Editing Gene flow (section)

=== Allopatric speciation ===
[[File:Speciation modes edit.svg|thumb|Examples of speciation affecting gene flow.]]When gene flow is blocked by physical barriers, the outcome is [[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 natural. Examples include impassable mountain ranges, oceans, or vast deserts. However, in some cases, they can be man-made, such as the [[Great Wall of China]], which has hindered the gene flow of native plant populations.<ref name="pmid12634804" /> 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.<ref name="pmid12634804">{{cite journal |vauthors=Su H, Qu LJ, He K, Zhang Z, Wang J, Chen Z, Gu H | title = The Great Wall of China: a physical barrier to gene flow? | journal = Heredity | volume = 90 | issue = 3 | pages = 212–9 |date=March 2003 | pmid = 12634804 | doi = 10.1038/sj.hdy.6800237| s2cid = 13367320 }}</ref> {{Failed verification|date=November 2023}}This is due to ''Ulmus pumila'' having wind-pollination as its primary means of propagation and the latter-plants carry out pollination through insects.<ref name="pmid12634804" /> {{Failed verification|date=November 2023}}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.