumulated even if the formation of the more stable structures is statistically less probable than the formation of less stable structures. The sorting is similar to selection, however, only the stability of sorted entities can be the criterion of  evolutionary success. The most important difference between the sorting and the natural selection is the absence of any role of heritability in the former mechanism.

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Spandrels

Spandrels are architectural elements that develop not through the intention of the architect, but as a consequence of objective, e.g. geometric. Pendentives are given as a special case of spandrels in evolutionary literature; these are spherical triangular areas permitting the placing of a circular dome over a square room. The best known case (at least for evolutionary biologists) consists in the pendentives in the Basilica of St. Mark in Venice. At the present time, these pendentives bear pictures of the four evangelists and thus seem to be an essential and intentionally created element of the artistic decoration of the cathedral. In actual fact, they were not created to bear these paintings, but because this was the most rational structural solution for joining a four-walled base with its cupola ceiling. It is typical that the pendentives in the Basilica of St. Mark acquired an adaptive significance, i.e. that mosaics of four evangelists were placed on them later, several centuries after the creation of the basilica. Many biological structures, even highly organized structures with a highly favourable effect for the survival of the organism, are in fact spandrels, not adaptations.

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Speciation

One of the characteristic features of life on Earth is its extreme diversity, which is manifested in the existence of a great many very distinct species (diversity in the narrow sense of the word) and the differences between these species (disparity) It is obvious, and this also follows from the character of paleontological findings from the Proterozoic, that the biodiversity of organisms was incomparably less at the beginning of evolution and that its increase, including the increase in the number of individual species, occurred only gradually during evolution. The process, during which one or more new species are formed from a single old species, is called speciation. A number of kinds of speciation are known at the present time, which differ substantially in their mechanisms. It is assumed that some occur very frequently, while others are quite rare and there can be serious doubts about their very existence. This chapter will be concerned with the individual types of speciation as they can be encountered in the contemporary evolutionary literature. Particular attention will be devoted to the mechanisms of speciation in species with sexual reproduction. There are two reasons for this: to begin with, these species greatly predominate in nature (although not in the number of individuals and possibly not even in the total amount of biomass) and also because only for them does speciation also require the formation of reproductive isolation barriers between the old and new species. And it is the mechanisms forming reproductive isolation barriers that constitute a complicated and very interesting aspect of evolution.

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Speciation allopatric

A new species can be most readily formed by gradual evolution outside of direct contact with the parent species, i.e. by allopatric speciation (Fig. XXI.3). If, for example, a geographically isolated population is formed, which branched off from the population of the original species, and this population is reproductively isolated from the parent population for a sufficiently long time, genetic changes can gradually accumulate in its gene pool that finally lead to phenotype and subsequently also ecological differentiation of the two populations. If the two populations come into contact before they are sufficiently differentiated, the two species can again merge into a single species. Otherwise, the two species can exist sympatrically next to one another (if there was differentiation of their niches) or one of the species can force the other species out of the location or even globally.
If an originally uniform population is divided by a barrier (mountain range, river, for aquatic organisms a strip of land) into two comparably large populations and these populations differentiate in time both genetically and phenotypically, then this is called vicariant speciation or dichopatric speciation. On the other hand, if only a very small population splits off from the parent population and then gradually develops into a new species, this is termed peripatric speciation (Mayr 1999) (se Fig. XXI.3). Empirical data and the results of theoretical analyses, for example comparison of the differences in the sizes of the ranges of occurrence of young sister species, indicate that peripatric speciations are apparently more common than vicariant speciations (Barraclough & Nee 2001); however, sometimes quite the opposite is stated {8917}. In addition, it seems that these types of peripatric speciations more frequently lead to the evolution of species that have different phenotypes than the parent species. For example, mutually related species and geographical races of kingfishers of the genus Tanysiptera, which occur on tiny islands in the vicinity of New Guinea and which probably originated by peripatric speciation, differ in their phenotype substantially more than the related species and races occurring on New Guinea (Mayr 1963).
The more frequent occurrence of peripatric speciations can have a quite prosaic cause. The splitting off of small populations, e.g. by introduction outside of the original range of occurrence or splitting off of tiny subpopulations on the fluctuating edge of a range of occurrence can occur far more often than division of the original range by a newly formed barrier. In most cases, these new subpopulations disappear or merge with the main population after some time. However, a certain percentage of them can lead to the formation of a new species.
There can be at least two reasons for greater phenotype differentiation of species formed by peripatric speciation. Populations at the very edge of the range of occurrence and even more so populations formed by introduction outside of this range mostly find themselves in different natural conditions than those in which most of the populations of their species live. Thus there are also different selection pressures acting on them and, as a consequence, their genotype also substantially changes with time. However, if the original range is divided into two parts of approximately the same size, the natural conditions in the two parts will probably be rather similar. Thus, the sister species will be differentiated more by the action of evolutionary drives and genetic drift than by the action of different selection pressures. Thus, the differences between the species will very frequently tend to be selectionally neutral and need not substantially affect the phenotype.
The second reason for the greater differences in species formed by peripatric speciation can lie in the founder effect and following transition of a species from the evolutionary frozen to a plastic state. The existence of the founder effect was derived in the middle of the last century by Ernst Mayr (Mayr 1963). He stated that species cohesion exists in sexually reproducing species because their gene pool represents an integrated whole – an adaptive gene complex, in which approximately constant representation of the individual alleles is spontaneously maintained through a form of genetic homeostasis. If a new allele appears in the gene pool, either by penetration from an external gene flow or formed directly on site through mutation, it will not be capable of functioning as well in the context of the other alleles present in the gene pool of the population as the original alleles, which have been repeatedly tested in all the possible combinations. Thus, it will be eliminated in the population in time. Massive penetration of foreign alleles, e.g. as a consequence of merging of two originally separate populations, can even lead to a drastic reduction in the average fitness of the members of the population. This will be caused both by the fact that migrants coming from different conditions and their progeny will have phenotypes that are poorly adapted to local conditions and also by the fact that crosses that have emerged, bearing untested combinations of local and foreign alleles, can have reduced viability and fertility – alleles derived from distant populations will not be sufficiently mutually compatible. If an originally uniform population divides into two daughter populations, each of them will bear approximately the same gene pool, in which the frequencies of the individual alleles will remain mutually interconnected and the overall composition of the gene pool will thus remain stabilized. Thus, the gene pools will have only limited ability to evolve. The action of strong selection pressures can force the frequencies of the individual alleles to deviate somewhat form the original values; however, the greater this deviation, the greater will be the resistance of the gene pool of the population to the particular selection pressure. Following reduction of the selection pressure, the frequencies of the alleles will return to the original value (see the genetic homeostasis effect, IV.9.2). In contrast, a small population formed by splitting off from the large population will take only a small part of the overall genetic polymorphism with it and, subsequently, they will lose most of the remaining polymorphism through drift. Even if the population rapidly increases in size in the new environment without competitors, a great many alleles will be completely missing in it or will occur randomly, as a consequence of the founder effect, with very unusual frequency. This can completely disturb the homeostatic stabilization of the composition of the gene pool and the population will respond much more readily and willingly (and irreversibly) to selection pressures. In addition, in a population with drastically reduced polymorphism, new alleles, for example formed by mutations, will always find themselves (i.e. in each newly born individual) in the context of an almost identical set of alleles. Thus, their selection coefficients will not change from one generation to the next, i.e. will not, for example, oscillate between positive and negative values. Thus, the selection of new alleles can be far more effective in a non-polymorphic population than in a polymorphic population (Flegr 1998, Flegr 2010) and the newly formed daughter species will be able to differ substantially from the parent species.

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Speciation branching

The process in which speciation occurs in that one species as a whole gradually changes anagenetically into a different species, i.e. the phenotype traits of its individuals change, is termed phyletic speciation. Phyletic speciation leads to an increase in the total number of species in the paleontological record, but the biodiversity existing in nature at the particular moment does not change, as one species simply changes into a different species. In contrast, in branching speciation, one parent species divides into two or more daughter species, which further evolve separately in their phenotype traits. If a new species is formed by phyletic speciation in a single evolutionary line, the individual species following one after another are designated as chronospecies and the disappearance of the older species, i.e. their conversion into new species, is termed pseudoextinction.
            In some cases, cladistic speciation (Gould 2002) is differentiated as an independent type of branching speciation; in this type, the phenotype traits of the parent species do not change; a daughter species is split off from it and differs in both its phenotype and genotype (Fig. XXI.1). The name cladistic speciation is not entirely a happy one. If a parent species gradually splits off a number of daughter species, then the dendograms formed using cladistic methods will show all the species branching off from a single point, i.e. this will correspond to polytomy at the given point. Simultaneously, it is the goal of cladists to create a dichotomically branched dendogram, i.e. a dendogram on which only two branches will branch out from each point.
            The differentiation of speciation into phyletic speciation and branching speciation is justified at the theoretical level, but can be rather problematic in practice. Phyletic speciation in species consisting of a great many individual populations, i.e. apparently the majority of species, can frequently occur in only one or a few populations. Thus, at a given moment, populations with the individual species and populations with the new species coexist in nature. If the two types of populations coexist for a sufficiently long time, the process will be considered to correspond to branching speciation. If, on the other hand, the population with the original species soon becomes extinct, for example its members succumb to competition from the members of the new species, then this will seem to be phyletic speciation. Thus, in practice, we are capable of differentiating between the two kinds of speciation only for species with unstructured populations, i.e. in species that perhaps don’t even actually exist.

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Speciation by coevolutionary lift

- Sexual selection can basically occur through two different mechanisms. The members of a certain sex, in the vast majority of cases males, can do battle for access to sexual partners or they can be selected as their sexual partners by the members of the opposite sex on the basis of more or less arbitrary criteria.
In the latter case, objective preconditions are formed for the manifestation of a continuously acting tendency towards the creation of new kinds of species by the coevolutionary lift mechanism (autoelection). Amongst females, there is a certain constant variability in the genes determining the traits according to which they will select their sexual partners. As soon as a new trait appears in the population and, simultaneously, in the population of females a gene for preference for this trait emerges, favorable conditions are formed for positive assortative mating of the bearers of this trait and the bearers of the genes for its preference, and thus for branching off and partial reproductive isolation of a certain part of the population. If the presence of the new trait were to reduce the attractiveness of the particular male for the other females and increase his attractiveness only for the subpopulation of females bearing the gene for its preference, the presence of the two complementary genes will constantly increase in the population. The reasons for the increase in the frequency of genes for preferred trait in the population are obvious. However, the genes for preference for the particular trait are also at an advantage. A bearer of the trait (male) is most probably also simultaneously a latent bearer of the gene for preference for this trait. His father was most probably a male who was also a bearer of this trait and therefore his mother was a female that most probably preferred this trait, as she selected a bearer of this trait as her sexual partner. The genes that cause that the female prefer the bearer of a certain trait thus also assist in its preferential spreading, to be more exact spreading of its own copies, which are most probably contained in the genotype of preferred individuals. The fact that there is a simultaneous increase in the frequency of both traits in the population accelerates the coevolutionary lift process because of positive feedback and the gene for the new trait and the gene for its preference are fixed in the population.
            The evolution of the system does not end with fixation of the two complementary genes. As soon as all the males in the local population carry the new trait, females that lose the ability to recognize the presence or absence of this trait in their sexual partners cease to be “penalized”. Thus, the gene for preference for the particular trait can gradually disappear from the population. In contrast, males that would lose the particular trait will be selectionally “penalized” as long as at least some females remain in the population that bear a functional gene for preference for the relevant trait. Consequently, males will have a tendency during evolution to gradually accumulate and fix genes for marked secondary sexual traits, while in females the genes for preference for certain sexual traits will tend to periodically fluctuate and accumulate in the population maximally in the form of more or less selectionally neutral polymorphism.
The evolution of male secondary sexual traits and female preference traits can be important from the viewpoint of speciation, for example, when the species has a discontinuous range of occurrence and a number of various traits, according to which the females select their sexual partners, are gradually independently fixed in each individual population. As these traits are fixed in separated populations, some of them can be mutually exclusive or the differences between the males in two populations can be so great that the males of one population can become completely unattractive for the females of the other population. Thus, internal prezygotic reproduction barriers can be created as a consequence of the action of sexual selection through female choice.
If two new sexual traits spread simultaneously in a single population, which can occur quite frequently in species with large unstructured populations, three basic alternatives can occur. If these are traits whose expression in a single individual is mutually excluded, and if the population is otherwise panmictic, it is highly probable that the trait that is preferred by a greater percentage of females will predominate in the population. The bearers of this trait have a greater chance of reproducing than the bearers of the alternative trait. However, the suppression of one of the traits does not mean that its bearers and their progeny, i.e. all its alleles located in other loci, would be eliminated from the population by selection. As the gene flow barriers formed by preference for sexual partners are mostly permeable, i.e. individuals sometimes make a mistake in selection of the right sexual partner, and the individual genes can “move” between the chromosomes bearing various alleles for the preferential and preferred genes through genetic recombination, probably only the genes for the particular trait, genes for its preference and also alleles in immediately neighboring loci will be eliminated.
The second possibility occurs if the traits are also mutually exclusive, but the population is spatially structured and new traits spread towards one another from two different areas. A more or less sharp boundary in male phenotypes will tend to exist at places where the two populations meet. If a male passes into the territory in which females preferring bearers of the opposite trait predominate, he will be at a disadvantage in sexual selection and will leave fewer progeny. Simultaneously, there need not be practically any barrier between the two forms for the passage of other genes than genes for the sexual trait and genes for its preference. As soon as a male in the territory of the other form crosses with the local female form, some of its sons will inherit from the female the genes for the alternative form of the sexual trait and some of his daughters will inherit the gene for preference for this trait, so that they will not be at a disadvantage in competition with the other members of the population and will be able to freely pass their genes down in the second part of the population.
The third possibility occurs when the expression of both traits is not mutually exclusive. In this case, both traits will probably be fixed.
The importance of sexual selection in speciation is not entirely clear at the present time. However, meta-studies have indicated that, of 15 formerly published comparative studies, twelve demonstrated a positive correlation between the level of sexual dimorphism and species diversity in the relevant taxon (Panhuis et al. 2001). This could mean that species in which this mechanism is important also actually exhibit a greater rate of speciation. On the other hand, from the viewpoint of anagenesis, the coevolutionary lift mechanism could explain the fact that there are very great differences between related species in traits visible on the surface, i.e. in coloration, patterns or structure of the surface. This mechanism could, in fact, be responsible for “address phenomena”, whose existence was pointed out in the mid-twentieth century by A. Portmann (Portmann 1960). It is very striking that most organisms have a tendency towards “self presentation”, i.e. have various ornaments, bright colors or structures on their surfaces. In contrast, most internal organs are usually uniform in color, aesthetically uninteresting or even ugly. It is frequently very difficult to imagine a biological function that these surface structures, which are part of the self presentation of the members of a certain species, could fulfill for their bearers, e.g. some marine turbellarians or opistobrancial gastropods. It is quite possible that, in actual fact, these traits do not provide any advantage for their bearers, but only provide an advantage for those genes that encode their formation, and the genes that enable the members of the other sex to recognize these traits. Any mutation can be fixed by genetic drift. However, the probability of fixation of a new mutation by drift is very low. Mutations advantageous for the aspect of survival can become fixed by natural selection; however, this mechanism is also not very efficient in polymorphous populations of species with sexual reproduction (see IV.9). In contrast, practically any mutation that is capable of sufficiently obvious manifestation in the appearance of its bearer can be very efficiently fixed in sexually reproducing species through the coevolutionary lift mechanism. Thus, coevolutionary lift could be responsible for a large portion of self-presentation phenomena manifested in living organisms and thus for a large part of the aesthetics of living nature.

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Speciation chromosomal

The morphology or numbers of chromosomes in the karyotype change by chromosome mutation. In sexual reproduction, especially during meiosis, the new and old karyotypes need not be compatible, i.e. all the pairs of homologous chromosomes need not be capable of forming regular bivalents. As a consequence, meiosis need not occur successfully or can lead to the formation of aneuploid germinal cells. This results in partial or complete sterility of hybrids, which form an effective postzygotic reproduction barrier. As soon as homozygotes with the new karyotype appear in the population, they can cross together without any problems. The formation of a new species by chromosome mutation is usually termed chromosome speciation.
The importance of chromosome speciation is a frequent subject of discussions. They are often based particularly on the fact that most even very closely related species differ in their karyotype. It is sometimes estimated that 90-98% of speciation is accompanied by a karyotype change (White 1978). A great many biologists conclude from this that chromosome mutations have a fundamental, possibly key importance in speciation. However, a number of biologists object that the karyotype differences between related species could be caused by the fact that this trait mutates quite frequently and the mutations can spread rapidly by meiotic drive within a panmictic population. It is sufficient if a certain chromosome variant, for example a chromosome formed by the fusion of two other chromosomes, has a greater chance of being transferred to the oocyte in the cells of the heterozygote than to the polar body, and it will spread very effectively in the panmictic population even if it will reduce fertility, and thus the fitness of its host, to a certain degree. New chromosome mutations can spread very rapidly within one species, so that all the members of the species are mostly karyotypically uniform. However, as soon as the gene pool is divided into several parts through any type of speciation, the new chromosome mutations can no longer cross the borders between the gene pools and a different mutation is fixed in each of them - the types become karyotype differentiated. Thus, interspecific differences in karyotypes can be rather a consequence than a cause of branching speciation.
The main difficulty with chromosome speciation (at least if this were to occur sympatrically in a panmictic population) is that, immediately after their formation in the population, mutants occur with low frequency and thus reproduce almost exclusively with individuals with the original, incompatible karyotype. Thus, they are at a considerable selection disadvantage compared to the original form as the members of this form, to the contrary, encounter almost only the much more common individuals with compatible karyotype. Thus, there is only a relatively low chance in a panmictic population that a chromosome mutation could lead to the formation of a new species. In contrast, the situation is far more favorable in a spatially structured population, for example in immobile organisms, such as plants. Mutant individuals have a good chance of always reproducing with their neighbors, which will often be their relatives and thus carriers of the same mutation. Thus, they need not be at a great disadvantage compared to members of the original form. Thus, the population of mutants can gradually spread from a certain place within the range of the original species. Low-mobility organisms, in whose karyotype a certain type of chromosome mutation frequently occurs, can thus very easily and frequently undergo stasipatric speciation and form complexes of mutually neighboring, geographically separated and phenotypically rather similar or identical species in a certain territory (King 1993).

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Speciation cladistic

see Speciation branching

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Speciation dichopatric

see Speciation alopatric

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Speciation ecological

Ecological speciation is a form of speciation that has come into and fallen out of favor (Via 2001). Ecological speciation is generally termed sympatric speciation in discussions related to this subject. As sympatric speciation also includes a number of types of instant speciation which, understandably, have completely different mechanisms, this established terminology is not really appropriate. Ecological speciation very frequently has the character of parapatric speciation. A great many species of organisms have a rather broad ecological valence, while the environment in which they occur has a heterogeneous and discontinuous character from the viewpoint of the prevailing conditions. Thus, it is advantageous for the population to specialize on various local environmental conditions by evolving various ecological forms, each of which is adapted to a certain type of environment or certain strategy for utilization of these conditions. The numbers of the individual ecological forms can fluctuate in the population in dependence on the character of the environment in which they occur. If the individual types of environment are also separated spatially and the organisms are not very mobile, there is a greater chance that the members of a single ecological form will preferentially reproduce together and will only occasionally cross with members of the opposite form.
 The existence of large phenotype differences between the members of the two forms, as a result of which they need not recognize one another as members of the same species, can have similar consequences. In some cases, such a situation can even lead to differentiation of the parent species into two daughter species, which divide up the original ecological niche and then each of them will have lower ecological valence than the original species. The effect of disruptive natural selection, caused by the original differentiation into two ecological forms, is frequently further reinforced by the fact that the crosses of the two forms have a transition phenotype, which is suboptimal in both types of environment. This creates a selection pressure on the creation of further, this time prezygotic (for example ethological) mechanisms that could reduce the probability of crossing between the members of the two ecological forms. The main problem in functioning of ecological speciation is the necessity of preventing recombination, which would break the connection between the genes responsible for the ecological differentiation between the two forms and the genes responsible for their prezygotic reproductive isolation. Mathematical models have shown that the formation of a new species is favored by a situation in which only a small number of genes is responsible for prezygotic isolation and a medium number of genes is responsible for ecological differentiation (Kondrashov & Kondrashov 1999; Via 2001).
 To the present day, only a few systems have been described in which the mechanism of ecological speciation is assumed to play a role. Primarily, this applies to the historically documented case of speciation of the apple maggot fly Rhagoletis pomonella in America, which moved from the hawthorn to apple trees in the middle of the nineteenth century (Bush 1969). Other probable candidates for ecological speciation include cichlids in one of the African crater lakes {12524}, palms in Lord Howe Island {12340} and possibly also American freshwater sticklebacks.

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Speciation ethological

Sexually reproducing species must have evolved specific mechanisms enabling mutual recognition of sexual partners. Only in this way is it possible to ensure that the members of a single species will recognize one another in nature and reproduce together. Traits according to which the members of one sex recognize the members of the other sex of the same species or according to which the members of a single hermaphroditic species recognize one another can be subject to evolutionary changes through the action of genetic drift and selection. As soon as a certain part of the population creates a new means of recognizing sexual partners, preconditions are created for the branching off of a new species by ethological speciation. The differences in these mechanisms that could evolve, for example allopatrically, can form very effective, internal, prezygotic reproduction barriers that are capable of ensuring the coexistence of two evolving species even if they secondarily meet in a single territory.

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Speciation extinction

Extinction speciation is another example of instant speciation (Fig. XXI.2). A great many species form an extended linear series of individual populations in their area of occurrence, where exchange of genetic material (sexual reproduction) occurs solely or almost solely between neighboring populations. Geographically more distant populations can thus be so phenotypically and genetically different that their members are not capable of fertile crossing and can occupy different ecological niches in nature. Nonetheless, they must be considered to be the members of a single species, as exchange of genetic material is mediated by a number of mutually neighboring populations, which geographically connect them. Ring species are an interesting example of species with this type of population structure. Their range of occurrence frequently forms a relatively narrow line of mutually neighboring populations that, for example, encircle a certain geographic obstacle (Californian salamanders of the Ensatina genus) or that even extend in a strip around the entire Earth (the lesser black-backed gull (Larus fuscus) and the herring gull (Larus argentatus)). These species spread in the past in one or both directions from the place of their original occurrence until their populations met secondarily in a certain area and closed the range of occurrence to form its present-day ring shape. Thus the phenotypically and genetically most distant populations met at the site of encounter and their members act as well defined species; in the ideal case, they do not cross and even do not compete much. Extinction speciation occurs when one of the inner populations becomes extinct, interrupting the gene flow from one end of the range to the other.

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Speciation gradual

Most speciations take quite a long time and these are called gradual speciations. Allopatric speciation is usually an example of this kind of speciation (see XXI.3). Genetic differences gradually accumulate between two populations of a single species occurring in two spatially separated territories, leading in time to phenotype differentiation of the members of the two populations and simultaneously to the formation of reproductive isolation barriers in sexually reproducing species. If the genetic differences accumulate only through the effect of genetic drift, the formation of sufficiently effective reproductive isolation barriers can take quite a long time. For example, in drosophila, the time required for accumulation of a sufficient number of genetic changes was estimated using the molecular clock at 1.5 – 3.5 million years (Coyne & Orr 1989). However, a great many examples of flora and fauna are known in which speciation does not occur even in situations where their American and Asian areas of occurrence were geographically separated for more than 20 million years. It has been stated that mammals lose the ability to cross after 8 million years of divergence, while birds and frogs retain it for 55-60 million years {9992}. If natural selection also contributes to speciation, then the progress of speciation can be substantially faster. For example, several hundred species of cichlids in Lake Victoria probably evolved from a single ancestor over 100 thousand years {9905} but, according to some ideas only over12 thousand years (Johnson et al. 1996).
            However, there are types of speciation that can occur almost in an instant. Polyploid speciation is an example of instant speciation; as a consequence of a cell division disorder, a tetraploid individual is formed from a diploid, usually flora, species. Because of their polyploid genome, the members of the new tetraploid species are simultaneously phenotypically different from their diploid predecessors and, because of their different phenotypes, also have different ecological requirements. The old diploid and new tetraploid species thus need not compete and can coexist permanently in a single territory.

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Speciation hybridization

The hybridization of two different species frequently yields crosses that have phenotype traits different from both the original species and even exhibit better viability in some habitats. If crossing occurs repeatedly between these species, interspecific crosses can have great importance in the particular ecosystems. In subsequent filial generations or on recrossing with the parent species, however, both their viability and their fertility are reduced as a result of irregularity in the separation of chromosomes derived from two different species. If we ignore the possibility of transition to a purely asexual means of reproduction, there are two basic ways in which hybridization can lead to the formation of a new fully fledged species.
The first means of hybridization speciationis called recombination speciation. The individual recombinants derived from crossing of hybrids of the F1-generation may occasionally contain individuals that are normally fertile and have different ecological requirements than the original species. If sufficiently strong reproduction barriers are also created between these individuals and the original species, they can form the basis for the emergence of a new species.
Hybrid polyploidization is another means of hybridization speciation. The emergence of fertile individuals through hybrid polyploidization, i.e. the emergence of a fertile alopolyploid, is even easier than its formation by polyploidization of a nonhybrid individual, i.e. than the formation of a fertile autopolyploid. In autopolyploids, the double chromosome set is derived from a single species. For example, in autotetraploids, all the chromosomes are present in the cell in four copies and, in meiosis, tetravalents can be formed instead of bivalents. The presence of these structures can seriously disturb the progress of meiosis and thus reduce the fertility of the polyploid. In contrast, with alopolyploids, the two original sets are derived from two different species so that the relevant homeological chromosomes mostly do not pair together and, rather than tetravalents, twice as many regular bivalents are formed during meiosis. As a result, alopolyploids can be fully fertile.
Polyploidization mostly occurs in that the first (reduction) division does not occur during meiosis, yielding diploid gametes. Tetraploids are only rarely formed by the meeting of two rare diploid gametes. Mostly a diploid gamete first encounters a haploid gamete and a triploid is formed. This then forms a triploid gamete as a consequence of a disorder in reduction division. A tetraploid individual is formed only by fusion of a triploid gamete with a haploid gamete. Thus, the triploid stage is a frequent intermediate step in the evolution of a new species by polyploidization; this has two haploid chromosome sets from one species and the third from the other species. Once again, it is necessary to bear in mind that this type of speciation comes into consideration primarily for species without chromosomal sex determination through differentiated sex chromosomes, where the ratio of the gene dose located on the sex chromosomes and on the autosomes must be retained, and thus primarily for some taxons of plants and fish. Hybridization speciation can be of relatively great importance for these taxons. According to some estimates, up to 11% of all species diversity in plants exists as a result of hybridization (Barraclough & Nee 2001).

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Speciation instant

see Speciation gradual

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Speciation microalopatric

see see Speciation sympatric

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Speciation parapatric

Basically, parapatric speciation forms an intermediate link between allopatric speciation and sympatric speciation (Gavrilets 2000; Pennisi 2000a). Parapatric speciation could occur, for example, in not very mobile species whose members form mutually neighboring local populations within their range of occurrence. Crossing occurs between the organisms within these populations, while the gene flow between populations is much less and tends to be mediated by isolated migrants. The fact that the individual populations are constantly in contact means that there is a continuous gene flow between their members, even during speciation. This, of course, complicates their phenotype differentiation and the formation of post-zygotic reproductive isolation barriers. However, if the gene flow is sufficiently limited, for example because the ranges of occurrence of the two populations come into contact only at a certain restricted site, the individual populations can nonetheless adapt to the local conditions of their environments. Crosses born as a consequence of penetration of migrants into the territory of a foreign population exhibit a combination of the traits of the two populations and consequently have suboptimal phenotype and lower fitness in both environments. They are thus gradually eliminated from the population, again reducing the gene flow between the two populations.
The effectiveness of parapatric speciation, similar to the effectiveness of gradual sympatric speciation, is sometimes doubted. Mathematical models indicate that even very small gene flow, for example, exchange of a single individual per generation between the two populations, is mostly enough to prevent genetic differentiation of a new species. However, in actual fact, these conclusions are valid only for differentiation occurring through the action of genetic drift. If more effective processes participate in the differentiation of the two populations, such as evolutionary drives or selection, the intensity of the gene flow would have to be much greater to prevent differentiation.

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Speciation parasitic

Postzygotic reproductive isolation barriers can be a direct product of the activity of some parasitic microorganisms (Breeuwer & Werren 1995). The best known and apparently the most widely spread such parasites are bacteria of the Wolbachia genus, which parasitize in the cells of a large number of species of arthropods, especially insects, and in nematodes. It has been estimated that up to 75% of all species of arthropods are infected by them (Stevens, Giordano, & Fialho 2002). One of the many manifestations of the presence of parasites is the formation of reproductive incompatibility between infected males and uninfected females of the host species. Wolbachia cannot be transmitted by male sex cells. Thus, if this parasite finds itself in the body of a male, it has only a very few ways of increasing its inclusive fitness. One of these is induction of reproductive incompatibility between infected males and uninfected females. If an infected male copulates with both infected and uninfected females, only the infected ones produce viable (and infected) progeny and their percentage in the population and thus also the total number of parasites (potential relatives of the parasite that induced the reproductive incompatibility in the male) will increase (Fig. XXI.13). The mechanism of induction of incompatibility is not known in detail. It was originally supposed that the sperm of infected males contain a toxin that, after fertilization, destroys the zygote formed from eggs that do not contain the antitoxin formed by Wolbachia in infected females. The new results, however, suggest that Wolbachia changes the timing of certain steps in the division of the male cell nucleus, which may lead to desynchronization of divisions of pronuclei of male and female origin in a fertilized ovum and therefore disruption of division of the zygote {10891}.
Various strains or various species of Wolbachia frequently exist in the host population and their toxins and antitoxins (or their desynchronization mechanisms) need not be compatible. Thus, if one Wolbachia variant spreads in part of the population and a different one in another part of the population, the incompatibility of their toxins and antitoxins may lead to an impermeable reproductive barrier between the two subpopulations and can facilitate speciation. The fact that Wolbachia is really responsible for the existence of reproduction barriers can be verified by “curing” the relevant host species by administration of antibiotics (Fig. XXI.14). Following this procedure, the mutual reproductive incompatibility of the members of the two populations generally disappears (Mandel, Ross, & Harrison 2001). According to some theories, Wolbachia are the main motor for speciation in insects and also the reason why a great many of its taxa are extremely rich in species (Rokas 2000).

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Speciation peripatric

see Speciation alopatric

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Speciation polyploidization

Polyploid individuals are readily formed in some taxons and a new species can be formed by polyploidization speciation from these individuals under favorable circumstances. In species with differentiated sex chromosomes, the balance between autosomes and the sex chromosomes is frequently disturbed in polyploid individuals, so that polyploidization, for example by doubling of a chromosome set, yields individuals with serious disorders in the development of the sex organs and thus reduced or even zero fitness. In species without differentiated sex chromosomes, especially in plants, these individuals are frequently fertile and can lead to the emergence of a new species (Rieseberg 2001). It has been estimated that polyploidization speciation is responsible for 2-4% of all speciation in vascular plants (about 7% amongst ferns) (Otto & Whitton 2000).
Crossing between tetraploid and diploid individuals then yields triploid individuals, which are often infertile. During meiosis, part of the chromosomes do not find unoccupied homologous partners with which they could form bivalents during meiosis and thus participate in the formation of trivalents or even remain unpaired as free univalents. Especially univalents are unevenly distributed amongst the daughter cells or their presence can completely block the completion of meiosis. Thus, reproductive barriers exist between polyploids and diploids, which can lead to the emergence of a new species under favorable circumstances. This speciation is greatly assisted by the fact that, due to the larger size of cells of polyploidy plants, the phenotype traits of polyploids and diploids can differ substantially (Otto & Whitton 2000). This can contribute to differentiation of the ecological niches of the two forms and facilitate their prolonged or even permanent sympatric or microallopatric coexistence in a single territory.
 see also Speciation gradual

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Speciation recombination

see Speciation hybridization

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Speciation stasipatric

see Speciation chromosomal

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Speciation sympatric

Sympatric speciation is the opposite of allopatric speciation. In sympatric speciation, a new species is formed in the same territory as that occupied by the parent species. Occurrence in the same territory at the time when speciation occurs is a necessary but not a sufficient condition for the particular speciation to be considered to be sympatric. If the members of the new and original populations basically do not meet in the particular territory, this would not be sympatric speciation. The main difference is that allopatric speciation is accompanied by the formation of internal reproductive barriers between differentiating species in the presence of already existing reproductive barriers, while sympatric speciation occurs in their absence. For example, if a parasite transmitted by direct contact happens to jump over to a different species of host living in the same territory as its original host, the two populations of parasites will continue to exist sympatrically in their range of occurrence. In actual fact, no interactions need occur between the parasite populations on the original and new host species, specifically because of the minimum of physical contact between the members of the two host species, and thus there will be no exchange of genetic information or competition for resources. The two parasite populations can gradually diverge into independent species. However, these will certainly not be sympatric speciation, as internal reproductive barriers would be formed in this case after the formation of external reproductive barriers. Similar cases, when the species live in the same territory but have island ranges at different places, are mostly called microallopatric. In true sympatric speciation, the members of the two populations must constantly meet during the evolution of the new species.
A large fraction of instant speciation, for example polyploid speciation or hybridization speciation, has the character of sympatric speciation. In these types, one-step formation of reproductive isolation barriers precedes, or even causes, phenotypic and therefore also ecological differentiation of a new species. It is understandably rather questionable whether it makes sense to differentiate sympatric and allopatric speciation in cases of instant speciation. However, sympatric speciation also includes gradual ecological speciation, which occurs through the action of disruptive natural selection or ethological speciation. These gradual speciations are accompanied by the accumulation of changes that eventually gradually lead to complete reproductive separation, where constant gene flow between the populations consistently prevents accumulation of differences. The possibility of formation of a new species through gradual sympatric speciation thus continues to be a matter for discussion.

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Speciation vicariant

see Speciation alopatric

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Species

Organisms form natural groups of more or less similar individuals. These groups of individuals can be differentiated on the basis of different phenotype traits and can simultaneously be ordered in a natural hierarchically organized system, i.e. taxonomic system, in which lower-order groups can be gradually associated, on the basis of common traits, to form higher-order groups (taxons). While the differentiation of higher-order taxons is, to a substantial degree, a matter of convention or explicit agreements amongst taxonomists, i.e. professionals concerned with this subject area, most biologists now consider that basic taxonomic units exist in the context of the entire hierarchical system at a certain, very low level of hierarchical ordering, where these units are, in some way, natural, i.e. they exist in nature independently of man and his conventions. Current opinion has it that these units are species. Within species, it is, of course, possible to divide individuals into subgroups of more or less similar individuals – subspecies, geographic races, ecological forms. However, delimitation of these intraspecific taxons tends to be a matter of convention and there basically exist continuous transitions to individual variability.

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Species - biotope recognition concept

Within some taxons, the species differ primarily in the type of biotope that they chose for the lives of their members in the typical case. For example, it is known that individual species of birds divide up the landscape that they inhabit in great detail. For example, some species are strictly bound to the lower level of forest stands, while others are linked to the middle level and others, for example, to isolated bushes outside of continuous forest stands. Simultaneously, it is not very probable that such a detailed division of the biotope would be a consequence of disruptive selection, possibly supplemented by evolutionary character displacement (see XXI.4.4). For example, bird species bound to the lower level of a forest stand could probably inhabit isolated bushes just as well, but are not encountered there under normal circumstances, even when potential competitors bound to this type of biotype are missing.
            Some authors are of the opinion that this speciation of individual species and thus their primary mutual limitations are a result of differences in the characteristics according to which the members of the individual species recognize their normal biotope (Storch & Frynta 1999). These characteristics were chosen more or less at random in the evolution of the particular evolutionary line and basically only very loosely reflect the suitability of the particular biotope for the life of the given species. As indicated by the relevant models, the evolution of characteristics employed by the individual species for choice of a suitable biotope is a cumulative process and is, to a substantial degree, a one-way process – characteristics gradually accumulate during the evolution of a particular developmental line rather than some of them being ignored by younger species. This necessarily leads to a situation where the species gradually divide up the available biotopes in great detail. Thus, specialization of the individual species is not primarily related to the diversification of their phenotypes as a consequence of diversification of natural selection, but rather to more or less autonomous diversification of their cognitive apparatus and the relevant concept of a species can be denoted as the biotope recognition concept.

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Species biological

While the typological definition of a species is currently used most in practice, the definition of a biological species is used most often in the theoretical area; this is also sometimes denoted as the isolation definition of a species. According to it, species are groups of interbreeding natural populations that are reproductively isolated from other such groups. Similar to the previous case, this method of distinguishing species can be employed only for sexually reproducing organisms. Although it does not follow directly from the wording of the definition, it is generally tacitly agreed that crossing can occur to a certain degree between individuals that belong to two different species. However, under natural conditions, the frequency of this crossing or the fertility of the crosses is so low that the consequent gene flow between the gene pools of the two species is weak and selection or even genetic drift are capable of maintaining the integrity and mutual differences between the genetic compositions of the gene pools of the two species. The definition of a biological species can be quite readily understood intuitively. On the other hand, it is very difficult to use it in practice to define the boundaries between actual species. This need not be a drawback of the actual definition of a biological species, but can be a simple consequence of the fact that sharp and unambiguously definable boundaries between species do not exist (cf., for example the nominalist definition of species undergoing evolution over time). Because populations and species develop over time, it is apparent that the boundaries between the individual species are also not absolute and invariable and are, at the very least, moving in time.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species Buffon’s

Buffon’s concept of a species is apparently the oldest definition of a species as a natural taxon (Mayr 1982). According to this concept, the criterion of belonging to the same species is the ability to productively reproduce, i.e. the ability of a pair of individuals of the opposite sex to produce fertile progeny. As soon as the members of two different forms become capable of fertile reproduction, it is necessary to consider them to be members of the same species, even if their phenotypes are very different and they do not cross in nature – for example because their areas of occurrence do not meet. It is apparent that this method of distinguishing species can be employed only for sexually reproducing organisms.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species cladistic

Cladistic species can be either related lines in populations located on a phylogenetic tree between two points of branching or all the terminal branches of a phylogenetic tree. In other words, according to the cladistic concept, a species is formed at the instant of speciation and disappears at the instant of the next speciation or at the instant of extinction (Ridley 1989). According to this concept, the existence of a species is completely independent of anagenetic processes – until speciation occurs in a particular related line, i.e. to branching off of a daughter species, it continues to be considered to be a single species, even though the phenotype of its members changes and develops over time. Amongst other things, cladists recognize only branching speciation – the formation of a new species by splitting of an older species into two or more daughter species – and do not recognize the existence of chronospecies and the possibility of the existence of phyletic (anagenetic) speciation – the formation of a new species through the action of anagenetic processes occurring within a single line (see XXI.1). On the other hand, as soon as a daughter species splits off from the original species, cladists consider it to be a different species from this moment, even if the phenotype does not in any way change.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species cohesion

Variability is constantly generated within any species through random mutations. The genotype and thus also the phenotype spectrum of each species should be constantly expanding, which should also lead to gradual reduction of phenetic distances and thus to obscuring of the boundaries between different species. However, such processes do not seem to occur in nature. So far, no example is known where two phenotypically identical species would be formed by gradual convergence of two phylogenetically unrelated species. One of the possible explanations could be that mechanisms of species cohesion exist for the individual species, i.e. mechanisms that act against the broadening of the phenotype and apparently also the genotype spectra of the relevant species and that prevent the fusion and merging of two species (Templeton 1989; Templeton 2001). Several possible mechanisms of species cohesion have been proposed up to the present time. They have the common property that they are responsible for the active process of maintenance of the similarity between the members of a single species and simultaneously indirectly for the existence of differences between the members of different species.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species cohesion concept of species

According to the cohesion species concept (Templeton 1989) a species is the largest delimited population that functions as an internal mechanism ensuring mutual phenotype cohesion of its members. Phenotype cohesion of a population is understood to mean maintenance of mutual similarity of its members even when the average appearance of individuals in the population changes in time and the population develops as a whole.
            It is apparent that, in a sexually reproducing species, the exchange of genetic information between members of the population, occurring during sexual reproduction, functions as a mechanism capable of maintaining mutual similarity of the members of the population. In other cases, cohesion is ensured by the existence of a common species-specific mechanism of recognition of sexual partners. Thus, the cohesion species concept encompasses the  biological species concept and also the ethological species concept.

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Species cohesion in asexually reproducing organisms

The basic mechanism of species cohesion, sexual reproduction, cannot function in asexually reproducing organism; however, simultaneously, a different process that is capable of ensuring species cohesion can exist here, i.e. genetic draft (Gillespie 2000; Gillespie 2001). Genetic draft, which is also known under the older name hitchhiking, consists in elimination of genetic (and thus also phenotype) polymorphism from the gene pool together with an allele that is subject to selection (see IX.4.4.1). The effects of this process are very limited amongst sexually reproducing organisms. They are related only to elimination of polymorphism in genes in close genetic linkage with a gene that is the actual object of selection. The effect of draft is manifested most strongly here in the sections of the genome in which genetic recombination does not occur, for example, in the non-recombining parts of the sex chromosomes. It is assumed that genetic draft is responsible for the absence of intraspecific polymorphism, which is typical for these genome sections. The importance of genetic draft is much greater in asexually reproducing species, as all the genes of the individual are in absolute genetic linkage. Thus, if a single advantageous mutation appears in the population at a certain moment, that has a sufficiently large selection coefficient and sufficient luck to become fixed, the alleles of all the genes that occurred in the genome of the mutant are also fixed. All the other alleles occurring in the population at the given moment are, on the other hand, eliminated (Fig. XX.4). If the fixation of the new alleles is so fast that there is no time for the formation of new genetic variability in the progeny of the particular mutant, the entire species can become genetically and, to a considerable degree, also phenotypically uniform. Over time, new mutations will accumulate in the gene pool of the species and genotype and phenotype polymorphism will be renewed. However, over shorter or longer intervals, new, advantageous mutations will repeatedly “sweep out” this polymorphism and thus renew the gene and phenotype uniformity of the species.
Continuous elimination of negative mutations will understandably contribute to the maintenance of the uniformity of members of an asexual species. As soon as negative mutations appear in the genome of an individual and become an object of gradual elimination through the action of negative (purifying) selection, all the copies of alleles occurring in the genome of the individual are also fated to disappear. Of course, this disappearance occurs only for copies occurring in the particular individual, so that the final effect on the overall polymorphism of the species or population will be less than for positive mutations. However, because negative mutations occur far more often than positive mutations, their overall importance for maintenance of species cohesion can actually be greater than the effect of positive mutations.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species cohesion in sexually reproducing organisms

Most described species reproduce sexually. If the organisms in a certain population reproduce sexually exclusively or almost exclusively with one another, then it is probable that, sooner or later, they will differ in their phenotype and genotype from the members of groups with which they do not reproduce or with which they reproduce (cross) only exceptionally. Simultaneously, it is not important whether the members of given groups do not reproduce together because “they don’t want to” (for example, they reproduce preferentially with individuals that look or smell similar to themselves) or because they are not capable of doing so (for example, because natural barriers occur between their areas of occurrence, e.g. a river or mountain range). The gene pool of sexually reproducing species necessarily develops as a single unit (Mayr 1963). If a new mutation is formed in a gene of a certain species, then its evolutionary fate does not depend only on how it affects the fitness of the individual in whose genome it is formed, but primarily on how it will affect the fitness of most of its future offspring, i.e. how well it works in combination with the alleles of all the genes that occur in the particular gene pool with the greatest frequency. Thus, in sexually reproducing organisms, constant testing occurs, not only of how new mutations affect the fitness of the individual in whose genome they are momentarily located, but also in the long term primarily of how these new mutations are capable of cooperating with the other alleles occurring in the particular gene pool. This ensures that the individual organisms within the species will not be able to diverge too much through chance or through the variability of the selection pressures acting on them. Thus, a species might not be able to evolve in the most effective way; for example, it cannot simultaneously adapt to several types of environment encountered by its members, but rather it evolves as a whole.
            Genetic and thus phenotype divergence of individual populations can occur in species with a structured population and low intensity of gene flow across the area of occurrence (Ehrlich & Raven 1969). A relatively large percentage of species of animals and plants belong in this category. Consequently, some authors are of the opinion that species cohesion is determined by only a certain subset of genes, i.e. selectionally advantageous genes, whose spreading across the area is ensured by quite small gene flows. Thus, the individual populations can mutually diverge in most other (selectionally neutral) genes without disturbing the overall cohesion of the species (Rieseberg & Burke 2002).

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Species cryptic

see species morphological

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species ecological

According to some concepts, only a limited number of potential niches exist in each environment and thus the number of species that can adapt their phenotypes to these niches is also limited. If, through mutations, an individual deviates too much from the phenotype that is optimal for the niche of a particular species, it or its progeny will sooner or later be eliminated from the population. This theory, which became the basis for the concept of an ecological species, has the advantage that it can also be employed for species that do not reproduce sexually or for species where the frequency of sexual processes or gene flow between the individual populations within the area of occurrence of the particular species is so low that it could not suffice for ensuring species cohesion (Ehrlich & Raven 1969). On the other hand, the model of an ecological species has the disadvantage that its basic starting point, i.e. the assumption of a limited number of niches occurring in nature, does not much agree with current knowledge in ecology. Ecological data and conclusions following from other biological disciplines tend to indicate that there are an enormous number of potential niches in nature, of which only a small portion is actually occupied. Thus, it tends to seem from an evolutionary point of view that the individual species do not select their niches from a previously existing limited choice, but rather that they actively create them themselves. Until a giraffe is formed in evolution, the relevant niche basically does not exist; to be more exact, the given resources objectively existing in nature can be utilized in an almost unlimited number of ways and can become part of an enormous number of basically different niches. If a niche is formed in nature, i.e. if a species emerges in evolution that occupies it and begins to utilize its resources, a number of originally existing potential niches can simultaneously disappear and, at the same time, a number of new potential niches can, on the other hand, be formed.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species ethological

see Species recognition concept of species

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Species evolutionary

A number of concepts of species are concerned with the potential for delimiting species vis-a-vis one another at a single time level. However, especially paleontologists are frequently faced by the problem of how to define the boundaries of species in time or, to be more exact, in the fossil record including samples of organisms over a long period of time. A number of sometimes mutually compatible and, in other cases, incompatible concepts of species attempt to define such delimitations. The evolutionary species concept is the most general concept that encompasses most cases (Simpson 1951). According to this concept, an evolutionary species is a related line, i.e. linear or branching sequences of the population that are related by the ancestor-progeny relationship, which develops separately from other similar lines and which has its specific evolutionary function (role) and specific evolutionary tendencies. This is obviously a phenomenological definition, i.e. a definition that describes the given phenomenon but does not consider the reasons for the existence of distinct species or the mechanisms that keep their members together (in their phenotype). According to this definition, a species is considered to be a group in populations occurring at various places in space and at various moments in time that have the same function in evolution, for example that led to the formation of a new species amongst their members at a certain instant, and that have the same evolutionary tendency, i.e. the phenotype of their members changes in the same way in evolution. At first glance, it may seem that this definition is so general that it can be of little assistance in delimiting species in practice. In this respect, it does not differ much from the typological conception of a species – however, there we are used to its generality and mostly are not even aware of its phenomenological character.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species morphological

 

It has been found for a number of morphological species that they actually consist of a complex of two or more species that can fundamentally not be differentiated on the basis of morphological traits. In some cases, they can be differentiated on the basis of ethological and ecological traits and in some cases on the basis of the geographic occurrence. However, sometimes the main or only indication of the existence of cryptic species (sibling species) is the existence of a reproduction barrier between their members. This can be manifested either in direct study of reproduction of the members of the particular “species” or in study of the genotype composition of the population. In the latter case, a certain combination of alleles does not occur in the population, although its occurrence would have been expected on the basis of the frequency of the individual alleles in the gene pool of the population. It is understandable that the absence, for example, of heterozygotes in a particular locus does not necessarily indicate the existence of cryptic species. Some heterozygotes could, for some reason, not be viable and there could be a strong selection pressure against them in natural populations. However, if the absence is related to a greater number of genotypes and a greater number of loci, the existence of cryptic species becomes a very probable explanation of the given phenomenon.
            The existence of cryptic species constitutes a complication, not only from the viewpoint of the theoretical definition of a species, but from a factual standpoint. Cryptic species generally have overlapping ecological niches, so that it is a mystery how they can occur in nature over prolonged periods and with such a high frequency. It is quite possible that they could correspond to newly emerging species that will also differ morphologically in the future. However, it is also possible that this is only a transitory state that will end either with the disappearance of one of the almost identical species or renewal of gene flow between the given species.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species phenetic

see Species typological

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species phylogenetic

A phylogenetic species is most frequently defined as the basic (smallest possible) set of a population between which the ancestor – progeny relationship exists and which can be distinguished from other such sets on the basis of a diagnostic trait (Nixon & Wheller 1990). This definition encompasses both the requirement on monophyly of the given set in the population (otherwise some of them would have ancestors outside of the population set) and also the requirement on the presence of a phenotypic trait, on the basis of which it is possible to differentiate its members from the members of other species. This trait can, of course, be a certain combination of individual traits that, in themselves, i.e. in other combinations, can also exist in other species. Simultaneously, the trait need not be present in all the individuals in the population and can occur, for example, only in the members of one sex or only at a certain stage in development, e.g. in larvae or in adults. The definition of a phylogenetic species does not permit a decision to be made on whether a certain population is a species. The main contribution of the concept of a phylogenetic species is, however, that it allows delimitation of what is certainly not a species. Thus, higher taxons cannot be species as this is not the smallest possible set in the population fulfilling both criteria, and even a certain phenotype variant cannot be a species, for example all albino individuals, because they split off repeatedly, independently of mutual relatedness and do not, together, form a monophyly. Even a monophyletic group in the population, which would not differ in any trait from other similar groups in the population, cannot be a phylogenetic species. Thus, according to the concept of a phylogenetic species, the formation of a new species requires anagenesis – an evolutionary change in phenotype.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species recognition concept of species

The species recognition concept is a variation of the definition of a biological species (Paterson 1985). With a certain degree of simplification, this concept could also be termed the concept of an ethological species. Its proponents consider that the existence of specific mechanisms is actually the cause of the existence of species in sexually reproducing species; these mechanisms enable the members of a certain species to recognize suitable partners, i.e. members of the same species of the opposite sex. Amongst gonochorists, males and females can recognize members of the opposite sex on the basis of other traits and thus using other mechanisms. Altogether, the mechanisms of recognition of members of the same species but of the opposite sex form a specific mate recognition system (SMRS). As soon as a random modification of SMRS occurs in part of the population, its members will begin to preferentially reproduce together and only rarely with members of the population with the original SMRS variant. If this reproduction barrier is sufficiently impermeable and if there is sufficient time, the two subpopulations that originally differed only in their SMRS will also differ genetically and phenotypically and separate into two independent species. Consequently, it seems useful to consider that a species consists in a population of individuals sharing a specific mate recognition system. Of course, in its pure form, the concept of an ethological species can be valid only for animals. However, for plants pollinated by insects, traits employed by pollinators to recognize suitable flowers could play an analogous role.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Species typological

- The typological definition of species is not easy to defend theoretically but nonetheless is used most frequently in practice. This is primarily a result of the fact that the commonest task faced by the taxonomist is that of determining the species of a particular individual, i.e. deciding whether a certain individual belongs in a particular predefined species. Even in cases when the goal of the taxonomist at the particular moment is not to assign an individual to a known species, but to define the boundaries of new species, an attempt to assign all the studied individuals to known species is the first and an absolutely necessary step.
Most species are defined as typological species, i.e. on the basis of phenotype traits characteristic for their members, i.e. on the basis of properties that occur in the members of the given species and simultaneously are not present in the members of other species. In species with separate sexes and sexual dimorphism, with several developmental stages in their life cycles or with phenotype-differentiated casts, traits occurring in any of the life forms of this species can be used. Both morphological traits and physiological, biochemical and ethological traits can be used to define the boundaries between species. However, morphological traits are used most frequently to define species and, in these cases, they are designated as morphological species. If a large number of mostly mutually interchangeable traits are used to define species rather than a small number of key traits and the species are delimited by the exact methods of numerical phenetics, then the species are sometimes designated as phenetic species.
Some species are monotypical, i.e. they are represented by a single form in their entire area of occurrence. Other species are polytypical; they occur in several and frequently a great many phenotypically different forms within their area of occurrence or at a single site. In the first case, the species can be defined by a list of the traits that its members must have; in the second case, however, the individual traits are usually mutually interchangeable, so that membership in a certain species tends to be determined by the presence of a certain combination of traits and not the simple presence or absence of one particular trait or a single combination of traits. In the most complicated cases, a decision cannot be made on the membership of an individual in a particular species on the basis of its phenotype, as the presence of a certain combination of traits simply determines the probability with which a particular individual will belong to a certain species. If, for example, an individual bears a combination of alleles that occur only very rarely in one species and, on the other hand, relatively frequently in another species, it can be assumed that it will more probably be a member of the latter species. If a larger number of individuals from the same population is available, the frequency of the occurrence of the individual forms of various traits can be found and thus the species membership of these individuals can be determined with greater certainty (Ayala 1983) (Fig. XX.5).
            Typological definition of species should not be confused with definition of species using type specimens. In the description of a new species, it is extremely important to place type specimens in a collection, i.e. specific individuals (for microorganisms a specific culture), to which a description, and thus also the naming of the particular new species, is related. If it is found in the future that the description was ambiguous and that it can actually apply to two or more species, the existence of the type specimen allows us to decide which of the original species is described by the original description and thus which of the species should continue to bear the relevant name. The possibility of using type specimens follows directly from the manner of definition of typological species. Nonetheless, the typological definition of species need not always be related to the existence of a type specimen; in a great many cases only a description of its traits forms the basis for definition of a species.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Specific mate recognition system

see Species recognition concept of species

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Spiteful behavior

It could be expected that spiteful behavioral patterns will become fixed in evolution in the same way as selfish behavioral patterns because, in sexually reproducing species, the spreading of a biological trait in the population is determined by how much it increases the effectiveness of spreading the allele responsible for this trait  (compared to the other alleles of the same gene) and, in asexually reproducing organisms, it is decided by how much the trait  heightens the individual fitness of the carrier of a particular allele (compared to the average carriers of other alleles in the population) (Hamilton 1970). At first sight, it seems that it does not matter whether the individual achieves an increase in its relative fitness by increasing its absolute fitness or by lowering the absolute fitness of its competitors, other individuals in the population. Nonetheless, observations in nature show that spiteful behavioral patterns are quite rare (Dobson, Chesser, & Zinner 2000; Foster, Wenseleers, & Ratnieks 2001).  I will intentionally ignore the most trivial explanation that the individuals “selflessly harm” other members of the population so skillfully and inconspicuously that, in most cases, a naive and idealistic biologist cannot observe it.  In any case, it would be appropriate to mention that our shared experience with the behavior of the representatives of a certain well-studied primate species indicate that this possibility should not be ignored.
            The simplest explanation of the evident absence of spiteful behavior is that all three mechanisms of the evolution of altruistic behavior mentioned in the previous section, i.e. group selection, kin selection and reciprocal behavior, simultaneously act as a barrier against the egression and spreading of spiteful behavioral patterns. A technically different, but at least equally important, reason for the absence of these behavioral patterns is that, in consequence, not only the bearer of the trait, the Vandal, but also other individuals in the population (more specifically those who are at the particular moment not directly affected by the spiteful behavior) profit from it.   These “innocent bystanders”, whose relative fitness increases thanks to lowering the absolute fitness of the individual affected by vandalism, are moreover at an advantage in relation to the bearer of spiteful behavior. They do not have to expend any strength on spiteful behavior and do not expose themselves to the risk of possible revenge from the vandalized victims.
            Theoretic models show that spiteful behavior can spread in a population mainly when vandals can recognize the degree of their genetic affinity to the victims and direct the spiteful behavior primarily towards unrelated individuals. In this context, it is sometimes discussed whether certain elements of behavior of individuals infected by some parasites could be interpreted and their origin be explained as spiteful behavior of the infected host (Rozsa 1999; Rozsa 2000). If the individual’s fitness is lowered, because it has been infected  with a parasite, then the best thing it can do is to infect other individuals in the population. Theoretical models show that, if it infects other individuals in the population regardless of the degree of their genetic relationship, i.e. regardless of the probability of their sharing the copies of the same alleles, this behavior will be selectively neutral, i.e. it will not lead to any change in the inclusive fitness of the individual.  If the infected organism preferentially harms the individuals that are not related to it, the gene for this behavior may spread in the population.
It is well known that individuals infected by certain kinds of parasites have higher motility and are able to migrate over longer distances (Poulin 1994a). According to some concepts, this may be a result of manipulative activity of the parasite, which is favored by higher motility of the infected individuals, because motile hosts can infect more so-far healthy organisms (Randolph 1998). According to another hypothesis, it can be an expression of spiteful behavior of the infected host, which is trying to infect as many still healthy non-relatives in the population as it can, thus heightening its own inclusive fitness.

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Spontaneous disturbances theory of mass extinctions

The mutual relationships between the individual species within an ecosystem form a complicated network of positive and negative relationships. Theoretical analysis of similarly complicated systems has shown that a complicated temporal or spatial structure is frequently formed spontaneously in them, i.e. without input of information from external sources (Kauffman 1993). The existence of temporal structures can be manifested, for example, in that apparently spontaneous disturbances (defects) appear periodically in the system, i.e. sudden changes in the states of a greater number of elements. The energy for these disturbances can be derived from inside the system and thus it can be exhausted after a longer period of time, or can come to the system from the surroundings. The intensity (extent) of the disturbance that occurs in the system at a certain moment is simultaneously not directly dependent on the amount of energy that comes into the system from its surroundings at the given moment, i.e. on the magnitude of the external stimulus. In dependence on the momentary state of the system, a weak stimulus from the surroundings can cause a large disturbance while, at other times, a very strong stimulus can cause only a minimal disturbance.
If the logarithm of the intensity of the disturbance is plotted against the logarithm of the frequency of disturbances of a given intensity, a straight line is very frequently obtained (Fig. XXII.6). In this case, we say that a power law governs the distribution of the frequency of disturbances of a certain intensity. This type of distribution of the frequency of disturbances of various intensities differs substantially from normal or Poisson distribution, in which the magnitude of the disturbance would be limited from above, to be more exact the frequency of disturbances exceeding a certain value would be completely negligible. For disturbances respecting a power law, disturbances of low intensities also predominate and the frequencies of larger disturbances gradually decrease; however, their frequency and especially effect on the behavior of the system can certainly not be neglected. A striking property of this type of distribution is that its shape does not depend on the scale employed. If the frequency of disturbances is first measured on a time scale of years and subsequently on a scale of millions of years and we forget to describe the axes in the obtained histograms, it is not possible to distinguish them later on the basis of their shapes.
            The described behavior of complicated systems is reminiscent of the evolution of biodiversity occurring on a paleontological scale in a great many respects, including the shape of the distribution of the frequency of disturbances of various magnitudes. The mechanical model of such a system is a pile of long-grain rice, onto the top of which a thin stream of more rice is constantly poured. This phenomenon can be best observed if this pile of rice is formed between two panes of glass or at least against the inner wall of an aquarium. Over time, it can be observed that variously large avalanches of grains run down the surface at irregular intervals, analogous to the waves of extinction. It is not possible to predict when an avalanche will start. However, for each type of rice, it is possible to calculate the characteristic distribution of avalanches of various sizes and, on the basis of this distribution calculated for a shorter period, statistical techniques can be employed to estimate the period before an avalanche of even greater intensity will occur, i.e. an avalanche greater than that which occurred during the original, shorter time interval. This method, which is based on the statistics of extreme values, is employed, for example, by seismologists to estimate the time before an earthquake of a certain intensity will occur, on the basis of seismological records covering a shorter time interval. Apparently not only variously large avalanches of rice and mass extinctions that occurred during the history of life on Earth, but also earthquakes, volcanism, atmospheric phenomena and similar processes also have similar frequency distributions.
            Some authors assume that the reason for the similarity in the distribution of mass extinctions and other types of regularly occurring natural processes is that a similar state is characteristic for ecosystems on a long-term scale as that which can be encountered for the mentioned pile of rice, i.e. a state that is mostly termed self-organized criticality (Bak, Tang, & Wiesenfed 1988; Tang & Bak 1988). In general, a region of the state space of a certain system in which there is a sudden change in its behavior, for example from ordered to chaotic, is critical. In the case of self-organized criticality, the system spontaneously remains, i.e. has an attractor, in the vicinity of the critical point or points from which it can suddenly change to two different states, for example to the avalanche – quiescent or mass extinction – background extinction states. Under the influence of minor cumulative changes, e.g. accumulation of grains on the top of a pile or the extinction or emergence of a species, local disequilibrium occurs that, when it exceeds a certain level, is suddenly eliminated by the sliding of an avalanche of grains to the bottom of the pile or the extinction of a greater number of species. If the system is outside of the critical region, the magnitude of the disturbance is limited from above. If it is in the critical region, disturbances can occur with an extent that affects the whole system.
If the changes in the extinction rate over time were actually a consequence of self-organized criticality, this would mean that the intensity of extinction would basically not be related to the intensity of external stimuli and background and mass extinction would have the same cause. Study of mass extinction on the basis of models of self-organized criticality is a favorite pastime of theoretical biologists. Some published models, for example the Kauffman NK-models of Boolean networks, require that the ability for their transition to a state of self-organized criticality be facilitated in advance by suitable choice of parameters – for the NK-models the average number of inputs per element (Kauffman 1993). Understandably, suitably adjusted parameters that make it possible for a system to pass into a state of self-organized criticality can ensure both natural selection and sorting from the standpoint of stability. If systems with the correct number of inputs and outputs per element have greater evolutionary potential than other systems, then it is quite natural that they will be encountered in nature. Other models spontaneously enter a region of self-organized criticality of their state space from almost any arbitrary initial state (Bak & Sneppen 1993; Bak, Tang, & Wiesenfed 1987; Boettcher & Paczuski 1996; Head & Rodgers 1997; Vanderwalle & Ausloos 1995; Vanderwalle & Ausloos 1997).
For the above-described models, the value of the exponent determining the shape of the distribution of the frequency of extinction of various magnitudes in the relevant equation is different from the values measured on the basis of paleontological data; however, models exist that have values of this exponent corresponding to the empirical data (Newman & Roberts 1995; Sole & Manrubia 1996). However, a distribution of the frequency of disturbances corresponding to the power law can also be obtained on the basis of models that do not anticipate the participation of self-organized criticality but which, for example, assume that the source of the individual extinction lies in the interaction of random changes in the external environment with evolutionary processes occurring in the biosphere (Newman 1997). In addition, it is apparent that the behavior of sufficiently complicated models is so highly variable that paleoecological (and also any other) processes can be very readily modeled when the parameters are suitably adjusted. However, with such variable systems it is very difficult to demonstrate that their behavior is actually determined by processes following from the existence of self-organized criticality and that they cannot be explained, for example, by the effect of random disturbances arising from the surrounding environment.

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Spontaneous mutations

Spontaneousand induced mutations can be differentiated according to the causes of their formation.The existence of induced mutations was already demonstrated in the 1920’s, when the effect of radiation on organisms was first studied.Since then, it has been shown that a great many physical and especially chemical factors can cause the formation of mutations in organisms.The exact mechanisms of their action are known for a great many factors, i.e. the specific course of the chemical reaction that leads to replacement of one nucleotide by another, to insertion, deletion or fission.

In fact, the existence of spontaneous mutations was doubted for some time.Some biologists assumed that all mutations are actually caused by external effects, mutagens, occurring in the environment.However, it was gradually demonstrated that there are some categories of mutations that are actually spontaneous.The most typical example consists in mutations that occur through inclusion of the incorrect nucleotide during DNA replication as a consequence of statistically random transitions of individual bases to less common structural forms, i.e. tautomeric transitions (Cox 1976).The individual bases are present in these forms for only a short time (10–5–10–4 s), after which they spontaneously return to the usual form (Fersht 1980).Bases in the unusual forms can form hydrogen bonds with the wrong nucleotides so that, during replication, the wrong nucleotide is included in the synthesized  chain with a certain, non-zero probability.For example, the enol form of T pairs with C (instead of with A) and the imino-form of C pairs with A (instead of with G) (Fig. III.5).The enzyme DNA-polymerase exhibits 3’-5’-nuclease activity, so that it is mostly capable of eliminating these mutations; a large part of the remaining mutations are repaired by subsequently acting  specialized repair systems, which are capable of differentiating which of the unpaired nucleotides is located in the newly synthesized chain and thus which must be repaired.However, some mutations are not repaired and remain a permanent part of the DNA.It is assumed that the frequency of mutations in human beings is 10–10 /nucleotide/cell cycle.During a human lifetime, mutation affects each 4th base in the DNA of brain cells and each base is replaced on an average of 5x as a result of repair processes (Holmquist & Filipski 1994).s

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Stem group

see Monophyly

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Stereochemical hypothesis

of the origin of genetic code From the viewpoint of discussion of the evolution of the genetic code, it could also be important that a correlation exists between certain properties of aminoacids and the corresponding triplets of the genetic code (Tolstrup et al. 1994). At random, we can, for example, mention the high positive correlation of the index of hydrophobicity of aminoacids and the 3’-dinucleotides of the corresponding anti-codons, or the negative correlation of the chemical reactivity of aminoacids in the formation of peptide bonds and the content of nucleotides G and C in the corresponding codon. According to some authors, these correlations indicate that, at the beginning of the evolution of the genetic code, direct stereochemical interactions between the aminoacids and the corresponding codons and anticodons also participated in assigning of codons to the individual aminoacids (Woese 1965; Shimizu 1982). The original stereochemical hypothesis of the formation of the genetic code tends, however, to be gradually abandoned at the present time or has undergone substantial changes (Alberti 1999; Di Giulio 2001).

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Sterility of hybrids

see Reproductive isolation postzygotic

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Sterility of hybrids

see Reproductive isolation postzygotic

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Stimulus enhancement

see Cultural traits transmission of

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Stochasticity of evolution

See Determinism and stochasticity of evolution.

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Stopping the Muller's ratchet hypothesis of advantage of sexuality

Sexual reproduction or, to be more exact, the genetic recombination that accompanies it, is currently considered to be an important mechanism through which the population gets rid of most slightly negative mutations in the DNA. In asexually reproducing organisms, mutations accumulate in the genome that, in most cases, worsen the properties of the encoded proteins. Mutations that reduce the viability of an organism in a drastic manner can be removed together with their carriers by natural selection. However, a large proportion of the mutations are almost neutral in their effects, so that there is very limited potential for natural selection here. Mathematical models indicate that gradual worsening of the average viability of individuals in the population must necessarily occur through the effect of slightly negative mutations occurring randomly in all the genes of the individual members of the population. This irreversible process is called Muller's ratchet (Muller 1964). A ratchet is a toothed wheel with a pawl that ensures that the wheel can rotate in only one direction (for example, in the equipment for stretching a volleyball net). The existence and importance of this process in the individual types of organisms is still a subject of discussion at the present time.

            It should simultaneously be realized that the accumulation of slightly negative mutations can also have an accelerating trend. It can happen that mutations can also occur in the genes for enzymes active in DNA replication and reparation. Thus, the potential for positive feedback also arises: deterioration of the quality of the DNA-polymerase – lower precision of replication – more mutations (amongst others also in the gene for DNA-polymerase) – deterioration of the quality of the DNA polymerase, etc. According to some theories, the ageing of multicellular organisms is caused by just this process (Orgel 1973; Kirkwood 1977).  In asexual reproduction, it cannot happen that the daughter individual would carry fewer mutations than the parent individual. However, in sexual reproduction, genetic recombination leads to individuals that can have more or fewer mutations than their parents. Individuals with a large number of mutations can be eliminated by natural selection, while individuals with a smaller number of mutations can be placed at an advantage. This process can be especially effective under the conditions of positive epistasis, i.e. when the detrimental effects of the individual negative mechanisms are not simply added up, but multiplied together, i.e. when, for example, the concurrent occurrence of four negative mutations in the genome leads to more than twice the reduction in the fitness of the particular individual compared to the occurrence of two mutations (Fig. XIII.7). In this case, even the elimination of a relatively small number of unfit individuals from the population is accompanied by the elimination of a large number of negative mutations from the particular gene pool (Kondrashov 1988).

            The existence of sexual selection leads to a further increase in the effectiveness of removal of negative mutations from the population (Agrawal 2001a; Siller 2001). Because males have a greater variability in fitness compared to females (see Chap. XIV), females can preferentially choose, as their sexual partners, very fit males that apparently have only a few negative mutations in their genome. The results of mathematical modelling indicate that the existence of males and the associated existence of sexual selection substantially reduce the average mutation load on members of the population.

            It is thus apparent that sexual reproduction at the level of populations or species, but not at the level of the individual, can be a form of adaptation towards stopping or at least retarding Muller’s ratchet, i.e. to stopping or even reversing the otherwise irreversible process of accumulation of slightly negative mutations (Kondrashov 1988; Howard 1994).

 

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Stratigraphic correlation method

see Fossils

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Structuralists

See  Evolutionary constraints.

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Substitution rate

The frequency with which mutations are fixed in a certain position or in a given DNA section per time unit in evolution is called the substitution rate. This rate is generally expressed as the number of fixed mutations in the given position per year. The substitution rate must not be confused with the mutation rate; however, it can be concluded that these two rates are numerically equal for neutral mutations (see V.3.3). The mutation rate, i.e. the number of mutations occurring in the given position per time unit for all the members of the population, depends primarily on the accuracy of replication, the efficiency of reparation processes, the intensity of the action of mutagens and the mutability of the sequential motif in the given position of the DNA chain. In contrast, the substitution rate depends not only on the mutation rate at the given site, but also on the intensity and direction of selection that act on the mutation in the given position and in its vicinity. Simultaneously, the substitution rate for selectively neutral mutations does not depend on the size of the population (see V.3.3) (which suggests that genetic drift rather than genetic draft drives the DNA evolution in the studied populations). With growing population size, the number of newly formed mutations in a given position in the population increases linearly, i.e. the mutation rate linearly increases; however, simultaneously, there is a linear decrease in the probability that the newly formed mutation will be fixed by genetic drift.

It must be, however, emphasized that the percentage of mutations that fall in the category of selectively neutral does depend on the size of the population. It is not possible to draw a sharp line between selectively significant and selectively neutral mutations. Basically, only a minimum of them has a selection coefficient equal to zero; most mutations have a negative or positive selection coefficient. It is generally accepted that those mutations whose absolute selection coefficient value is less than 1/Ne, where Ne  is the effective size of the given population, act as effectively neutral in the given population. This means that a greater percentage of mutations fall in the category of effectively neutral mutations in a small population than in a large population. As most mutations have a negative selection coefficient and only a minority have a positive selection coefficient and because the probability of fixation of negative mutations is substantially smaller than the probability of fixation of positive or selectively neutral mutations, the total number of mutations fixed by drift over a time unit, i.e. the substitution rate, is greater in a small population than in a large population (see also V.5).

 

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Suicide of a host

In a great many cases, the pathogenic manifestations of parasitization tend to be caused by the defense mechanism of the host rather than the actual parasite activity. It is no exception for a host to die as a result of hyperactivity or autoreactivity of its immune system, while individuals with partial immunosuppression overcome the infection without difficulties. In most cases, it is apparently only a matter of failure of the relevant defense mechanisms, which are optimized for defense against certain species of parasites and that function disproportionately and counter-productively in defense against other parasites. However, it is also possible that, in at least some cases, this is an evolutionary adaptation on the part of the host, permitting elimination of infected individuals from the population and thus reducing the potential for spreading the parasite.
It is evident that a similar ability to “commit suicide” can emerge only through group or species or also kin selection. If an infected individual were occasionally capable under normal conditions of recovering and reproducing, then the strength of individual selection acting against the emergence of suicidal behavior would be so strong that the probability of its evolutionary formation would seem negligible. However, in some situations, the conditions for the formation of similar behavior are far more favorable. For example, populations of butterflies bound to food coming from a rare plant survive at a single site for a long time, so that the individuals are very closely related. In this case, a caterpillar can increase its inclusive fitness if it commits suicide following attack by a parasite or parasitoid, e.g. in that it would let itself be caught by a bird (Trail 1980). This kind of behavior has actually been observed for caterpillars of the butterfly Harris' Checkerspot (Chlosyne harrisii).
            In eusocial insects, the nonsexual casts do not participate at all in reproduction and express all their fitness in assisting sexual individuals. Here cases are also known that can be interpreted as voluntary suicide of parasitized individuals. Amongst bumble bees of the Bombus genus, individuals infected by parasitic flies of the Conopidae family stay out of the nest, both reducing the probability of transmission of the infection inside the nest and also increasing the probability that they will die (Poulin 1992; Muller & Schmidhempel 1992). However, according to some authors, the lower temperature outside the nest retards the development of the parasite and thus prolongs the survival time of the infected individual; for details, see (Poulin 1995a). Superinfection and virulence- The possibility of optimizing (and thus also of reducing) the rate of reproduction and the pathogenicity of a parasite is not limited only by the formation of genetic variability directly within the infrapopulation of parasites. It is limited even further by the possibility of multiple infection of a single host by genetically unrelated strains of parasites, i.e. the possibility of superinfection (Fig. XIX.8). In both cases, the genetic variability increases in the infrapopulation and systematic selection of “selfish” individuals multiplying at a greater rate than would be optimal from the viewpoint of the entire population (Bonhoeffer & Nowak 1994).
An increase in the growth rate within an infrapopulation and the related increase in the pathogenicity of the relevant parasitosis as a consequence of superinfection is of great importance from the standpoint of epidemiology. If, for example, a suitable epidemiological intervention manages to reduce the incidence of infection, i.e. the number of individuals infected per time unit, not only is the prevalence of infection, i.e. the number (percent) of infected individuals in the population, reduced, but also, as a consequence of reducing the frequency of superinfection, there is usually also a reduction in the pathogenicity of the relevant parasite. The trends in intestinal bacterial infections in countries where good water purification has been introduced are a typical example. This step was taken in the U.S.A. in the first quarter of the 20th century; in the 1930’s the “virulent” (i.e. highly pathogenic) strains Shigella dysenteriae had already been replaced by less pathogenic strains S. flexneri; again, in the 1950’s these were replaced by even more benign strains of S. sonnei. Waste water treatment plants were introduced and the pathogenicity of dysentery decreased sooner in England, while both the introduction of waste water treatment and decrease in the pathogenicity of dysentery occurred later in Poland. Similar developments were also observed for Salmonella typhi and Vibrio cholerae.
            An increase in the probability of superinfection connected with increased genetic diversity of the parasite population is apparently the main reason for the great danger presented by hospital infections (Ewald 1994). In the U.S.A., infections acquired in a hospital, i.e. nosocomial infections, are the fourth most common cause of death. For example, at the present time, salmonellosis acquired outside a hospital environment is practically never fatal. However, salmonellosis acquired in a hospital is a cause of death in approximately one of seven infected patients and this can increase to one in three in some epidemics. Similarly, Staphylococcus aureus bacteria normally infect about 40% of the population but are not particularly harmful to their carriers. However, the prevalence of these bacteria reaches approximately 70% in hospitals and infection is accompanied by highly pathogenic symptoms that are frequently fatal to patients.
            There is a substantial increase in the virulence of parasitosis during military conflicts. Wars are frequently accompanied by the concentration and movement of large numbers of persons and a general decrease in hygiene standards. In some cases, there have been enormous concentrations of sick people at a single place and thus the creation of ideal conditions for selection of especially virulent strains of parasites. For example, during the Ist World War, an evacuation hospital in France had 340 beds for patients with respiratory diseases; as many as 824 people passed through per day, and a total of 34,000 people passed through over a three month period. The influenza epidemic that broke out in 1918 three months before the end of the war and which lasted approximately half a year had approximately a 10-fold higher fatality rate than influenzas in times of peace. During this period, 20 – 40 million people died, i.e. far more than those that died directly as a result of the Ist World War. After about three years, the death rate from influenza returned to normal, i.e. to a value of less than 0.1%. Similarly, during the American Civil War, 3% of those infected died from diseases with diarrhoea in the first year. Towards the end of the war, the fatality rate from diarrhoeal diseases increased to 20%. Simultaneously, this was apparently not a consequence of the reduced resistance of soldiers following from suffering during the war because, for example, the fatality rate from malaria was the same throughout this period. Basically, up to the IInd World War, far more people died from infectious diseases during military conflicts (in recent times, primarily typhus and epidemic typhus) than as a result of war wounds. For example, it has been estimated that 10-20 times more soldiers died from parasitic diseases than in battle during the American War of Independence (Ewald 1994).

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Supergenes

If a group of genes is located next to one another on the chromosome then, in a short time scale, they act as a single gene during genetic processes. This is advantageous in cases when the relevant gene participates in the formation of traits that exist in two or more forms that are advantageous for their carrier, where other forms of the trait that could be formed by random combination of the alleles in the relevant loci would be disadvantageous for their carrier. The occurrence of genes in closely neighbouring loci is then denoted as a supergene.

The best known cases of supergenes were described in study of mimesis in butterflies. Sometimes, a certain type of butterfly imitates several various kinds of poisonous or bad-tasting butterflies. A number of genes affecting the individual traits of the pattern are required for the creation of the relevant pattern on its wings. These genes are located close to one another on the chromosome, so that the parent passes on the relevant combination of alleles to its progeny together. Thus, the progeny consist almost entirely of individuals that inherit the relevant supergene from the father (and imitate one bad-tasting species of butterfly) or who inherited this supergene from the mother (and imitate another species of bad-tasting of butterfly). Individuals that inherited the recombined genotype, whose phenotype would thus not be similar to either of the imitated species and whose mimetic defense against predators would thus be reduced, occur in the progeny of the particular species only rarely.

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Superposition principle

see Fossils

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Supertree

see Condensed tree

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Symbiogenesis

Symbiogenesis refers to the formation of a new species of organism by the integration of two unrelated organisms that live for some time in some form of symbiosis, most probably parasitism or mutualism, into a single organism. If both symbionts begin to reproduce together in a coordinated manner, i.e. so that each daughter organism of symbiotic origin begins to inherit from its parents only the genetic material of both symbionts, the evolutionary fates of the two original species become so interconnected that they sooner or later merge into a single species. Evolutionary dissolution of one species in another species, for example a microscopic parasite or mutualist in its macroscopic host, is sometimes term the Cheshire cat effect (unfortunutly, this term is also used for at least two unrelated phenomena). The relevant literary sources state that, under suitable conditions, a Cheshire cat can gradually disappear and, in the last stage, only its smile remains and, after a certain time, this also disappears. If both symbionts that form a common symbiotic organism produce independent progeny and a new symbiotic organism is formed each time (or at least frequently) through new integration of both symbionts that grew from embryos produced by two unrelated and independent individuals, both species will most probably preserve their species identity (Fig. XXIII.3). This is the frequent case of the symbiosis of fungi and vascular plants. The best known opposite case is the formation eukaryotes, occurring through gradual integration of the members of several unrelated lines of prokaryotic organisms. Completely unrelated phylogenetic lines of organisms can merge through symbiogenesis.

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Synapomorphies

In the reconstruction of cladogenesis, all the homologous traits are not similarly useful. Systematic biologists have intuitively known these facts and have respected them for ages. However, it wasn’t until 1950 that Willi Hennig in his book “Grundzüge einer Theorie der phylogenetischen Systematik” explicitly stated the requirements that cladogenesis be reconstructed exclusively on the basis of a single category of homologous traits – termed synapomorphies. A trait is understood to refer to any structure, function or behavior that occurs in various species in at least two different forms. From an evolutionary standpoint, the individual forms of a certain traits are not equivalent; one of them, the plesiomorphic form, for short plesiomorphy, is evolutionarily older in the particular phylogenetic line and the other forms were formed secondarily all at once or in a certain order as a consequence of anagenetic changes in the original form of the trait. These evolutionarily derived forms are termed apomorphic forms, abbreviated apomorphies. If several species (or higher taxa) within the studied phylogenetic line inherited certain apomorphies from their common ancestor, this apomorphy is termed a synapomorphy; in contrast an autapomorphy is an apomorphy that no other species shares with the given species. The distribution of synapomorphies within the given set of studied species is the best guide for reconstruction of cladogenesis. Even if two species share a large number of plesiomorphies, they need not be closely related in the particular line (Fig. XXIII.6). This could be only a consequence of the fact that the particular species did not change much during evolution, in contrast to other species, for example because it lives in the same environment as the common ancestor of the given line. In contrast, if two species share a large number of synapomorphies, this is most probably a result of the fact that they have a common ancestor that is simultaneously not the common ancestor of any of other studied species.

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Synchronized evolution

The entire process of molecular drive occurs somewhere deep at the molecular level and thus lies partly outside our field of attention. Thus, it might seem that this process is not biologically very important. However, the situation may be somewhat different. Common mutations affect only individuals in the population. The fate of an isolated mutation is frequently decided more by chance than by its biological properties. In contrast molecular drive - induced changes in the genome affect many individuals in the population almost simultaneously (synchronously). This is as if all the members of the population were to mutate simultaneously in the same way. If a mutation proliferated in the population by molecular drive is also manifested in some way in the phenotype, then the same change in the phenotype will occur almost simultaneously in a large number of individuals of the given species, so that some authors speak of this as synchronized, concerted or coincidental evolution (Dover 1986). If the proliferation of a certain mutation is assisted by molecular drive, then evolution fixation of the new evolutionary trait is far more probable than if it were to occur only through the action of selection or genetic drift.

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Syngamy

At a certain stage in sexual reproduction, fusion, i.e. syngamy, occurs of two sex cells bearing a single copy of the genome, i.e. two haploid gametes. The newly formed zygote is then necessarily diploid and, prior to entering the normal asexual cell cycle, can either renew its haploid state and further cyclically alternate between the haploid (in the G1-phase) and diploid (in the G2-phase) states, or can remain diploid and alternate between the diploid state (in the G1-phase) and the tetraploid state (in the G2-phase) for the time of its sexual reproduction. It is necessary to state that the ploidy of the cells is even greater in a great many differentiated tissues of multicellular organisms.

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Syngenesis

In rare cases, two originally independent lines can merge to form a single line, which can then branch apart. Fundamentally, there are two basic mechanisms of syngenesis, i.e. the formation of a new phylogenetic line by the merging of two older lines: symbiogenesis and interspecies hybridization (Fig. XXIII.2). Symbiogenesis refers to the formation of a new species of organism by the integration of two unrelated organisms that live for some time in some form of symbiosis, most probably parasitism or mutualism, into a single organism. If both symbionts begin to reproduce together in a coordinated manner, i.e. so that each daughter organism of symbiotic origin begins to inherit from its parents only the genetic material of both symbionts, the evolutionary fates of the two original species become so interconnected that they sooner or later merge into a single species. Evolutionary dissolution of one species in another species, for example a microscopic parasite or mutualist in its macroscopic host, is sometimes term the Cheshire cat effect (unfortunutly, this term is also used for at least two unrelated phenomena). The relevant literary sources state that, under suitable conditions, a Cheshire cat can gradually disappear and, in the last stage, only its smile remains and, after a certain time, this also disappears. If both symbionts that form a common symbiotic organism produce independent progeny and a new symbiotic organism is formed each time (or at least frequently) through new integration of both symbionts that grew from embryos produced by two unrelated and independent individuals, both species will most probably preserve their species identity (Fig. XXIII.3). This is the frequent case of the symbiosis of fungi and vascular plants. The best known opposite case is the formation eukaryotes, occurring through gradual integration of the members of several unrelated lines of prokaryotic organisms. Completely unrelated phylogenetic lines of organisms can merge through symbiogenesis.
Two lines can also merge through interspecies hybridization, i.e. accidental crossing of the members of two different species. However, in contrast to symbiogenesis, this mechanism can occur only in closely related species with sexual reproduction. The effect of symbiogenesis and hybridization on the topology of the phylogenetic tree is the same; in both cases, the cladogenesis scheme can have a recticular structure instead of a tree structure at some places.

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Synklepton

- see hybridogenesis

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Synonymous mutations

see Point mutations in the protein-encoding DNA

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more

Systematic biology

 Systematic biology is concerned with the study of biodiversity. Systematic biology is mostly considered to be a synonym of taxonomy and both terms will be used in this text in the same sense. However, some authors consider systematic biology to be broader subject encompassing all the aspects of study of biodiversity, including the diversity of biological structures and functions, while taxonomy is understood in a narrower sense as a discipline attempting to catalogue all species, to arrange these species in systems of usually hierarchically ordered groups and naming of these groups in accordance with the rules and recommendations of taxonomic nomenclature. see also Taxonomy.

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The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more
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