VII.2.2 Gene flow helps maintain the genetic polymorphism of a population.
During its history, each population is exposed to the effects of natural selection, which constantly eliminates individuals whose phenotype, and thus also genotype, is not appropriate to local conditions. Genetic drift has a similar effect on the gene pool of a population. These two processes constantly reduce the amount of genetic polymorphism in the population’s gene pool. A genetically uniform population is in a worse position when it is required to respond evolutionarily to fast, often just short-term changes in the environment and can, in response, only resort to mutation as a source of selectable genetic variability. The gene flow constantly enhances the genetic polymorphism of local populations because, via migrants, it keeps supplying them with alleles that they may have contained earlier but that disappeared as a result of local selection pressures or genetic drift. Due to the fact that local populations exist under slightly different conditions and are therefore exposed to different selection pressures, the composition of their gene pools can also be expected to differ. An allele that is not useful in one environment and is therefore eliminated from the gene pool of the corresponding population by natural selection may be useful in a different environment and may therefore frequently and consistently occur in the gene pools of other populations. As a result, migrants are very likely to introduce alleles that are not present in the host population or that are infrequent.