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

Genetic drift is one of the most important mechanisms contributing to changes in the composition of a population’s gene pool. If a population disintegrates into several partial populations isolated in terms of reproduction, the drift effect gradually changes the frequency of alleles in each of these populations. As genetic drift is a stochastic process, allele frequencies in the populations move in different directions. A mathematical model of the genetic drift suggests that genetic diversification should occur very rapidly in populations. However, studies of real populations of the most varied animal and plant species have shown that the frequencies of alleles that can, for some reason, be considered selectively neutral are, in fact, very similar in different populations (Lewontin 1974). It can be demonstrated that the uniformity of selectively neutral alleles within a metapopulation is most likely to be the result of gene flow. Calculations show that even a surprisingly small number of migrants can prevent subpopulations from diversifying genetically through genetic drift (Wright 1931). If we take two populations, each with size N, with an average frequency of the various gene alleles equal to p, subject only to the effects of genetic drift, not selection, and exchanging a certain share m of their genes via migrants in each generation, then the average difference d in the frequencies of the relevant alleles between the populations, or more precisely its absolute value, can be calculated as follows:   

For example, if we take populations of 10 000 individuals that exchange 10 individuals in one generation (m= 0.001) and that had an initial average allele frequency of 0.5 then, at equilibrium, the average difference in allele frequencies will equal 0.156. As m in the equation represents the ratio of the number of migrants to the population size, then Nm term is equal to the absolute number of migrants and the effects of gene flow consequently do not depend on the size of the population but only on the absolute number of migrants per generation. It follows that, in terms of neutralization of the impact of genetic drift, the same number of migrants will have a comparably strong effect on a population of 10 thousand and on a population of 10 million. Although this number will introduce a relatively smaller share of foreign genes into a large population, genetic drift in this population is also proportionally slower than in a small population. 

            As early as in 1931, Sewal Wright deduced that the exchange of 1-2 migrants between partial subpopulations can prevent genetic differentiation and thus speciation of subpopulations within a metapopulation as a result of genetic drift and will ensure that the metapopulation develops synchronously as a single evolutionary unit (Fisher 1958). This conclusion has also been verified experimentally, for example, by studies of differentiation in red flour beetle populations (Schamber & Muir 2001).