Natural selection is defined as the process of uneven transfer of alleles derived from particular individuals to the gene pool of the following generations through their progeny.This process can occur in a number of quite different ways, and thus it is possible to differentiate several basic types of natural selection and also their combinations.The individual types of selection can be studied from the standpoint of their impact on the course of evolution, i.e. on the speed and direction of changes that they cause in the gene pool of the population, and from the standpoint of the level at which the selection acts (alleles, individuals, populations, etc.).
One of basic preconditions for the functioning of natural selection is the existence of heritability of the properties of organisms (I.9). Over time, organisms can develop complicated adaptive structures and patterns of behaviour only if randomly formed mutations and phenotype manifestations of these mutations and their consequences for the biological fitness of the individual are transferred from the parent organisms to their progeny. This precondition is fulfilled for organisms reproducing asexually – an individual with a certain mutation produces progeny whose genome contains copies of the same mutation and, if further mutation does not occur in the progeny that would somehow change the manifestations of the original mutation, the phenotype manifestations of this mutation and their impact on the biological fitness of the individuals will be the same as for the parent organism. However, a very different situation occurs in organisms with sexual reproduction. In these organisms, the progeny do not receive a copy of the genome of their parents, but rather their zygote is formed with a unique genome through combination of the genes derived half from the mother and half from the father. Although the newly formed mutations are also transferred (with a probability of 50%) from the parents to the progeny, their impact on the phenotype and thus on the biological fitness of the individual is usually fundamentally different than for the parent organism. Compared to asexually reproducing organisms, sexually reproducing organisms have substantially limited heredity of phenotype properties as, because of epistatic interactions between the individual genes, the same allele in the context of various genomes can cause the formation of completely different phenotype traits. Similarly, they have substantially limited heritability of biological fitness as, in the context of certain phenotype traits, a single trait can increase the biological fitness of its bearers, while it can reduce it in the context of other traits. This means that a great many mutations cannot become fixed in the population because, while they can contribute to increasing the biological fitness in the genomes of some individuals, and are thus preferred by natural selection here, in the genomes of the progeny of these individuals they can, on the other hand, reduce their fitness and their frequency is then reduced by natural selection. The degree to which the heritability of properties in sexually reproducing organisms only reduces the effectiveness of the functioning of Darwinist evolution and the degree to which it prevents its functioning is a question that has not yet been resolved.
Attempts to come to terms with the problem of the apparent existence of biological evolution under the conditions of low heritability of traits and biological fitness in sexually reproducing organisms are exemplified in the theory of the selfish gene (Dawkins 1976)and thetheory of frozen plasticity (Flegr 1998).
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