The overall frequency of the alleles of the individual genes does not change in any way when a large population is divided into several smaller populations. However, small populations are endangered not only by the inbreeding effect, but some of its alleles are fixed much more rapidly through genetic drift. One of the two alleles is fixed at random in each of the smaller populations so that the frequency of the individual alleles in the overall population once again does not change. Then the Hardy-Weinberg law does not hold for the whole population as the frequency of homozygotes in the overall population will be higher and that of heterozygotes will be lower than would correspond to the frequencies of the individual alleles, i.e. the Wahlund effect is active here (Wahlund 1928). This is caused by the fact that fixation of a particular allele will occur in the individual subpopulations so that heterozygotes will not be formed in these subpopulations at all (Fig. V.4). In the extreme case, after a certain time, a situation could occur where one of the alleles is fixed in each subpopulation so that heterozygote individuals would not occur in the overall population. In practice, such a situation would require that gene flow would not occur at all between the individual subpopulations, i.e., migration of individuals from one population to another would not occur or any migrants in the population would not be able to cross with members of the domestic population. This situation will probably not be common in nature, but could occur, e.g., when an originally continuous water reservoir would be divided through a reduction in the water level in a lake.
The absence of heterozygotes in a real population is, however, frequently a consequence not of spatial fragmentation of the population, but rather the unrecognized presence of two or more cryptic species, i.e. species whose members have extremely similar or even indistinguishable phenotypes but cannot reproduce together. In a great many cases encountered in nature, the absence or a reduction in heterozygotes is caused by other mechanisms that are not related to the action of genetic drift, for example disruptive selection, parthenogenetic reproduction, etc.
Wall effect is a factor that probably plays an important role in biological evolution. It can be responsible for various evolutionary trends, for example, in process of increase of complexity of organism during evolution. If individuals can move in an arbitrary direction, the population as a whole more or less remains in one place. However, if an impermeable barrier (wall) prevents their movement in a particular direction, the population gradually moves away from it. In evolution, a barrier can be created, e.g., by the minimum complexity necessary for the functioning of a living system.
An interesting product of coevolution is the formation of warning (aposematic) coloration in species whose members are dangerous or inedible. Such species frequently exhibit a very striking phenotype, including not only obvious coloration, most frequently alternating stripes of contrasting colors (black with yellow or orange) but also a number of other traits and patterns of behavior that, together, make the members of this species more visible. The advantage of warning coloration for dangerous species is obvious as it reduces the danger of confusion with other, innocuous species and thus reduces the risk of attacks by predators. However, the evolution of the formation of an aposematic phenotype constitutes a certain problem. Until predators learn to avoid a aposematic prey, the more visible individuals more readily become the prey of predators than cryptic individuals. The evolution of an aposematic phenotype can be assisted if aposematic individuals occur in nature in clusters, i.e. always a greater number of individuals together, than if these are species occurring as lone individuals (Fig. XVIII.7). In this case, the predator will find the aposematic prey easily, but will attack only some individuals in the group; when it discovers that they are inedible or dangerous, it will leave the other members of the cluster alone (Alatalo & Mappes 1996).
In addition, it is advantageous for aposematic individuals if they are mutually similar, i.e. if only one or several typical phenotypes occur in nature (for example, the above-mentioned black and yellow stripes), which will designate the members of the particular species as belonging in the category of dangerous or inedible creatures. In this case, the other species more readily learn that the creatures with this phenotype are not suitable prey and that they should not be attacked. Here, it is not important whether the individual predators learn to recognize the aposematic phenotype during their lifetimes (Marples, Vanveelen, & Brakefield 1994) (which, in the case of recognition of lethal snakes it technically rather difficult in the absence of social learning – even the stupidest predator can fail to confuse such a snake with prey only once) or whether this is a case of evolutionary learning, in which the gene pool of the predator over time fixes randomly formed mutations determining congentital ability of the members of the particular species not to attack species with aposematic phenotype (Coppinger 1969).
At the present time, it is not very clear to what degree Müllerian mimicry and thus actually historical randomness, and to what degree other factors participate in the uniformity of an aposematic phenotype, especially in the uniformity of warning coloration. It is possible that the particular warning coloration was formed as one of a great many possibilities sometime in the past and, since then, it has been advantageous to use it for mutually unrelated species, as a great many species of predators are already capable of recognizing it and avoiding its carriers. However, it also may be that sensory drive on the part of predators also played a role in its formation. We should not overlook this alternative; it is quite possible that the combination of yellow (orange) and black stripes is objectively the most easily distinguishable optical signal for purely physical or neurological reasons.
It is obvious that the aposematic phenotype became an object of frequent imitation by innocuous species. Compared to other forms of Batesian mimicry, however, imitation of aposematic species brings the innocuous species a substantial disadvantage, especially at the beginning, when the similarity is not perfect – the individuals are very visible and a predator can still differentiate them from the imitated species (Fig. XVIII.8). Consequently the evolutionary emergence of this form of mimicry (imitation of an aposematic phenotype) is less probable than the formation of some other type of Batesian mimicry (imitation of a dangerous or inedible species without the typical warning colouration). Even in cases where the imitation of an aposematic species originally emerged as normal Batesian mimicry, the members of the innocuous species are regularly exposed to substantial selection pressure for formation of defense mechanisms against predators, for example, for synthesis or accumulation of a chemical that will make them inedible or unappetizing Thus, Batesian mimicry can secondarily change into Müllerian mimicry. In practice, both types of mimicry anyway form the ends of a more or less continuous series.
The last two evolutionary mechanisms that should be mentioned in a discussion of the formation of an aposematic phenotype in dangerous animals are sorting by stability and species selection. In the case of correlation between an aposematic phenotype and dangerousness or inedibility of the members of individual species, there is an unequal probability of extinction of the species with this correlation and without it. Stated simply, brightly coloured species, whose members are not dangerous or at least are not unappetizing or inedible, are at far greater risk of extinction than brightly coloured species with some of the above protective characteristics. Thus, according to this hypothesis, bright coloration is formed randomly in various species; however, in nature, only those of them that are simultaneously unappetizing, inedible or dangerous can survive.
see Greater robustness of oogenesis hypothesis
A fundamental reason why Lamarckian mechanisms cannot act as an important, generally active evolutionary factor is that they could function only in organisms with unseparated germinal and somatic cell lines.In the 19th century, the foremost German biologist August Weismann already pointed out that Lamarckian evolution can mostly not function in multicellular animals because the germinal and somatic cellular lines are sharply separated basically by an impermeable genetic barrier.In the most important groups of animals, early ontogenesis differentiates the lines of cells from which the sex cells will be formed.Only sex cells are immortal in the evolutionary sense and transfer their genetic information to further generations through the progeny.In contrast, all the somatic cells forming the body of an animal are mortal; even if they were to undergo some useful modification or mutation, this feature would disappear with the death of the individual and could in no way affect the course of evolution.Thus, if thick skin is formed on the foot soles of a person, his germinal cells would never learn of this; if a gene for dihydrofolate reductase multiplies in the liver cells, this change will not be manifested in the progeny.
Weismann not only described the existence of the barrier between germinal and somatic cells, but also attempted to demonstrate it experimentally.However, he was not very fortunate in his choice of experimental system.His experiments, in which he cut off the tails of mice over a great many generations to demonstrate that mice would continue to be born with tails of the same length, yielded the expected result (i.e., no evolutionary response); however, only a very enthusiastic person could consider that this demonstrates the impermeability of the Weismann barrier.However, it should be recalled that, in the 19th century, a number of “experiments” tended to have the nature of demonstration of the existence of a phenomenon and Weismann’s experiments fulfilled their role very well in this sense.By the way, Weismann was a Jew, so he didn’t have to look far for clear empirical proof of the absence of heredity of a similar surgical operation, performed systematically over hundreds of generations ....
In conclusion, it should be recalled that this separation of the germinal and somatic lines is not so strict in the representatives of a great many groups of organisms, including the representatives of most animal species.The Weismann barrier does not exist at all for a number of taxa, or the germinal line is differentiated later in ontogenesis.However, it is interesting that species of animals with early differentiation of germinal cells, i.e. primarily arthropods and vertebrates, have enjoyed the greatest success in the evolution of animals, particularly in the number of split-off species.Some scientists are of the opinion that the actual function of early differentiation of the germinal line is to prevent intra-organism competition amongst the individual cell lines and thus permits the formation of large complicated organisms consisting of a great many cells that can accumulate a large level of genetic variability through somatic mutations during ontogenesis (Buss 1987).Differentiation of the cells of the germinal line does not occur at all in plants and fungi.This could be connected with the existence of cell walls that limit movement of the cells within the the body of the organism and thus substantially limit the scope for intra-organism competition.Because a flower can be formed in a plant through differentiation from somatic tissues, acquired traits can be inherited in plants.For example, if cells better adapted to growth in a certain temperature regime come, in time, to predominate in the tissues of woody species, this property can be transferred to further generations through the seeds from flowers formed by differentiation of these tissues (Pineda-Krch & Fagerstrom 1999; Flegr 2002) (Fig. III.11).However, it should be recalled that, once again, this is not Lamarckian evolution, as adaptation of cells to a certain temperature regime did not occur through adaptive mutations, which would occur as a reaction to conditions or to the behaviour of the organism, but through the Darwinistic mechanism of survival of those tissue cells (or branches of the tree) that are best adapted to the local environment as a consequence of a random change, somatic mutation, somatic recombination or epigenetic change (II.8.1).
The energy invested into reproduction can be divided into two parts. These consist in the energy that a member of one sex cannot transfer to a member of the other sex, for example the energy required for production of its own gamete, and also the energy that can be transferred to a member of the opposite sex, e.g. the efforts invested in care for offspring.
The male and female do not have the same starting conditions in the battle for the smallest investment. The production of macrogametes is, in itself, an expensive matter, while the production of microgametes is relatively inexpensive in the typical case. If the male departs from the female following copulation and leaves the “choice” of whether to invest energy into embryos and offspring to the female, he basically has little to lose, because he invested very little into the production of microgametes. In contrast, the female has already invested more into the production of macrogametes (in mammals also into the development of the embryo) and thus can mostly not consent to reject the role of exclusive caregiver.
The result of the evolutionary game of “who’s the lazier parent” would thus seem to be decided in advance. However, the situation is somewhat more complicated. Males are not capable of agreeing on joint optimal strategy, they do not compete only with females, but with even greater intensity with one another. This can be effectively exploited by their evolutionary adversary, the female. If she manages to force the male to invest a great deal of energy into reproduction before the actual act of copulation, she balances out her own handicap and thus creates preconditions for fairer distribution of energy invested after copulation. Thus, it is advantageous for the female to prolong the precopulation phase of reproduction, to prolong the phase of courtship and delay copulation, for example until a nest is built or stocks of food are accumulated, or to require that the male provide gifts prior to the actual copulation (Wedell 1993; Leimar, Karlsson, & Wiklund 1994) (Fig. XIV.8). With each joule and each hour that the male expends in the precopulation phase of reproduction, the reproductive strategy based on unselective reproduction and zero care for progeny becomes less advantageous and the chance of more even distribution of parental care increases for the female.s
However, intrasexual competition occurs not only among males, but also among females. Thus, the course of the evolutionary game of the lazier parent can be very complicated and its outcome is not easy to predict in advance. It depends on the intensity of the intrasexual competition among males and among females, the cost of producing the individual types of gametes, the cost of bringing up offspring and a great many other factors related to the ecology of the particular species. The game can end up anywhere between biparental and uniparental care for offspring, where the winner, the lazier, certainly need not always be the male.
History of evolutionism – post-neo-Darwinist period
Prisoner’s dilemma-similar game, the so called wolf’s dilemma, belonging into a broader category of “common welfare” games, can be modeled in the laboratory using the methodology of experimental games. Compared to the prisoner’s dilemma game, the reward for mutual cooperation is even higher than the reward for one-sided betrayal, although the risk of betrayal is higher because of the greater number of participants. We seat twenty experimental subjects in separate cabins before the keyboards of a computer terminal and acquaint them with the following rules: the first to press a key gets – completely anonymously, without the other players knowing – $ 4. If no one presses a key during 10 minutes, each participant gets $ 20. It is highly probable the game will be short and we will only have to pay a $ 4 reward. Betraying and receiving a small reward immediately after starting the game, before someone else finds the right solution, is regrettably the most rational solution. (In any case this is not guaranteed; don’t ask me for compensation if you run into a cooperative group and will have to pay out $ 400 in rewards.)
see History of evolutionism - neo-Darwinist period