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Actual reproductive rate

see Basic reproductive rate

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Adaptations

Complex biological structures that have been built up by natural selection (that has used random mutations as the raw material) and therefore currently fill some vital function. They can be divided into two categories, adaptations and exaptations. Adaptation encompasses structures that fulfill the same function from the very beginning and were formed and developed as a consequence of the action of the same selection pressure as is acting on them at the present time. See also exaptations.

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Adaptive landscape

Biologists study a great many evolutionary processes on the models of an adaptive landscape.The American biologist Sewall Wright introduced the model of the adaptive landscape into evolutionary biology at the beginning of the nineteen thirties. In actual fact, this consists of two quite different models, between which not even their author always sufficiently differentiated. This is an abstract model of either the environment and the individual organisms or the environment and populations. In the former case, the evolution of organisms in the environment can be conceived, e.g., as a three-dimensional topographical map, in which the x and y coordinates correspond to two qualities of a hypothetical organism (e.g. body weight and maximum speed of movement); in the second case, the frequency of the alleles are on the x and y axes at two different loci and the average biological fitness of the given population is on the z axis. Let us now return to the first model of an adaptive landscape. The shape of the surface of an adaptive landscape, the system of hills and valleys, is given independently of the properties of the organisms and determines the distribution of future niches in the particular environment. The projection of the points onto the surface of the adaptive landscape, i.e. coordinate z, determines the fitness of each individual organism (characterized in our model by properties X and Y). It is apparent that various combinations of X and Y are variously advantageous from the standpoint of natural selection. Mutations, i.e. a change in coordinate x or y, move the organism from one place to another, also shifting their projections onto the surface of the adaptive landscape. Only mutations that shift the projection of the organism uphill in the plane of the adaptive landscape, to places with larger coordinate z, can be fixed by natural selection. It is apparent that, under suitable circumstances, in time, organisms climb up to the peaks of the individual hills.

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Adaptive radiation

The formation of a key evolutionary innovation is a very effective way in which an anagenetic change can increase the probability and thus the frequency of speciation; this consists in a trait that enables its carriers to occupy a new adaptive zone, i.e. utilize a certain set of niches that were not accessible to them until this time. If this set of niches is very extensive, adaptive radiation can occur in the particular line. During adaptive radiation, the relevant line very frequently splits into a large number of different species, each of which can lead to the formation of an independent phylogenetic line (Fig. XXVI.4). These species divide the newly available niches up amongst themselves. If the new niches include an important resource that is simultaneously part of the niches of an already-existing species, the new species can have a substantial negative impact on the species utilizing these original niches. They can reduce the area of their occurrence, utilize their ecological niches and thus reduce the sizes of their populations or can even cause their extinction. Homoiothermy in mammals and active flight in birds are apparently examples of anagenetic changes leading to adaptive radiation.

Adaptive radiation can occur through a quite different mechanism. If a member of a certain species enters a territory where there are no members of a great many taxa, for example if it gets to a newly formed volcanic island far from the mainland, it can rapidly undergo series of speciations and its descendants can occupy niches that are already occupied by other species on the mainland. The phylogenetically highly related but ecologically very diversified species of Darwin’s finches on the Galapagos and the Drepanididae on the Hawaiian Islands evolved in this way (Craddock 2000) (see Fig. XXVII.3). The king of the hill effect is very frequently active in both macroevolution and microevolution (see XXII.5.5). As soon as an ecological niche (or even a habitat) is already occupied by a certain species (for a habitat, a certain population), this species cannot be simply forced out by a species (population) whose members penetrated there at a later time. The factor favoring the current “king of the hill” consists in the greater number of the original species, which enables it to better resist any random fluctuations in the size of the population, and also its momentarily better adaptation to the local conditions. The newly arrived species (population) might, in the future, be able to adapt to the given conditions just as well or even better; however, competition from the “king of the hill” provides no scope for this.

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Address phenomena

see Speciation by coevolutionary lift

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Advantage of sexual reproduction

Advantage of sexual reproduction Reproduction is one of the properties of living systems that is essential for biological evolution.However, sexual reproduction is a modern product of this evolution, in a certain sense a luxury product and, as far as the mechanism of its emergence goes, a somewhat problematic product.Simultaneously, this type of reproduction clearly predominates in nature at the present time, at least in the number of species in which it occurs.It has been estimated that, amongst multicellular eukaryotes, only one species in a thousand is parthenogenetic (unisexual), i.e. reproduces through unfertilized eggs, or asexual, i.e. reproduces (entirely) without producing sex cells, for example through somatic clones (Simon et al. 2003).Asexual reproduction (agamogenesis or monogony) as an sole means of reproduction is encountered only in some groups of unicellular organisms (even here, however, research in the future will, in a great many cases, lead to the discovery of sexual processes) and also as a secondarily formed means of adjusting to specific living conditions and strategies in various unrelated taxa.In a number of fauna taxa, some form of parthenogenesis, i.e. some form of reproduction through unfertilized eggs, emerges as a consequence of parasitization by vertically transmitted microparasites, such as bacteria of the Wolbachia genus (Koivisto & Braig 2003).If these parasites cannot be transferred to progeny by sperm, they are very frequently capable of “manipulating” the female so that she does not produce – from their point of view – valueless males, but rather multiplies only parthenogenetically.Similarly in plants, parthenogenesis (denoted here by the term agamospermy)frequently emerges through the activity of “selfish genes” occurring in the genome of mitochondria and plastids, i.e. organelles transferred to progeny through eggs but not pollen in “higher plants”.
            The fact that asexual reproduction constitutes a secondary state in most taxa can be derived from the fact that sexual species occur amongst the relatives of asexual species and the asexual species represent only mutually isolated terminal twigs on the phylogenetic trees, i.e. individual species and genera, but not extensive branches of mutually diverse higher taxa, i.e. families, orders, classes and phyla.An exception mentioned in the evolutionary literature is the class of bdelloid rotifers (Bdelloidea, phylum Rotifera), a taxon with a phylogenetic age of at least 35 – 40 million years, whose 360 described species, divided into four families and 18 orders, apparently reproduce apomictically without participation by males.Analysis of the genome of four members of this group indicated that their originally diploid genome apparently changed over time to a genome that was functionally haploid and that sexual reproduction most probably never occurs in their life cycle (Welch & Meselson 2000).However, the newest results again throw doubt on this conclusion and indicate that recombination (and thus apparently also sex) occurs with very low frequency even here {9956}.Because the members of other classes of rotifers reproduce sexually, it is certain that asexual reproduction occurred secondarily in bdelloid rotifers.
While the disadvantages of sexual reproduction are mostly quite obvious, this is not as unambiguously true of its advantages.Simultaneously, the fact that, for a great many species and entire large taxa, such as birds and animals, it is the only means of reproduction, indicates that it should be an evolutionarily extremely advantageous process.The fact that asexually reproducing mutants do not gradually predominate in populations of sexually reproducing species also indicates that this is also an evolutionarily stable strategy (ESS) and that, in a population of sexually reproducing individuals, an asexually reproducing mutant is apparently in some way “penalized”, placed at an evolutionary disadvantage.
            At the present time, a number of hypotheses attempt to explain the emergence and persistence of sexual reproduction.The first group of hypotheses assumes that sexual reproduction increases the evolutionary potential of the particular biological species.The second group of hypotheses assumes that sexual reproduction brings an advantage to its carrier, i.e. increases its direct fitness or inclusive fitness.The third group of hypotheses corresponds to a model that assumes that sexual reproduction can be maintained by the mechanism of an evolutionary trap and does not bring its carriers any advantage, while the fourth group assumes that sexual reproduction could have been forced on organisms from outside, and provides an advantage to other subjects of biological evolution at their expense. See alsoNegative heritability of fitness model,  Lottery model,  Elbow room hypothesis, Repairing mutations hypothesis, Stopping the Muller's ratchet hypothesis, Simultaneous selection hypothesis, Genetic elite hypothesis, Sisyphean genotypes hypothesis, Tangled bank hypothesis,  Mills of God model, Evolutionary trap hypothesis, Evolutionary constraints hypothesis, and Manipulation hypothesis of the origin of sexuality.

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Ageing of the phylogenetic line

Changes in time in the character of cladogenesis within a certain phylogenetic line are a striking and simultaneously difficult-to-explain macroevolutionary phenomenon. The paleontological record contains a number of phylogenetic lines that first continued in a state of evolutionary calm for a longer time after their formation, underwent speciation with only minimal frequency and their members also accumulated only a minimum of anagenetic changes. This was followed by turbulent evolutionary activity with splitting off and mutual phenotype diversification of a large number of species in a short period of time. On the phylogenetic tree, this phase appears as sudden radiation of a large number of phylogenetic lines from practically a single point. The period of radiation is again followed by a period of evolutionary calm, with a minimum of new branches, and evolution occurs only within the formerly split-off branches and tends to have the character of adjustment of the adaptation to the conditions of the environment rather than the creation of new diversity. In this period, specialized forms emerge in the line, frequently with large body dimensions that readily become extinct during the next mass extinction event. The last phase in the existence of a phylogenetic line from the viewpoint of its cladogenesis consists in evolutionary stagnation and decline – the species undergo speciation more slowly than extinction and thus the entire line gradually dies out. In some cases, rejuvenation can occur in any phase of the phylogenesis of the line, i.e. new radiation occurs in one of its branches and lines formed during this event then gradually replace most of the lines formed during the previous radiation (Fig. XXVI.2).

            The described process was sometimes interpreted in the past as maturing and ageing of the phylogenetic line. However, it is very probable that this is actually only a superficial analogy with the life cycle of a biological individual. In the life of a biological individual, i.e. in the section defined on the one hand by its birth and on the other hand by the physical destruction of its body, there actually do exist physiological processes determining the alternation of developmental phases, during which the viability of the individual and its ability to reproduce first increase and then decrease, with a proportional change in the probability of its death. However, this is not the case in the life of a phylogenetic line. Only individual species are formed and become extinct (and thus it is, at the very least, theoretically quite possible that the individual species can age, see XXVI.5), but the phylogenetic line encompassing all the species of organisms on the Earth lasts without interruption from the moment of emergence of life on this planet and only gradually branches over time. The individual lines (phylogenetic sublines) do, of course, disappear through extinction; however, the newly formed branches have, at the moment of their formation, the same age as all the branches existing on the Earth at the given moment. The individual taxa, monophyletic sections of the phylogenetic line defined on the phylogenetic tree by a taxonomist on the basis of an important trait, i.e. on the basis of the attained level of anagenesis, can, of course, emerge and disappear. The evolutionary emergence of a new taxon corresponds not to the formation of a new individual, but only to the formation of the relevant new anagenetic traits in a species within a long-existing phylogenetic line. Simultaneously, various taxa are delimited on the basis of very diverse traits. There is absolutely no reason to assume that the formation of basically an arbitrary trait would automatically initiate the process of ageing of the relevant phylogenetic subline. However, the anagenetic traits themselves, or rather the complex of anagenetic traits that characterize a new taxon, can age, e.g., can freeze, see below.

            It is highly probable that the “life cycle” of a taxon is not a result of its ageing and real changes in the traits of the relevant species. It is more probably a manifestation of mathematical laws following from the existence of fluctuations in speciation rates within a single phylogenetic line and also a consequence of the manner in which we obtain input data. A taxon that did not undergo at least one period of radiation has a very low chance of surviving in nature long enough for us to be able to observe it in the paleontological record and thus of being included in our considerations. There is low probability that a taxon will be subject to constant radiation; radiation is apparently always caused by some specific, relatively rare anagenetic change (see XXVI.2.1). The probability of radiation of a certain line is simultaneously apparently negatively affected by the overall biodiversity in the environment (see, e.g., the king of the hill effect, XXII.5.5, and the discussion of the model of the ecological network, XXII.7). As a consequence, a global balance is apparently maintained between the rate of speciation and the rate of extinction. However, this means that a taxon that does not undergo radiation in time becomes extinct. Obviously, once a taxon has become extinct, it can no longer undergo radiation and can thus not return to the paleontological record.

An alternative explanation is provided by the theory of frozen plasticity, according to which all evolutionary lines gradually age (and thus increasingly old lines emerge) {15429}. Old lines have reduced probability of evolutionary thawing and formation of new fundamental anagenetic features – and thus important evolutionary radiation, as more and more traits that were originally capable of thawing following peripatric speciation become permanently frozen. This phenomenon could explain why, for example, the great majority of disparities, the great majority of animal strains evolved in the Cambrian and, since that time, the number of basic body plans has tended to decrease on the Earth {12783}.

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Allelle

Variant of a particular gene, see Gene.

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Allen’s rule

clinal variability

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Alli effect

An important complication that the emergence of sexual reproduction brought was the Alli effect, i.e. the phenomenon of the dependence of the effectiveness of reproduction on the density of the population, accompanied by a threshold population density necessary for effective reproduction. If an individual of a sexually reproducing species comes to a new territory, sooner or later it will die without establishing a new population. Even when this individual consists in the legendary “fertilized female” or even a small population of individuals of both sexes, it is highly probable that they will die out without establishing a basis for a long-term population. Amongst sexually reproducing species, there is always a threshold density below which the population cannot permanently exist. If the size of the population or, more precisely, the density of the population, i.e. the number of individuals occurring in a certain space, decreases below this value, the population is fated to die out. Individuals of the opposite sex or their gametes are not capable of finding one another sufficiently effectively. Assuming that organisms have not created special ethological mechanisms, the frequency of meeting of individuals is directly proportional to the square of the population density. In this case, the transition between sufficient and insufficient population density can be very sharp and the difference between the actual size of the population, determined, for example, by the resources in the environment, and the threshold density can be very small. Thus, sexually reproducing organisms are permanently exposed to the risk that, on a random fluctuation in the size of the population, they will get below their threshold population density and die out.

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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

Alopolyploidization

see Mutations at the level of the entire chromosome set

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Altruistic behavior evolution of

Altruistic behavior has always been subject to heightened interest from evolutionary biologists.  The origin and prolonged endurance of altruistic behavioral patterns cannot be explained by the mechanism of individual (intraspecific) selection; consequently its relatively frequent incidence in nature long remained a mystery to biologists.

            Historically, the first suggested mechanism of evolutionary origin of altruistic behavior consisted in group selection. According to this hypothesis, altruistic behavior belongs among traits that were not fixed by individual, but by group selection, because they favor groups (sub-populations), where bearers of these traits appear, at the expense of groups with fewer or no bearers of altruistic behavior. The effect of group selection was long thrown into doubt, mainly because the influence of individual selection in favor of egoists (free riders) inside the groups easily prevails over the influence of interpopulation selection in favor of groups with altruists (see IV.8.2). Today, the opinion prevails that, in a normal (structured) population, consisting of a larger number of continuously emerging and fading sub-populations, this mechanism can be relatively strong and, under some conditions, can even prevail over the influence of individual selection. It can therefore be expected that, in at least some cases, group selection is responsible for the origin  and maintenance of altruistic behavior (Alexander & Borgia 1978; Shanahan 1998; Wilson 1975a).           

            Another mechanism that may be responsible for the origin of altruistic behavior is kin selection (Hamilton 1964a; Hamilton 1964b).  As the theory of interallelic selection (the selfish gene theory) stresses, the decisive criterion for fixation or loss of a mutated allele is not how the allele contributes to the fitness of the individual in whose genome it is located, but how it contributes to spreading the copies of the allele in the particular locus inside the gene pool of the population (Dawkins 1976).  Some alleles may contribute to spreading their copies within the gene pool so that their bearer increases, through his altruistic behavior, the fitness of the other bearers of the same allele, usually his relatives, at his own expense (Fig. XVI.5). Once again, it seems that the existence of some patterns of altruistic behavior can be explained by this mechanism.

            Another explanation of the existence of altruistic behavior assumes that it is often actually a case of reciprocal altruism (Trivers 1971; Axelrod & Hamilton 1981). In reciprocal altruism, the individual employs a particular pattern of altruistic behavior only vis-a-vis those members of the population from whom he can, in the future, expect returning of the relevant altruistic behavior. Ethological studies mostly show that individual members of the population constantly “keep a record” of how each member of the population altruistically behaves towards them or towards other members of the population and, according to the degree of his altruism, they behave or don’t behave altruistically towards him.

            In species with a sufficiently well-developed neural system and sufficiently well-developed social structure of populations, behavioral patterns have become fixed that lead to punishing less altruistic individuals or even to punishing individuals who do not participate in punishing less altruistic individuals (Gintis, Smith, & Bowles 1901; Okamoto & Matsumura 2000). These behavioral patterns, of course, greatly promote the existence of altruistic behavior. The subject of reciprocal altruism will be further treated in the section devoted to competition of strategies in the sphere of games with repeated interactions between players (XVI.5.3).     

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Altruistic behaviour

see Selection Intraspecific and  interspecific

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Anagenesis

Ongoing anagenesis in a certain phylogenetic line of an organism leads to changes in the phenotypes of the relevant species. These changes can occur from one generation to the next within a single species without splitting the original species into two new species. If a large number of changes have accumulated in such a species, from a certain moment the altered form can be designated as a new species formed by phyletic speciation. Thus phyletic speciation is caused by anagenesis alone. However, there is frequently a close connection between anagenesis and cladogenesis – the anagenetic change is connected with the formation of a new species. Anagenetic changes regularly occur in only some individuals of the original species that are reproductively isolated from the rest of the population. In this case, branching speciation occurs and the anagenetic changes in the phylogenetic line occur on the basis of gradual formation of new species, exhibiting new properties, and the extinction of the original species. Without anagenetic changes, the individual species would not differ phenotypically and we would not be capable of subsequently differentiating the individual events of cladogenesis. See also Phylogenesis.

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Anagenesis in parasites

Parasites can be divided into ectoparasites and endoparasites. Ectoparasites live on the surface of the bodies of the host or only in their “dwellings” (nidocolous parasites). In contrast, endoparasites live directly in the organs or tissues of the host species. A conspicuous anagenetic tendency is sometimes observed for endoparasites, i.e. a trend towards simplification of the body structure and individual physiological functions. This trend sometimes even goes so far that an originally multicellular organism becomes a more or less unicellular organism. A typical example is Myxozoa, a group of organisms parasitizing mostly on fish, in which the methods of molecular taxonomy have recently shown that these are not protozoa, but extremely reduced metazoa (Smothers et al. 1994; Siddall et al. 1995). A great many kinds of parasitic crustaceans have comparably greatly reduced body structure. Viruses are located at the other end of the scale of organismal complexity and they could well provide us with similar surprises in the future. It cannot be excluded that it will be found in time that some viruses are actually extremely reduced bacteria. (Well, probably not, but it would be nice, wouldn’t it?)

The reason for the simplification of the body structure of endoparasites is primarily the fact that a great many physiological functions related, for example, to maintenance of homeostasis, are left to the host organism by the parasite. The internal environment of the organism is relatively stable and rich in some easily processed nutrients. Consequently, a great many functions that are vital for freely living organisms are superfluous for a parasite and it can gradually get rid of them during evolution.

            It is even evolutionarily advantageous for a parasite if it manages to simplify its body structure as much as possible. The more differentiated tissues and organs its body contains, the greater the number of kinds of proteins that it must synthesize and the more easily can the immune system of the host recognize its presence and foreign nature.

            The specific cause of the reduction of the nervous system lies in the extreme predictability and stability of the environment in which the parasite infrapopulation lives. As a result of this predictability, the body of a host is sometimes designated as a third type of environment (in addition to aquatic and terrestrial environments) (Sukhdeo 2000). While every individual of nonparasitic species lives under quite unique conditions, parasites in the bodies of organisms are always in exactly the same, although highly heterogeneous environment. The distribution of the individual organs in the body of the host is almost identical in all individuals of a particular species. Consequently, while the nervous system and learning mechanisms are necessarily of great importance in controlling behavior in nonparasitic organisms, in parasitic organisms, these functions can be taken over by inborn fixed patterns of behavior, initiated without regard to stimuli from the environment (see XVI.2).

            In the past, the existence of predominating trends towards reducing complexity in parasitic organisms was considered to be a quite obvious fact (Poulin 1995b). As was later properly emphasized by a great many authors, the reason for general acceptance of this fact was an elementary methodical mistake. Instead of comparing the complexity of parasites in a certain phylogenetic branch with the complexity of nonparasitic organisms of the sister phylogenetic branch, the complexity of parasites was frequently compared with that of their hosts. It is obvious that, when a cow is compared to its tapeworm, the cow must appear more complex. However, when the complexity of parasitic and nonparasitic flatworms were compared, the result was far less obvious.

At the present time, the opinion pendulum has swung to the other extreme. To the contrary, especially amongst parasitologists and cladists, there is tendency towards throwing into doubt the trend associated with simplification of parasites (Brooks & McLennan 1993). Parastiologists have a quite understandable tendency to “favor” the objects of their study, so that they quite willingly accept the opinion that Sacculina barnacles, a crustacean that is most reminiscent of the mycellium of honey funguses, are, in fact, just as complicated as river crayfish; while it is true that they are lacking some organs, they quite certainly have a large number of evolutionarily new features, such as various types of receptors, at a microscopic or ultra-microscopic level. Support for this opinion of parasitologists by cladists then follows from their programmed lack of interest in similarity, and thus in the appearance of studied organisms. Cladists attempt to reconstruct the cladogenesis of organisms entirely on the basis of their mutual relatedness, where they estimate this relatedness wholly on the basis of sharing of new evolutionary features, synapomorphies (see XXIII.6). A new evolutionary feature could consist in both the acquisition and the loss of a certain body structure. Simultaneously, the complexity of the particular structures is of no importance in the formation of evolutionary hypotheses on the relatedness of organisms. If the particular evolutionary change occurred in one stage, then the loss of a complex structure, such as an eye, is equally important as the one-stage loss or acquisition of any other trait. Thus, if cladists attempt to compare the character of the evolution of nonparasitic and parasitic organisms, they first count the number of new evolutionary features that arose independently in a certain time interval. In this comparison, there understandably need not be any difference between parasites and nonparasitic species. When a cladist goes into greater detail and compares the number of new evolutionary features in both organisms, characterized by the loss of an organ and acquisition of a new organ, they do not in any way differentiate between the complexity of the lost and newly acquired organs. Simply on the basis of mechanical comparison of the matrices of evolutionary changes, they then come to the conclusion that tapeworms are an extraordinarily rapidly and progressively developing group. While they have lost a few organs, such as eyes and a digestive system, they have, on the other hand, acquired a large number of organs (mostly various types of chemoreceptors). Thus, only 6% of evolutionary changes here corresponded to the loss of organs (Brooks & McLennan 1993).

            Because of the difficulty of understanding the concept of complexity (see I.3), it will still long not be possible to exactly decide to what degree and whether trends towards a decrease in complexity predominate overall in parasites. It is, however, obvious that parasites provide us with a great many examples of secondary reduction of organs that have lost their original function in their new environment. In any case, the character of anagenesis of parasitic animals, i.e. at the very least the occasional trend towards simplified body structure, and thus towards reduction of the overall complexity of the organism, indicates that biological evolution need not always be connected with an increase in the complexity of biological systems. As was already pointed out in Sect. I.2, an increase in the complexity accompanies biological evolution; however, in a number of cases this parameter is quite independent of biological evolution. The only trait that differentiates biological evolution from other types of evolution is thus the formation of adaptive traits.

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Anaximandra

see History of evolutionism - pre-Darwinist period

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Aneuploidy

see Mutations at the level of the entire chromosome set

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Antagonistic pleiotropy theory of aging

The antagonistic pleiotropy theory goes even further than theory of reduction of the effectiveness of selection during the life of an individual. This theory assumes that a great many mutations have pleiotropic effect, i.e. that they have a number of contradictory manifestations from the viewpoint of the viability of the individual. These manifestations of a certain mutation can sometimes occur simultaneously; however, they also can occur in different phases of the life cycle of the individual. If a certain mutation has a favorable effect on the viability at an early stage in the life cycle and simultaneously a negative effect at a later stage in the cycle, it will become fixed in the population because, as was already explained in the previous section, the effectiveness of selection is greater in the early stages of the life cycle of an individual. It follows from this theory (Pedersen 1995) that the deterioration in viability during ageing is caused by the manifestations of accumulated mutations with antagonistic effects, to be more exact, by those that provide a selection advantage for a younger individual and simultaneously a disadvantage for an older individual (Fig. XII.7) {11593}. A mutation increasing the ability to regenerate tissue in a young individual which must almost necessarily simultaneously increase the risk of cancer in an older individual is a typical example.

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Ape process

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

 

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Apomorphic traits

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.s see Synapomorphies

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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

Aposematic coloration

Warning coloration

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Apostatic selection

see Selection frequency dependent

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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

Arena hypothesis

Some authors are of the opinion that, under certain circumstances, the female can, to the contrary, create conditions for the most effective intergametic competition. It is known that the females of a great many animal species repeatedly copulate with a large number of different males. The arena hypothesis assumes that the purpose of this behavior consists in accumulation of sperm samples from a large number of males, thus creating conditions for effective competition between sperm derived from various individuals (Birkhead, Moller, & Sutherland 1993; Telford & Jennions 1998; Zeh & Zeh 1996). However, this hypothesis assumes that there is a positive correlation between the biological quality of sperm and the biological quality of the multicellular organism.

            The assumption that the biological quality of an individual is highly correlated with the competition potential of his sperm is rather bold; however, there is no reason why this should be rejected a priori. While organisms have created some mechanisms that prevent the emergence of competition between the sperm derived from a single individual (see, e.g., the above-mentioned inactivity of their genes), on the other hand the properties of the sperm are quite certainly determined by the quality of the genome of the male. Consequently, it can be expected that the quality of the sperm produced and thus its chance of success in the arena could depend on the biological quality of the males. Even if this were not true, it is still possible that sperm derived from various males compete in the arena independent of their quality but in dependence on their number. The situation is much more favorable in this case, as it can be expected almost with certainty that there exists a correlation between the quality of the male and the number of sperm that he can afford to produce.

            It has so far been verified only in a very few cases that promiscuity amongst females increases the fitness of the progeny. For example, it has been observed in the common yellow-toothed cavy (Galea musteloides) that females that were allowed to copulate with four males raised a larger number of healthy progeny that females that could copulate with only a single male; simultaneously, the size of the litter did not differ (Keil & Sachser 1998) (Fig. XIV.5).s

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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

Aristotle

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

Arms race

Coevolution of a predator and its prey, parasite and its host or two species competing for utilization of a certain resource or having a common predator, and even the coevolution of two mutualistic species very frequently has the character of a sort of “arms race”. A certain new evolutionary feature emerges in one of the species, increasing the fitness of its bearers at the expense of members of another species, and thus it gradually spreads in the population of this species (Fig. XVIII.1). Such a new evolutionary feature could, for example, consist in stronger jaw muscles, enabling a predator fish to crack the shell of a certain kind of snail and include it in its diet. This would create a selection pressure on the snail to create a stronger shell, or to synthesize a poisonous or terrible-tasting substance to defend itself against attacks by the fish. The emergence of such an adaptation in the snails would produce a selection pressure on the predatory fish which, sooner or later, would lead to the creation of a suitable counter-strategy, enabling it to somehow overcome the effect of the new evolutionary trait in the snail – for example, through the formation of even stronger jaw muscles or of sharper horny jaws, or the emergence of some suitable pattern of behavior that would enable it to break even stronger shells. A new round in the arms race can be started by either the “attacker” or the “defender” and the participants in this race need not be only a pair of species, but can even consist in large groups of mutually interacting species.

            A great many paleontologists assume that some rapid changes in fauna that occurred in certain short periods in the history of the Earth, with the simultaneous participation of an enormous number of species, emerged as a product of just such arms races. A certain new evolutionary feature emerged in one species, creating a selection pressure on a great many species in the environment; they had to somehow react adaptively to this pressure, other species had to react to their reactions, so that the wave of anagenetic changes could finally affect a great portion of the species in the particular ecosystem in a very short period of time (Morris 1995).

            Arms races can also occur at an intraspecific level and this can happen on a relatively short time scale in microevolutionary processes. The members of the male or female sex can increase their fitness at the expense of the members of the other sex. Under normal conditions, the speed of evolution of males and females is comparable, so that it cannot be observed that the members of one sex would gain an apparent advantage over the members of the other sex. However, by suitably arranging an experiment, we can ensure, for example, that the females will not be able to respond to the evolutionary drives of the males. The experiment is performed in that the males are allowed to interact ecologically, ethologically and sexually with the females of a certain strain for a number of successive generations, where only the males of the obtained progeny are allowed to survive. The females are replaced in the experimental population in each generation from the stock population (and are thus completely “naive” from the standpoint of interaction with the particular selected line of males). The results of such an experiment performed on drosophila indicate that the males gained a considerable advantage over the females after 41 generations (Rice 1996). They achieved copulation with the females of the particular strain much faster; following copulation, the time before the females began to copulate with some other male was prolonged and, in case of multiple copulation with their own and foreign males, only a small percentage of the eggs were fertilized by the foreign males. The females lost out in the evolutionary battle, not only in the limited ability to chose, from their point of view, the best father for their progeny, but also in shortening of their lifetimes following copulation with the test males (but not with control males) (Fig. XVIII.2).

            According to some authors, the coevolutionary battle between the sexes in sexually reproducing species is the main reason for the genetic divergence of allopatric populations and is thus indirectly an important motor for allopatric speciation (Martin & Hosken 2003).

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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

Atavisms

How comparative anatomy and embryology finds proof evolution theory?

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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

Autoelection

see  Speciation by coevolutionary lift

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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

Autopolyploidization

see Mutations at the level of the entire chromosombe set

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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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