see Genome evolution
see developmental canalization
Cause – see Goal-orientation.
The cell cycle is an aggregate of processes through which two or more daughter cells are formed from one parent cell. Temporally, this is delimited as the interval from the instant of commencing one cell division to the commencement of the next cell division. In unicellular, asexually reproducing organisms, the cell cycle is identical with the life cycle, while, in sexually reproducing organisms and multicellular organisms, the life cycle is more complicated and includes more or even a great many cell cycles. Duplication of DNA in a process of replication is a fundamental event in the cell cycle. DNA replication can occur (and in prokaryotes fundamentally occurs) simultaneously with cell growth. After a certain limiting size is attained, the cell divides into two or more parts, each of which takes part of the genetic material with it. Under these conditions, however, there is a danger that cell division will occur prematurely, i.e. at the time prior to completion of DNA replication. In this case, one of the daughter cells would bear incomplete genetic information and it or its progeny would be destined for death following dilution or consumption of the molecules that it would not be able to synthesize anew as a consequence of the absent genetic information.
see History of evolutionism – classical Darwinist period
see Selection of chemostat type and turbidostat type
A natural chimera is the opposite of a ramet, i.e. this is an individual whose body is formed by cells and tissues, derived from two or more genetically different organisms. Chimeras are regularly encountered in some plants. For example, the body of strangling fig tree, which, from an external viewpoint, looks like a single plant, is frequently formed by the combining and intergrowth of tissues belonging to several plants sprouting from a group of seeds that were transported to the host tree by bird droppings. The formation of chimeras is again a result of ecological causes. Under certain conditions, it is advantageous, i.e. increases the chance of survival of the organism, when genetically unrelated individuals, finding themselves in close proximity, do not compete together, but rather cooperate and together rapidly form the body of an organism that is capable of producing progeny. Amongst fig trees, the reason could be that the host tree is only a temporary resource and, if the ficus is not capable of rapidly utilizing it, i.e. climbing up its trunk to the sun, sooner or later it will die and, together with it, its parasite. Thus, in a certain sense, the fig tree is racing against time. If it does not form a sufficiently strong body fast enough, its host will die, either through natural causes or could even be killed by some other fig tree. The parasite would thus lose the opportunity of climbing up to the light, forming a massive crown with reproductive organs and producing enough seeds. Chimerism is encountered to a certain degree in other organisms, including humans. It has been observed that 8% of fraternal twins have chimeric blood, i.e. that part of the blood elements in the body of the individual are derived from the other twin. The professional literature contains cases where one the body of individual contained islands of tissue consisting entirely of maternal cells. Chimerism in humans is sometimes suspected of leading to serious defects, which could be true, e.g., in the development of autism (Pearson 2002).
Simple chromosome mutations very frequently spread in the population without their presence substantially affecting the fitness of their carriers. In fact, even heterozygote individuals, which frequently bear very drastic chromosomal mutations, frequently exhibit more or less the same fertility as homozygote individuals. In this case, a relatively large part of the population, usually in the territory where this mutation emerged, can exhibit the mutated karyotype. The territory in which this karyotype variant occurs can grow in time at the expense of the territory occupied by the original variant. In this case, karyotype variants are generally termed chromosome races. The members of new chromosome races freely cross with members of the original race and regular recombination occurs between their chromosomes. Although they differ in the chromosome number or morphology, they need not differ in the frequencies of the individual alleles on these chromosomes and therefore in their phenotype. However, if two different chromosome mutations spread simultaneously in the population, heterozygotes with reduced or even zero fertility can be formed at their site of contact. The chapter on meiotic drive (VI.3.5.1) described a case where two metacentric chromosomes with a single common arm formed by Robertson translocation, with participation by the same acrocentric chromosome in both cases, but in each case in combination with a different acrocentric chromosome, met at a certain place in the range of occurrence. In this case, complicated multivalents are formed in meiosis, which can substantially disturb the course of reduction division. The infertility of these heterozygotes can result in the formation of a very effective reproductive barrier between these two chromosome races and the races can gradually differentiate into completely separated species.
see Pseudoextinctions and Speciation branching
In this test, two mutants with mutations manifested in the same way in the phenotype of their carriers are first prepared or isolated from nature. If we are interested in whether both mutations are found in the same cistron, then a sufficiently long continuous DNA section bearing the mutation is transferred artificially, e.g. through transfection, or naturally, i.e. through crossing, from one mutant to cells containing the DNA of the individual with the other mutation. The obtained individual carries the given section in two copies, each of which bears one mutation. If the functioning of the particular gene is renewed in it, i.e. the original form of the trait corresponding to the unmutated form of the given gene is formed, it will be very probable that the relevant mutation will be located on two different cistrons. In the opposite case, this will correspond to a mutation on a single cistron. This test can be employed in this simple manner only if the wild form of the trait is dominant, i.e. when the presence of only a single copy of the given gene in the genome is required for its occurrence. Simultaneously, it is necessary to exclude (technically in advance or at least subsequently when evaluating the data) the possibility that the functioning of the given gene was renewed as a consequence of genetic recombination occurring directly in the studied DNA section. See also Gene
Cistron - see Gene.
The process of cladogenesis mostly takes place by gradual branching apart of the individual lines of organisms. If a speciation event occurs, two daughter species are formed in a certain moment from a single parent species. One of these species can be basically identical in its phenotype with the parent species for a long time. Nonetheless, in the first phase of reconstruction of cladogenesis, it is preferable to consider both daughter species to be new species. The evolutionary fates of the parent and daughter species differ substantially. All the genetic changes relating to the parent species can be passed on to both the daughter species and subsequently to both phylogenetic lines that can be formed from them by repeated speciation. In contrast, the genetic changes that occur after splitting off of the daughter species can affect only the progeny of one of the two daughter species and cannot be transferred to the sister phylogenetic line.
Speciation can occur in rapid succession in a certain phylogenetic line. In fact, in some cases, a certain species can decompose into a whole series of daughter species at a single moment. For example, such a situation can occur on a decrease in the water level in a lake and subsequent division of the original continuous population of a certain aquatic species into a number of isolated populations (Fig. XXIII.1). In this case, it is also useful, although not always technically feasible, to express the entire process of cladogenesis as a sequence of binary branchings of the particular phylogenetic line. In actual fact, the physical division of the population occurred at a single moment on a decrease in the water level in the lake; however, branching of the population probably occurred gradually, as the individual parts of the population occupied the individual parts of the original lake. Simultaneously, it is highly improbable that two populations that will, in the future, occupy two different parts of the lake and that led to the formation of the two new species would split off from the parent population at the same instant. Consequently, in reconstruction of cladogenesis we primarily attempt to express this process as a sequence of binary branchings of the particular phylogenetic line and, only if the available data do not make this possible, can we create a scheme of cladogenesis including multifurcation.
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.
A. Graham Cairns‑Smith formulated Clay hypothesis of the origin of life (Cairns-Smith 1982). He based this on the concept that the original structure that provided for transfer of information could have been a clay-type inorganic substance rather than an organic compound.
The microstructure of clay is formed by an irregular crystal, in which the individual series of silicate molecules lie above one another in regularly ordered layers. However, the overall structure of clay is in no way monotonous, as the layers copy the surface on which they lie and also contain a number of defects that are then copied in further layers of the molecule. The fact that the defects are thus copied ensures a certain mechanism of heredity. Clays containing various types of defects can be variously successful. Some grow and enclose further layers faster than other ones, some dry out faster and, after disintegration into smaller particles, can be readily dispersed by the wind and can thus “infect” other locations on which clays settle. A certain type of natural selection can thus occur between various types of clays.
Similar to nucleic acid in the genetic model of life, clays can also “learn” to cooperate with some other substances, for example with proteins, whose synthesis they can catalyze on their surface (Coyne 1985b; Ferris, Huang, & Hagan 1988; Ferris et al. 1996).
The Cairns-Smith hypothesis is certainly very interesting and inspiring; however, it is hard to imagine a way in which a transition could occur from a system of storing genetic information in a set of defects in pseudocrystalline clays to a system of storage of information in sequences of nucleotides in nucleic acids. The hypothesis also in no way resolves the problem of the evolution of the genetic code and proteosynthetic apparatus.
In a number of species, gradients exist in the area of occurrence in the degree of expression of certain phenotype traits. Simultaneously, these gradients need not be identical for the individual traits; a certain trait, e.g. body size, can change in the north – south direction while another traits, for example, hair color, can change in the east – west direction. In some cases, there exists a similar clinal variability in a certain trait in a greater number of even unrelated species. For example, body dimensions in warm-blooded animals often increase in the direction towards colder areas in the geographic area of occurrence (Bergmann’s rule), while the length of body appendages (for example, ears) becomes shorter in the same direction (Allen’s rule).
The mechanisms of formation and maintenance of this clinal variability are very diverse. Very frequently, geographic gradients of a certain factor in the environment are responsible for their existence, for example the average annual temperature or average annual precipitation. A biotic factor can also be responsible for clinal variability, for example a gradient in the population density of a competitive species limiting the width of the ecological niche of the species in question in a certain direction. If the ecological niches of two sympatrically occurring species thus overlap at least slightly, the clinal variability of one species can induce the emergence of clinal variability in another species in a similar manner. Finally, gene flow penetrating at a certain place into the gene pool of the population from the gene pool of another species, with whose members the representatives of the studied species can reproduce to at a limited degree, can be responsible for clinal variability.
The subpopulation of parasites in one host very frequently corresponds to a clone of genetically identical individuals. There is minimal variability within the clone and thus the effectiveness of individual selection that would favor selfish individuals is also minimal. If an individual in the subpopulation mutates towards a greater rate of reproduction, its progeny are at an advantage only in the first host; following transmission to another host, they yield a clone of identical individuals, so that any selectional advantage of the particular mutation disappears. To the contrary, the disadvantage connected with the deviation from optimum virulence is manifested and the entire daughter subpopulation finally produces less infectious stages than the subpopulation of unmutated individuals.
The advantage of the limited effectiveness of individual selection within an infrapopulation is apparently a cause of the fact that asexual reproduction is very frequently encountered in parasitic organisms, for example parthenogenesis and polyembryony (division of the embryo into several or even many embryos) which occurs in taxonomic groups in which wild species reproduce mainly sexually. Parasites sexually produce practically only the invasive stages, which leave the host for the outside environment. Whenever the progeny could compete with one another or with the parent individuals, they are produced by asexual reproduction, where new genotypes are formed only by the slow route of new mutations, and not by the rapid process of genetic recombination.
It is mostly stated that the reason for the emergence of secondary forms of asexual reproduction lies in the necessity of ensuring reproduction even when only a single parasite is capable of entering the host. However, this is inadequate to explain the repeated formation of asexual reproduction. The same goal could be achieved in a far simpler way by the emergence of hermaphroditism, the production of a microgamete and a macrogamete in a single individual. Although sexual reproduction is the commonest means of reproduction in nonparasitic eukaryotic organisms and thus it can be assumed that it is evolutionarily very advantageous, a great many parasitic species apparently are not capable of sexual reproduction or are capable of reproducing in their life cycle only in one of their hosts, i.e. in the definitive host. It is very probable that the selection pressure that, through interspecies selection, has led in parasitic species to the formation of this otherwise unexplainable “sexual abstinence” consisted in the necessity of limiting individual selection in favor of group selection.
Coacervate is a sort of primitive physical model of protocells (Oparin 1980; Miller, Schopf, & Lazcano 1997). Its most important property is the existence of a semi-permeable membrane that separates the inner and outer environment of the coacervate. Structures of the coacervate type are formed relatively easily in colloidal solutions with various chemical compositions through the effect of hydrophobic and hydrophilic interactions. If coacervates contain, in their interior, molecules exhibiting enzymatic activity (in experiments actual enzymes are used) and the medium contains the relevant substrate, for which the coacervate membrane is permeable, in contrast to the reaction products, then the reaction product must be gradually accumulated inside the coacervate. Water enters the interior of the coacervate through the effect of osmotic pressure until it becomes unstable and breaks and divides into a number of daughter coacervates. However, the basic problem is that the molecule with enzymatic activity is not reproduced during this cycle, so that these molecules gradually become more dilute during the division of coacervates. Thus, the cycle of growth and division of coacervates (Fig. X.2) is only a superficial analogy of the self-reproduction cycle of living organisms and it is not at all clear how a system capable of actual biological evolution could be developed on this basis. In addition, the coacervate hypothesis implicitly assumes the existence of a molecule with enzymatic activity but does not provide any idea about how these molecules could be formed
During evolution, organisms adapt, not only to the conditions of the abiotic environment, but also to the effects of the biotic environment, i.e. they create traits and patterns of behavior through which they react effectively to the presence and actions of the other species of organisms in their environment. Evolutionary adaptation to the biotic environment differs from adaptation to the abiotic environment in two primary aspects. To begin with, the biotic component of the environment changes much faster than the abiotic component. While it is true that some local changes in the climate take place quite rapidly, in most cases rapid changes are temporary or even cyclic in character and, after a certain period of time, the climate returns to the original state. The abiotic environment is usually quite stable on a scale of 106 to 107 years, which is the usual duration of the existence of a species. The individual species can react to temporary changes through temporary changes in their areas of occurrence, for example, through withdrawing to refuges, more easily and especially more rapidly than through evolutionary changes in their phenotype traits.
The other way in which adaptations to biotic and to abiotic environments differ fundamentally is in that the biotic environment is not passive, but also reacts effectively to ongoing evolutionary changes. For example, if a predator evolves a certain anatomic trait or a certain pattern of behavior that enables it to acquire a certain prey more effectively, then the newly emerging evolutionary pressure will sooner or later lead to the evolution in the prey of a trait that enables it to defend itself more effectively against the new hunting strategy of the predator. Rather than independent evolution of the individual species, mutually interconnected and mutually dependent evolution occurs in pairs of species or larger groups of species – coevolution.
While the abiotic factors in the environment are more or less stable on the time scale of biological evolution, or only fluctuate cyclically or randomly in time, changes in the biotic factors in the environment tend to have a cumulative character and are frequently irreversible. The irreversibility of changes in biotic factors is a result primarily of the irreversibility of macro-evolutionary processes, anagenetic and cladogenetic processes in biological evolution. Stochastic processes constitute an important factor in these processes, whether genetic mutations or mass extinction caused by catastrophic events in the environment. Although the environment otherwise changes periodically, for example when there is a periodic alternation of warm and cold periods, species react to these cyclic changes through irreversible evolutionary changes. For example, Dollo’s law expresses the irreversibility of evolutionary processes. According to this law, an organ that disappears during the evolution of a certain species never reappears in the future in its original form, even if the relevant selection pressures that led to its formation in the original species are renewed. It is not necessary to emphasize that, similar to all biological laws, Dollo’s law also has a number of exceptions.
The biological species that occur at a particular moment constitute a key factor in the evolution of the biosphere. The fact that these species undergo cumulative and irreversible evolution means that changes in the biosphere as a whole also have a cumulative and irreversible character.
Although most biologists probably agree that species exist in nature, there is no such agreement in opinions on why they exist or how the category of species can actually be generally defined and how the individual species can be mutually differentiated. If an attempt were to be made to reasonably differentiate the individual potential or existing theoretical concepts of species, we would probably first have to define a basic group of questions, to which the proponents of the individual concepts of species would give different answers. The first question that must be answered is whether species exist objectively in nature, independent of human beings, or whether they are mutually differentiated only by humans – taxonomists. The realistic concept of a species prefers the former, while the nominalistic concept of a species prefers the latter option.
If species do actually exist objectively in nature independent of humans, then another question must be posed: whether the existence of distinct species necessarily follows from the properties of living systems or from the properties of the environment in which they live, or whether their existence is only a consequence of a historical accident (see below).
The third question that must be posed is whether the reasons for the existence of distinct species are internal, i.e. whether they follow directly from the properties of the internal elements of living systems, or whether they are caused by external circumstances, i.e. properties in their surroundings, their environment. The first possibility is preferred by structuralists and essentialists, while the latter option is preferred by proponents of most theories of species cohesion. Fig. XX.1 depicts one of the possible divisions of various concepts of species.
The essentialist concept of species is based on the idea that that members of the same species objectively share a certain inner quality, essence, that sets them apart from members of other species. According to essentialists, the number of species or organisms and boundaries between these species is unambiguously determined beforehand, similarly as, for example, the number and shape of Platonic bodies are determined beforehand. Essentialists explain the existence of species internal variability as being a consequence of a different degree or quality of expression of this essence, where this unequal degree of expression has its origin in unforeseeable random effects of the surroundings on a particular organism. Philosophically, the essentialist concept is based in Plato’s world of ideas (where Plato’s own approach to classification of objects was based on other principles and it is improbable that he would consider himself an essentialist in the modern definition of this term). Similarly as transitions between triangle and rhomboids cannot exist in an ideal world, according to essentialists there cannot exist transitions between individual species. However, transitions between species can, of course, exist in our real (and imperfect) world. Here, however, it is frequently useful to differentiate between the essentialist and nonessentialist explanation of the difference between species (or between anything at all). For example, there are certainly several ways in which clocks can be formed. Simultaneously, it is hard to image a transitory type between pendulum clocks and crystal clocks – the differences are essential. In contrast, there can be quite continuous transitions between detergent powders and the fact that only a certain number of kinds of detergent powders are available in stores is not a consequence of the fact that transition kinds couldn’t be mixed or that transition forms would wash less efficiently. This is given by “external factors”, the business strategy of the manufacturer or seller or the laws of competition amongst individual market entities.
It is not probable that the essential concept of a species, at least in its original sense, would be very useful for description and explanation of the phenomenon of the existence of species in live nature. Species are not crystals or geometric objects, but products of a unique series of historical events, whether this series of historical events is considered to consist of biological evolution or biblical creation. This means that this development into the present-day form was affected not only by deterministic processes that necessarily followed from the properties of the particular system, but also, to a substantial degree, by chance. To assume under these conditions that the individual species should differ in a certain special category of properties, i.e. essential properties, is perhaps not completely naive, but is certainly very impractical. It is, of course, possible and, in fact, quite probable that any two species will differ from one another in a specific property, whether this is a particular biochemical or physiological property of their members or a certain aspect of their position in the ecosystem. However, this property is “essential” only in that it differentiates the two particular species. It is more than questionable whether the concept of a species based on “essential” properties defined in this way could be included in the category of an essential concept of species.
If we were to live in a stationary biological world, in which species would be invariable and each of them would be the product of an individual act of creation, without regard to whether through the action of supernatural forces or natural processes, the number of species would correspond to the number of these unique events. As every formation of a species would be a distinct event, the individual species would necessarily be distinct and it would not be necessary to further separately investigate the phenomenon of the existence of species. However, all the available data indicate that we are not living in a stationary biological world and that species do not emerge independently of one another. To the contrary, it is apparent that they are formed in the closest possible interdependence, that a species is formed from some other species. Under these circumstances, the existence of relatively sharp boundaries between species is a rather unexpected phenomenon which deserves a separate explanation. The historical concept of species offers one of the possible explanations for the existence of distinct species in a nonstationary world.
The historical explanation can consist, for example, in that the development of each species is characterized, for some reason, by the alternation of periods of evolutionary plasticity with long periods of evolutionary stasis, during which the species does not change even under the action of selection pressures (Eldredge & Gould 1972). Thus, the traits of the individual species would reflect the conditions that happened to prevail at the time when the particular species was evolutionarily plastic, i.e. most likely at the time and place of its formation (Flegr 1998, Flegr 2010). Under these conditions, the differences amongst the individual species would not be able to be blurred and obscured.
Another historical explanation of the existence of species in a nonstationary world could be based on the phenotype multidimensionality of living organisms. Every biological property of an organism can be considered to be a dimension in the multidimensional phenotype space. Compared to space with few dimensions, there is much lesser probability in such a multidimensional space that two individuals (for example two species) would accidentally meet, even if they are only a few steps (a few mutations) apart. There are so many directions in which individuals only one step apart can set out that it is highly improbable that they would run into each other in the future. Thus, even though the phenotype spectra of the individual species continue to expand in a multidimensional space, it is almost impossible for the two species to coalesce at some time. Thus, once formed, the species retain their distinctiveness for ever. However, this is a result of the fact that, compared with the number of evolutionary pathways that they can follow in a multidimensional phenotype space, the number of species is so small that it is highly improbable that phenotype variants would occur amongst them that would lie at the borderline between two existing species. For example, if phenotypes were to differ only in a few traits, the situation would be completely different and the phenotype spectra of the individual species could easily intersect as a consequence of mutations or through the effect of mutations and selection – on their unique pathways in evolutionary history, the various species would occasionally meet at a single place in the phenotype space.
The nominalistic concept of species assumes that a natural property of all biological objects lies in their variability. Nominalists consider that only individuals and not taxons realistically exist. According to nominalists, the sharp boundary between species is basically artificial and, similar to higher taxons, is only a matter of convention.
If the discussion between nominalism and realism is limited to the aspect of the existence of an objective boundary between species at a single level in time, specifically at the present time, most of the available empirical data will tend to throw doubt on the nominalist concept. While the delimitation of the boundaries between individual colors in a spectrum is a matter of convention and various cultures have resolved this task with different results, when delimiting the boundaries between species living in a certain territory, mostly not only scientists working independently using different methodical approaches, but most scientists and nonscientists, i.e. representatives of pre-industrial ethnic groups living in the particular area, will tend to agree in terms of rough characteristics (Gould 1979). This is, of course, only an argument against cultural relativism and it can, in addition, be objected that both the scientist and the “savage” are members of the same species and have, for example the same sensory organs and the same architecture of their brain structure. It will probably not be possible to perform a real experiment any time in the near future; nonetheless, it seems highly probable that a sufficiently motivated Martians equipped with the corresponding Martian instrument (e.g. a DNA sequencer) would differentiate terrestrial organisms into a similar system. Nothing personal, but the occasional excursion of molecular biologists into the field of systematic biology does not differ much from the above described hypothetical experiment.
It necessarily follows from the existence of individual variability of organisms that some individuals in the population will be more or less similar and thus that the boundaries between the individual species will be “fuzzy”; consequently, in some cases, it will be difficult to differentiate the individual species from one another. In contrast, the fact that, nonetheless, there are mostly objectively more or less definable boundaries between the individual species, does not necessarily follow from anything and thus is a special natural phenomenon that is contradictory to the starting premises of nominalism and that requires a separate explanation.
The discovery of evolution led to extension of the discussion between nominalists and proponents of other concepts about the boundaries between the species over time. The nominalist concept assumes that daughter species are formed in evolution more or less through continuous development from parent species, where the specific moment from which an individual in a particular related line can be considered to be a member of a new species is only a matter of convention. In contrast, a different concept of species assumes that, at a certain historical moment, a new species is formed in a particular related line, and is qualitatively different from the parent species. The decision as to which concept to favor is fundamentally complicated by the fact that a species is a set of a great many individuals and a great many related lines and no just a single entity. Individuals can differ substantially within a single species. Even if we were to come to the conclusion that a fundamental change occurs at a particular moment in a particular related line, which allows us to consider the particular individual to be a member of a different species, the frequency of the individuals with the particular property in the population is a continuous quantity that can gradually change over time or even fluctuate irregularly (Fig. XX.2). This means that the nominalist concept of species developing and splitting apart in time is quite legitimate and has a great many proponents at the present time.
In contrast to the more or less single nominalist concept, there are a number of realistic concepts of species (e.g. historical concept of species, essentialist concept of species, structuralist concept of species). They are all based on the concept that species and the boundaries between them are objectively present in nature, independently of man’s will. The individual realistic concepts differ primarily in what they consider to be the cause of the distinctness of the individual species.
Essentialism can, in a certain sense, be considered to be a certain, rather extreme form of structuralism. The structuralist concept of species is based on the idea that the phenotype of organisms is determined primarily by almost deterministic processes following from the properties of their structural elements and also the character of the mechanisms governing ontogenesis. In contrast, the action of external factors is assigned a secondary role, either in the basically deterministic processes of selection or in random historical processes of the type of drift and speciation. According to structuralists, there is only a limited number of ways in which a functional organism can be formed from biomolecules through existing ontogenetic processes. According to them, each species is a specific manifestation of one of these means. Thus ontogenic constraints play a decisive role in determining the phenotype of an organism and, consequently, the evolution of a new species. The Vavilov model of homologous series (Vavilov 1967), one of the oldest complete structuralist concepts of the character of species, was proposed in the 1920’s. The Russian geneticist N.I. Vavilov noticed that a certain combination of phenotype traits occurs in various genera of plants, permitting the individual species to be distinguished. On the basis of study of the morphology of the members of a certain genus, he was thus capable of predicting the existence of so-far unknown species in a different genus and a great many of these predicted species were, in fact, discovered (Fig. XX.3).
It is probable that developmental limitations can be of substantial importance in the evolution of viruses. For example, in these cases, the morphology of the virus capsid is actually determined to a major degree by the laws of crystallography rather than by the character of selection pressures acting on viruses. In more complex organisms, developmental limitations can very substantially predetermine the evolution (and thus also the phenotype diversity) of the individual traits (for example the possible shapes of mollusc shells (Raup 1962; Raup 1966)). The structuralist model could also explain the diversity of the patterns on the wings of butterflies and on the body cover in general. Another area where the structuralist explanation could be important consists in the nonadaptive variability in the individual traits between species within a single genus. As an enormous number of traits contribute to the phenotype of more complicated organisms, the total number of possible combinations of these traits is infinitely large. Thus, it is not very probable that structuralist laws would determine which species would be formed in evolution and which not. This factor tends to rather have a limiting and defining role; of the enormous number of phenotypes that would correspond well to the requirements of the environment, it eliminates a certain percentage of those that, in actual fact, cannot be formed for internal reasons. However, I am of the opinion that this will tend to be a small percentage, but cannot, of course, be certain of this. Which of the species amongst the almost infinite number of other possibilities are actually formed tends to be determined rather by chance, selection and evolutionary drives.
see Synchronized evolution
Reconstruction of phylogenesis by the maximum parsimony method very frequently yields several trees that do not differ at all or differ very little in the anticipated number of evolutionary changes. If there is no statistically significant difference amongst them, it is not possible to decide which of them best corresponds to the progress of cladogenesis of the relevant group. Similarly, in cases where the cladogenesis of a single group is reconstructed several times in succession on the basis of a different set of traits, mostly trees are obtained that differ in more or less important details. In these cases, it is correct to create one consensus tree on the basis of several trees, which summarizes the information contained in all the obtained trees.
The degree of consensuality can be selected according to the purpose that the study is to serve. If we require one-hundred percent consensuality, we must denote only those binary branches on the final tree that occurred on all the individual trees. If the order of branching off of species differed in some places on the individual trees, the corresponding place on the consensus tree is designated by multifurcation, i.e. a node from which more than two branches originate. The presence of multifurcation on a consensus tree does not mean that we think that the particular species underwent adaptive radiation in the past from one place at a single moment, but only that we are not capable of deciding on the order of branching apart of the particular species on the basis of our data. When creating consensus trees, we need not always require one hundred percent consensuality; for example, for some purposes 90%, 75% or even only a majority consensus is sufficient (Fig. XXIII.10). A supertree is a special type of consensus tree. A supertree summarizes information from trees created for partially overlapping sets of species.
A condensed tree differs in principle from consensus trees. A condensed tree is not created on the basis of a greater number of individual trees. Multifurcations on a condensed tree once again designate sites where we are not certain of the order of branching; however, this time this is not because of conflicting results but rather, for example, because of the absence of the relevant data (for example, lack of suitable synapomorphies).
The mechanism of motivation based on the pleasure-distress balance, i.e. a mechanism of “inner motivation” enables even signals very indirectly connected with satisfying a particular need to become a trigger for complex behavioral patterns. This – in consequence – makes it possible for the individual to react, not only to certain objects in the real world, but also to symbols that stand for the objects. It makes no difference whether these symbols are pheromones (i.e. chemicals primarily meant for communication between members of the same species), piles of droppings (rats’ markings for poisoned bait) or “Beware of the dog” warning signs on yard gates. The ability to mentally recognize symbols can finally lead to the emergence of consciousness, including self-consciousness. Consciousness and self-consciousness enable imagining situations and connections that have not appeared yet. Mostly, we can quite well imagine what would happen if we put our hand into a mad dog’s mouth without having to practically test the advantages or disadvantages of this kind of behavior.
see Condensed tree
see Point mutations in the protein-encoding DNA
– see Evolutionary constraints.
see Scientific nomenclature
The fact that some groups of parasites are closely adapted to a particular host species or small group of host species means that transmission from one host species to another is relatively rare in these parasites even on an evolutionary scale or occurs only within a group of related species. However, this means that, as the host species is subject to speciation, i.e. individual daughter species split off during evolution, the parasite must also undergo speciation. Cospeciation occurs in the parasite and the host. Enforced speciation of the parasite may be caused by genetic drift following separation of a subpopulation bound to both daughter species and thus after separation of their gene pools. In many cases, however, this is apparently also strengthened by natural selection, adaptation to two, now mutually different, host species. Of course, sometimes host speciation need not necessarily be followed by speciation of the parasite; a single species of parasite can attack a relatively broad group of mutually related species. However, such a situation generally occurs when the host species live in the same territory (sympatrically) or when the parasite is spread by intermediate host species, which have sufficient mobility and habitats encompassing the habitats of all the related host species.s
The fact that the sequence of parasite speciation must frequently copy or at least respect the sequence of speciation of the host taxon means that the cladograms (phylogenetic trees) of the host and parasite will be similar to a certain degree (Thomas et al. 1996) (Fig. XIX.19). Thus, if we know the phylogenetic tree of the host taxon, it is often possible to approximately estimate the phylogenetic trees of its parasites (and vice versa). This Fahrenholz rule was first apparently published in 1896 by Kellogg (Brooks & McLennan 1993). It is, of course, possible that, in some cases, the parasite can transfer to some other host organism, especially if the parasite is not particularly specialized, i.e. has a broader host spectrum. A parasite can also “miss the boat”, i.e. may be absent at the critical moment in the host subpopulation that, in time, results in the formation of an independent species, or could become extinct over time in a host species. In this case, the phylogenetic trees of the parasite and host need not be identical or can contain individual anomalies that are externally manifested, for example by crossing of the relevant branches of the cladograms. Empirical data indicate that better similarity is exhibited between the cladograms of hosts and their parasites when the relationship of the two organisms tends to be mutualistic and that the similarity is less in cases of actual parasitism. For example, the cladograms of Wolbachia bacteria in insects are far less similar to the cladograms of their hosts than those of Wolbachia bacteria and Filaria (Moran & Baumann 1994; Casiraghi et al. 2001). This is apparently a result of the fact that a coevolutionary “arms race” occurs between the parasite and the host, in which the host occasionally manages to successfully shake off its parasite. However, this success is usually only temporary; the empty niche can be secondarily occupied by a different species of parasite that is often not related to the original species.
see Two-fold ecological costs of males
see Two-fold genetic cost of sex
If we ignore the rather atypical and apparently derivative situation in species in which microgametes are transferred en block, for example, in the form of pollinia (some orchids) or spermatophora (some insects, salamanders), then we find that males tend to be r-strategists when compared to females. They produce the greatest possible number of gametes and try to ensure the formation of the greatest possible number of zygotes. In the typical case, they do not try to influence the further fate of these zygotes. In contrast, even through the formation of macrogametes alone, females must always invest a substantial effort into the future zygote, so they are mostly forced to adopt the role of K-strategists. They produce fewer gametes and try to care for their further fate so the greatest numbers of progeny live to reproductive age.
This different starting position of the two sexes generally means both a greater share of females in care for the progeny and also greater efforts that the females exert in selection of a sexual partner. While the optimum reproduction strategy of a male is unselective mating with the greatest possible number of females, females can affect their reproductive success primarily through selection of an optimum mating partner, a male with whom they will combine their genes in a coalition through production of common progeny.
The phenotypes of existing species are more or less adapted to the conditions of the environment in which they live. If they are to start to use a new niche and enable the formation of a new species by branching speciation, they must first adapt to this new niche. This evolutionary change can only rarely occur in a single step, i.e. can occur as a result of a single mutation. Mostly a number of gradual evolutionary changes are necessary for exploitation of a new niche; these change the phenotype corresponding to the requirements of the original niche to a phenotype corresponding to the requirements of the new niche. There are a number of intermediate stages between the terminal states of the particular evolutionary pathway, where the particular intermediate links, transition forms, will probably exhibit suboptimal phenotypes from the viewpoint of both the old and new niches. Thus, their carriers will be at a disadvantage compared to the carriers of the original phenotype, which will substantially retard the progress of speciation. The entire phenomenon can be elegantly described by the model of the adaptive landscape (see I.13). There are a number of peaks in the adaptive landscape to which the individual species can gradually climb through the action of natural selection. Each of the peaks corresponds to a potential niche. Some of the peaks are occupied (exploited niches), while others are empty. In order for the members of a certain species to move from one peak to another, i.e. in order for them to form a new species, they must be able to first “climb down” from their adaptive peak and overcome the valley in the adaptive landscape, i.e. they must survive with a suboptimal phenotype from the viewpoint of fitness for a great many generations (Fig. XXI.4).
In some types of speciation, suboptimality of the transition phenotypes is not a great drawback. For example, if part of the population finds itself in an isolated territory with very different conditions than those to which the phenotype of the original species was optimally adapted, the phenoptype of the population can begin to adapt to the new conditions even if the changes lead to temporary or permanent loss of fitness under the original conditions. Because the members of the population will not find themselves in the original conditions for a long time, their potentially worse competitiveness will not be a detriment. However, in some types of speciations, the newly formed species is in constant contact with the original species and, in this case, the reduced competitiveness of its members can be detrimental. Prevention of speciation that is based on worse adaptability of the individuals with transition phenotypes can be an important factor in those types of speciation where the new species evolves in close contact with the parent species. There are two basic factors that can facilitate the formation of a new species, to be more precise phenotype differentiation, under these conditions. The first factor is the multidimensionality of the actual adaptive landscape. While the landscape to which we are accustomed is three-dimensional, the adaptive landscape is actually multidimensional and each of these dimensions corresponds to one phenotype trait of a living organism. There are valleys between the individual peaks in a three-dimensional landscape that correspond to an area with suboptimal phenotypes in the three-dimensional model of the adaptive landscape. In contrast, in a multidimensional adaptive landscape, there are sorts of ridges or upland plains between the individual, often very distant peaks, along which the population can move, through the action of suitable selection pressure, from one peak to another without first climbing down to the valley.
Another factor that allows species to pass from one adaptive peak to another is genetic drift and the related possibility of neutral evolution. In small populations, to be more exact in populations with small effective size, selection has rather low efficacy. This means that, in small populations, individuals with suboptimal phenotypes, located in deep valleys of the adaptive landscape, can survive for long times and even predominate through the effect of genetic drift. Then, when the effective size of the population again increases, the progeny of these individuals can climb back to the surrounding peaks in the adaptive landscape, including formerly unoccupied peaks. Sewall Wright explained adaptive evolution and certain forms of speciations by his shifting balance hypothesis, which is based on the principle of alternation of the effect of genetic drift with that of natural selection (VII.3).s
see Meoitic drive
Cryptic choice hypothesis assumes that the processes occurring in the female reproductive organs are a continuation of sexual selection (Birkhead 1995). Through manipulation with the ejaculate, the female is capable of subsequently determining which sperm of the males with whom she recently copulated will finally fertilize the egg cells. Some authors have even suggested that, in this “decision-making”, the females are capable of “taking into consideration” the genotype of the individual sperm and their own genotype so that, in the final analysis, they allow the egg to be fertilized, for example, only by those sperm that ensure heterozygosity of the progeny for genes important for resistance, for example genes for MHC-antigens (Wedekind 1994; Lopez-Leon, Cabrero, & Camacho 1996; Stockley 1999) (Fig. XIV.3). Similar phenomena have also been observed for plants. Here, selection apparently occurs through affecting the rate of growth of the individual pollen tubes (Matthys-Rochon, Gaude, & Dumas 1987; Charlesworth, Schemske, & Sork 1987; Wendel, Edwards, & Stuber 1987).s
Unambiguous interpretation of the results of experiments studying cryptic choice is somewhat complicated by the ability of females to affect the viability of progeny through uneven investment into the individual embryos. It has, for example, been observed that the females of zebra finch (Taeniopygia guttata) that were able to copulate with more attractive males produce eggs with higher testosterone concentrations (Vogel 1999). In the traditional arrangement of experiments, i.e. without the use of artificial insemination, it is thus frequently not possible to decide whether the uneven success of the individual males is a result of cryptic choice occurring at the level of competition between sperm or classical sexual selection occurring at the level of the adult individuals. Results to date using artificial insemination indicate that it is possible that the cryptic choice mechanism also allows females to select males exhibiting quite specific phenotype traits, including the degree of expression of classical secondary sexual traits or body size (Evans et al. 2003) (Fig. XIV.4).
The fact that many behavioral patterns in animals do not develop by natural selection and are not genetically passed down through the generations means that their evolution does not obey the laws of biological evolution and follows the laws of cultural evolution instead. In cultural evolution, a possibility exists for horizontal passage of traits among unrelated individuals, along with (non-genetic) inheritability of acquired characteristics.
Another important feature of cultural evolution is that patterns fixed during cultural evolution may, in their consequences, be disadvantageous for their bearer as well as for the population and species. (Of course, this feature can also occur in traits fixed, e.g., by sexual selection.) Overlooking this important aspect of behavioral traits is probably the main flaw of classical socio-biology and, to a certain extent, also ethology. Both disciplines attempt to explain the origin of individual behavioral patterns from the narrow viewpoint of their contribution to the fitness of their bearers or to the efficiency of multiplication of the allele that is responsible for the particular behavioral pattern. Because of the specificity of the mechanisms of cultural evolution, this topic will be dealt with in a separate chapter (XVII).
Traits formed in biological evolution are transmitted genetically, most frequently in the form of information written in the primary DNA structure of the relevant species. Simultaneously, in a great many cases, the genetic relatedness of the population is not correlated with the similarity of the patterns of behavior transmitted in the particular group through imitation. For example, in individual populations of chimpanzees monitored for long periods of time in their natural environment, a great many patterns of behavior have been observed that were specific only for the population occurring in a certain territory. Systematic studies have demonstrated that, because of these differences, groups in populations of chimpanzees form a sort of cultural spheres that are strikingly similar to human nationalities. Simultaneously, the similarities of the individual cultures were related only very little to the genetic relatedness of their bearers (Whiten et al. 1999). A similar phenomenon was subsequently observed in orangutans (van Schaik et al. 2003). It is thus apparent that cultural patterns are transmitted to a substantial degree independently of gene flux.
Traits formed as a consequence of cultural evolution are transmitted nongenetically, by social learning, occurring in the simplest case by imitative learning (observational learning), i.e. imitation of the behavior of the individual members of the social group (Fawcett, Skinner, & Goldsmith 2002). If, for example, a rat observes that another rat prefers one kind of food of two possible kinds, then it also prefers this type of food for several days (Galef & Whiskin 2001). Only subsequently can individual experience with the two types of food predominate over the learned patterns of behavior and the animal begins to prefer the actually better type of food (Fig. XVII.2). In humans, imitative learning is the most important mechanism of cultural evolution and, according to some authors, it was its development that triggered and facilitated the rapid evolution of our species (Blackmore 2001). However, in most species, social learning does not have the character of imitating the behavior of another individual. Very frequently, the “imitated” individual only draws attention to the possibility of attaining a certain source through its behavior and the “imitating” individual then “finds” a way of getting to this source by itself (Reader & Laland 1999; Blackmore 2001). This mechanism, denoted as stimulus enhancement, is apparently also operative in probably the best known example of cultural evolution, adaptation of English tits to new ways of obtaining food (Hinde & Fischer 1952; Lefebvre 1995). Some time around 1921, blue tits in the Swaythling area learned to peck through the wax closings on milk bottles and peck up the cream from them. The habit gradually spread to other areas of England. Simultaneously, the individual birds used different techniques to open the lids. This indicates that only information on the fact that cream can be obtained by opening a bottle was spread culturally, but not the technique of how to open the bottle (Sherry & Galef 1984; Blackmore 2001).
However, in some species, in addition to stimulus enhancement, imitative learning also plays a role (Fawcett, Skinner, & Goldsmith 2002)(Fig. XVII.3). Imitation of behavior is mostly highly selective. Young animals very frequently imitate their parents or the behavior of other adult members of the population. Another fairly common means of learning consists in imitation of the behavior of the highest-ranking individuals in the social hierarchy. It has been observed, for example for Japanese macaque apes, that if only a low-ranking individual was capable of removing sand from grain (by throwing the mixture into water and collecting the floating grains from the surface), no one tried to imitate it. Only when a dominant male learned this from him did this skill rapidly spread to the whole group (Kawamura 1963). This means of accepting cultural traits is understandably advantageous – from the viewpoint of fitness it makes sense to accept behavior from successful, i.e. from higher ranking individuals.
see History of evolutionism - pre-Darwinist period
see Selection cyclic
see Selection cyclic