Although animals are thought to be more mobile than plants, pollen and seeds may be carried great distances by animals, water or wind. Mobility plays an important role in dispersal rate, as highly mobile individuals tend to have greater movement prospects. Gene flow is expected to be lower in species that have low dispersal or mobility, that occur in fragmented habitats, where there is long distances between populations, and when there are small population sizes. There are a number of factors that affect the rate of gene flow between different populations. In some cases dispersal resulting in gene flow may also result in the addition of novel genetic variants under positive selection to the gene pool of a species or population (adaptive introgression. For this reason, gene flow has been thought to constrain speciation and prevent range expansion by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to differentiation and adaptation. High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity. Migrants change the distribution of genetic diversity among populations, by modifying allele frequencies (the proportion of members carrying a particular variant of a gene). Gene flow is an important mechanism for transferring genetic diversity among populations. Populations can diverge due to selection even when they are exchanging alleles, if the selection pressure is strong enough. It has been shown that it takes only "one migrant per generation" to prevent populations from diverging due to drift. If the rate of gene flow is high enough, then two populations will have equivalent allele frequencies and therefore can be considered a single effective population. In population genetics, gene flow (also known as migration and allele flow) is the transfer of genetic material from one population to another. Transfer of genetic variation from one population to another Gene flow is the transfer of alleles from one population to another population through immigration of individuals.
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