<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>
two populations that have no gene flow between them are likely to
have developed genetic differences