<P> In population genetics, gene flow (also known as gene migration or allele flow) is the transfer of genetic variation from one population to another . If the rate of gene flow is high enough, then two populations are considered to have equivalent genetic diversity and therefore effectively be a single population . It has been shown that it takes only "One migrant per generation" to prevent populations from diverging due to drift . Gene flow is an important mechanism for transferring genetic diversity among populations . Migrants change the distribution of genetic diversity within the populations, by modifying the allele frequencies (the proportion of members carrying a particular variant of a gene). High rates of gene flow can reduce the genetic differentiation between the two groups, increasing homogeneity . For this reason, gene flow has been thought to constrain speciation by combining the gene pools of the groups, thus preventing the development of differences in genetic variation that would have led to full speciation . In some cases migration may also result in the addition of novel genetic variants to the gene pool of a species or population . </P> <P> There are a number of factors that affect the rate of gene flow between different populations . 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 . Mobility plays an important role in the migration rate, as highly mobile individuals tend to have greater migratory prospects . Although animals are thought to be more mobile than plants, pollen and seeds may be carried great distances by animals or wind . When gene flow is impeded, there can be an increase in inbreeding, measured by the inbreeding coefficient (F) within a population . For example, many island populations have low rates of gene flow due to geographic isolation and small population sizes . The Black Footed Rock Wallaby has several inbred populations that live on various islands off the coast of Australia . The population is so strongly isolated that lack of gene flow has led to high rates of inbreeding . </P> <P> Decrease in population size leads to increased divergence due to drift, while migration reduces divergence and inbreeding . Gene flow can be measured by using the effective population size (N e (\ displaystyle N_ (e))) and the net migration rate per generation (m). Using the approximation based on the Island model, the effect of migration can be calculated for a population in terms of the degree of genetic differentiation (F s t (\ displaystyle Fst)). This formula accounts for the proportion of total molecular marker variation among populations, averaged over loci . When there is one migrant per generation, the inbreeding coefficient (F s t (\ displaystyle Fst)) equals 0.2 . However, when there is less than 1 migrant per generation (no migration), the inbreeding coefficient rises rapidly resulting in fixation and complete divergence (F s t (\ displaystyle Fst) = 1). The most common F s t (\ displaystyle Fst) is <0.25 . This means there is some migration happening . Measures of population structure range from 0 to 1 . When gene flow occurs via migration the deleterious effects of inbreeding can be ameliorated . </P>

Admixture refers to a lack of exchange of genetic material between two or more populations