Editing Gene flow (section)

== Barriers to gene flow ==

=== 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.

=== Sympatric speciation ===

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.<ref>{{cite journal | vauthors = Savolainen V, Anstett MC, Lexer C, Hutton I, Clarkson JJ, Norup MV, Powell MP, Springate D, Salamin N, Baker WJ |author1-link=Vincent Savolainen| display-authors = 6 | title = Sympatric speciation in palms on an oceanic island | journal = Nature | volume = 441 | issue = 7090 | pages = 210–3 | date = May 2006 | pmid = 16467788 | doi = 10.1038/nature04566 | s2cid = 867216 | bibcode = 2006Natur.441..210S }}</ref> 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]]. In human populations, genetic differentiation can also result from [[endogamy]], due to differences in caste, ethnicity, customs and religion.