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Let's note two circumstances. First, the frequency of alleles usually is not in a state, answering to balance between straight lines and the return mutations therefore they are influenced also by other processes (for example, natural selection). Secondly, in the presence of straight lines and the return mutations of change of frequencies of alleles occur more slowly, than in that case when mutations go only in one direction, the return mutations partially compensate change of frequency of alleles as a result of direct mutations. It once again proves that for this purpose that mutations in itself would lead to a little considerable change of frequencies of alleles, a lot of time is required.

The interesting form of assortativny crossing is represented by inbreeding when crossing between related individuals happens more often than it would be possible to expect on the basis of casual crossing. Limitation in a choice of the partner leads to that at one individual two identical allelic genes having the general origins can meet.

Natural selection favors (and counteracts) to preservation of a genotype only through a phenotype. In this regard two outcomes of action of selection are allocated: positive selection (preservation "useful" genotype and negative selection (elimination "harmful" genotype. Thus, natural selection is the directed process, a driving force of evolution.

Mutations can occur and in the opposite direction at the expense of what there is compensation and preservation of a genetic variety. If And mutates in and with U speed, and and back mutates in And with a speed of V, then equilibrium frequencies of alleles will be equal

As a quantitative measure of selection the relative fitness (called by also selective, or adaptive value) is used. Fitness is a measure of efficiency of reproduction of this genotype, a measure of its contribution to the next generation.

Let's enter concept coefficient of selection of S, W = 1-S. S is proportional to decrease in reproduction of a genotype in comparison with neutral which fitness conditionally is accepted to unit. For neutral signs of S = 1, for neutral – S =

Directly we observe only phenotypes, but not genotypes or genes. Variability of a gene pool can be described by either frequencies of genes, or frequencies of genotypes. If we know a ratio between genotypes and phenotypes corresponding to them, on frequencies of observed phenotypes we can calculate frequencies of the corresponding genotypes.

Evolution can be considered as two-step process. On the one hand. there are mutations and recombinations – the processes causing genetic variability; on the other hand, drift of genes and natural selection – processes by means of which genetic changes pass from father to son is observed.