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

It very frequently happens that, after the end of a selection experiment, in which a population with a certain phenotype trait was selected, for example increased body weight, the population quite rapidly returns to the original values (see Fig. VIII.10). One of the possible explanations could be that, together with the alleles whose presence results in an increase in body weight, we unwittingly also selected alleles of genes that are genetically linked to them and that could have a negative effect on the viability of the individual (Sheppard 1958). Following interruption of artificial selection, natural selection causes a return of the frequency of these alleles to the original values and, together with them, the alleles affecting the body weight also return to the normal values.

It should be emphasized that genetic linkage is not the only possible explanation of this phenomenon. It is very probable that the frequencies of a great many alleles in the population are maintained at a certain level through frequency-dependent selection. If we force this population out of equilibrium through artificial selection and then interrupt the selection, the genetic composition of the population will gradually return to the original value through frequency-dependent selection. It is clear that if, during artificial selection, there is a decrease in the frequencies of some alleles down to zero and others increase to 100%, the original genotype composition of the population and thus the original phenotype of its population cannot be completely renewed.