[The role of natural selection in evolution].

To evaluate properly a role of natural selection, its effect should be considered in relation to different phases of the evolutionary cycle postulated earlier by the author. At the first stage of the cycle natural selection is directed towards organism's persistence to detrimental external factors and leads to an increased fitness (that is viability and fecundity) in every generation. At the next stage of the cycle natural selection occurs under conditions of intraspecific competition and is directed towards a more efficient utilization of food resources. At this stage natural selection leads to formation and divergence of intraspecific races and is carried out by "single" selection actions occurring now and then and consisting of the survival of rare mutants with an altered ecological potential. Such a strict selection for certain mutants occurs again during the periods of acute competition for food, the selected mutants being characterized by a decrease of fitness, the latter to have been restored by means of the "ordinary" selection within the intervals between crises. According to the model suggested, homozygotes for "detrimental" recessive alleles could be selected in diploids, as the mutants mentioned with altered ecological potential. At the end of the cycle, there is a kind of selection for hybrids in which ecological potential of specialized intraspecific races is combined. The genetic drift is considered as an inevitable consequence of the postulated mechanism of natural selection.