∞ generated and posted on 2016.12.15 ∞
Movement of alleles into or out of populations.
Migration, in a genetic or evolutionary sense, can be a consequence of movement of members of the same species from one population of that species to another, thereby entailing actual, physical movement. Such genetic migration, however, can also involve the movement of alleles between species, a process that is known, in different settings, as introgression or horizontal gene transfer (and also lateral gene transfer).
In the latter case, migration does not necessarily involve the actual, physical migration of organisms from one place to another but, instead, may entail matings that take place between members of different species in locations where the ranges of each participant happen to overlap (i.e., sympatric populations). Thus, one can think of genetic migration as migration of alleles from parents to offspring where the two parents are not really members of the same population.
Figure legend: Schematic representation of genetic migration, a.k.a., gene flow, as allele movement into as well as out of populations.
Migration is one of the violations of Hardy-Weinberg assumptions (see 1, 2, 3, 4, and 5). Thus, alleles can leave populations, as individuals move from one place to another relative to their gene pools, while individuals similarly can enter gene pools that they previously were found outside of it. In each case this potentially results in changes in allele frequencies within populations.
Note that the concept of genetic migration is closely associated with that of sex, whether the meiotic sexual reproduction that one observes in eukaryotes or the less-conspicuous or thorough gene exchange that one sees among microorganisms, e.g., bacterial sex. See also simply the concept of gene flow.
Figure legend: Shapes refer to differences in genotype among members of each population with underlying allelic differences. When an individual moves from one population to the other it carries its alleles with it. If such movement is unbiased then the impact of the movement will be to make the two populations more genetically similar, which at an extreme means that the frequencies of each allele will be identical going from one population to the other. It is difficult for the populations to in fact remain distinct, however, given such high levels of gene exchange. Note that migration need not be two way. In addition, populations can be of different sizes where substantial allele movement from a larger population to a smaller one can result in the smaller population's allele frequencies becoming dominated by the frequencies seen in the larger population.
The primary consequence of genetic migration is to make two populations more genetically similar: Greater similarly in terms of what alleles are present as well as the frequency of those alleles between the two populations. These two populations can be genetically similar already, such as the same species, or instead can be genetically dissimilar such as different species. The more similar two populations are then the more readily alleles may be exchanged. At the same time, however, the less readily that exchange may be detected when studying the populations involved since it is difficult to recognize the movement of alleles between populations that essentially are identical in both populations.