Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

In a great many species, the life cycle entails cyclic and acyclic alternation of phases of sexual and asexual reproduction.This phenomenon is typical for parasites and sessile organisms with zero or very low motility.Asexual reproduction generally appears in the life cycle when the individual has available a quite rich local source of nutrients, where this resource is not renewed and is quite rare in the environment, after it is exhausted the population that is dependent on it dies out and passes its genes on to future generations only through specialized progeny, in plants through seeds and pollen, in parasites through the invasive stages.The advantage of asexual reproduction in these situations consists in the fact that, because of genetic identity, selection of individuals cannot occur amongst siblings, which would give an advantage to those individuals that quickly and ineffectively use up the available stocks of resources.This is the same mechanism that is valid in maintenance of the integrity of a multicellular organism and in maintenance of the micro-evolutionary stability of altruistic behavior in colonies of eusocial organisms (see XII.6.1).However, in most modern organisms, asexual reproduction cannot function permanently; clones of organisms arising by asexual reproduction gradually age, i.e. their members have gradually decreasing viability and fertility.If sexual reproduction, probably accompanied by a reparation of a certain class of mutations or damaged DNA (see XII.3) do not occur, the entire clone will die out sooner or later (Pedersen 1995).

            From the viewpoint of long-term population dynamics, asexual reproduction is only a certain form of vegetative growth.Instead of organisms increasing their body size and increasing the number of their sex organs, as, for example, trees do, through vegetative reproduction, they produce separate, independently viable, genetically identical copies of themselves. A population of genetically identical organisms is called a genet and the individuals forming a certain genet are called ramets.Under certain conditions, the independence of ramets is advantageous, for example in parasites it allows the genets of the parasite to occupy and utilize the entire body of the host organism without there existing any mechanical interconnection between the individual ramets that would otherwise disturb the integrity of the host organism.

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).

The price of chimerism lies in the risk of gonadal parasitism (Rinkevich 2000).In gonadal parasitism, one of the partners preferentially and sometimes exclusively occupies the reproductive organs and produces all the sex cells of the chimeric organism.This danger is somewhat less in plant chimeras because of the existence of the cell walls and thus the related lack of motility of the cells within the organism; amongst animal chimers, encountered, for example, in a great many marine invertebrates, it is substantially greater.However, even in humans a case has been described of a woman whose somatic tissues were genetically uniform and thus were not of chimeric origin, but genetic tests of her four children showed that the sex cells in her ovaries were derived from her (fraternal) twin.I would happily be wrong, but I suspect that at least part of those convicted of rape, who are currently being set free with great publicity on the basis of DNA tests, could correspond to similar cases.

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ig. XII.11 Heritability of  various categories of traits. Morphological traits exhibit the greatest heritability, while the least is exhibited by traits related to life-cycle parameters. Data obtained in laboratory experiments (hatched columns) is shown separately from data measured in natural populations (grey columns). The values given for behavioural traits (columns five and six) are probably somewhat under-evaluated; the results of detailed studies generally indicate that multiply repeated behavioural tests yield substantially higher values, close to the values measured for morphological traits. The numbers of the individual columns correspond to the number of individual observations in the particular category. Modified according to Weigensberg a Roff (1996).

On a long-term evolutionary scale, possible cases of successful gonadal parasitism are quite common.In a great many taxa of vertebrates, including mammals and birds, the precursors of sex cells are not formed directly in the tissues of the future gonads, but rather travel to these organs from other parts of the embryo or even from extra-embryonic fluid during embryogenesis.Simultaneously, the places where the future precursors of the sex cells are formed differ substantially in the individual taxa.It thus follows, amongst other things, that the sex cells in various groups of vertebrates are not mutually homologous (Davison 1998; Davison 2001).