<P> F s t = 1 / (4 N e m + 1) (\ displaystyle Fst = 1 / (4N_ (e) m + 1)) </P> <P> The formula can be modified to solve for the migration rate when F s t (\ displaystyle Fst) is known: N m = 1 (1 / F s t − 1) / 4 (\ displaystyle Nm = 1 (1 / Fst - 1) / 4), Nm = number of migrants . </P> <P> When gene flow is blocked by physical barriers, this results in Allopatric speciation or a geographical isolation that does not allow populations of the same species to exchange genetic material . 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 . </P> <P> Barriers to gene flow need not always be physical . Sympatric speciation happens when new species from the same ancestral species arise along the same range . This is often a result of a reproductive barrier . For example, two palm species of Howea found on Lord Howe Island were found to have substantially different flowering times correlated with soil preference, resulting in a reproductive barrier inhibiting gene flow . Species can live in the same environment, yet show very limited gene flow due to reproductive barriers, fragmentation, specialist pollinators, or limited hybridization or hybridization yielding unfit hybrids . A cryptic species is a species that humans cannot tell is different without the use of genetics . Moreover, gene flow between hybrid and wild populations can result in loss of genetic diversity via genetic pollution, assortative mating and outbreeding . </P>

Two populations that have no gene flow between them are likely to