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In population genetics, gene flow (also known as gene migration) is the transfer of alleles or genes from one population to another. Migration into or out of a population may be responsible for a marked change in allele frequencies (the proportion of members carrying a particular variant of a gene). Immigration may also result in the addition of new genetic variants to the established gene pool of a particular species or population. There are a number of factors that affect the rate of gene flow between different populations. One of the most significant factors is mobility, as greater mobility of an individual tends to give it greater migratory potential. Animals tend to be more mobile than plants, although pollen and seeds may be carried great distances by animals or wind. Maintained gene flow between two populations can also lead to a combination of the two gene pools, reducing the genetic differentiation between the two groups. It is for this reason that gene flow strongly acts against speciation, by recombining the gene pools of the groups, and thus, repairing the developing differences in genetic variation that would have led to full speciation and creation of daughter species. For example, if a species of grass grows on both sides of a highway, pollen is likely to be transported from one side to the other and vice versa. If this pollen is able to fertilize the plant where it ends up and produce viable offspring, then the alleles in the pollen have effectively been able to move from the population on one side of the highway to the other. == Barriers to gene flow == Physical barriers to gene flow are usually, but not always, natural. They may include impassable mountain ranges, oceans, or vast deserts. In some cases, they can be artificial, man-made barriers, such as the Great Wall of China, which has hindered the gene flow of native plant populations.〔 One of these native plants, ''Ulmus pumila'', demonstrated a lower prevalence of genetic differentiation than the plants ''Vitex negundo,'' ''Ziziphus jujuba,'' ''Heteropappus hispidus,'' and ''Prunus armeniaca'' whose habitat is located on the opposite side of the Great Wall of China where ''Ulmus pumila'' grows. This is because ''Ulmus pumila'' has wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects.〔 Samples of the same species which grow on either side have been shown to have developed genetic differences, because there is little to no gene flow to provide recombination of the gene pools. Barriers to gene flow need not always be physical. Species can live in the same environment, yet show very limited gene flow due to limited hybridization or hybridization yielding unfit hybrids. Female choice can also play a role in hindering gene flow. Asymmetric recognition of local and nonlocal songs has been found between two populations of black-throated blue warblers in the United States, one in the northern United States (New Hampshire) and the other in the southern United States (North Carolina). Males in the northern population respond strongly to the local male songs but relatively weakly to the nonlocal songs of southern males. In contrast, southern males respond equally to both local and nonlocal songs. The fact that northern males exhibit differential recognition indicates that northern females tend not to mate with “heterospecific” males from the south; thus it is not necessary for the northern males to respond strongly to the song from a southern challenger. A barrier to gene flow exists from South to North as a result of the female preference. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「gene flow」の詳細全文を読む スポンサード リンク
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