Editing Allopatric speciation (section)

=== Reinforcement ===
{{Main|Reinforcement (speciation)}}
[[File:Allopatric Speciation with Reinforcement Schematic.svg|right|thumb|upright=1.5|In allopatric speciation, a species population becomes separated by a geographic barrier, whereby reproductive isolation evolves producing two separate species. From this, if a recently separated population comes in contact again, low fitness hybrids may form, but reinforcement acts to complete the speciation process.]]
Reinforcement has been a contentious factor in speciation.<ref name="eLS2012">{{Cite book | last1= Hvala | first1= John A. | last2 =Wood | first2 =Troy E. |chapter =Speciation: Introduction | title =eLS | title-link=Encyclopedia of Life Sciences | date =2012 | doi =10.1002/9780470015902.a0001709.pub3| isbn= 978-0470016176 }}</ref> It is more often invoked in [[sympatric speciation]] studies, as it requires gene flow between two populations. However, reinforcement may also play a role in allopatric speciation, whereby the reproductive barrier is removed, reuniting the two previously isolated populations. Upon [[secondary contact]], individuals reproduce, creating low-[[Fitness (biology)|fitness]] hybrids.<ref name=Hoskinetal2005>{{Citation |title=Reinforcement drives rapid allopatric speciation | author=Conrad J. Hoskin| author2=Megan Higgie| author3=Keith R. McDonald| author4=Craig Moritz | journal=Nature | year=2005 | volume=437 | issue=7063| pages=1353–1356 | doi=10.1038/nature04004 | pmid=16251964| bibcode=2005Natur.437.1353H| s2cid=4417281}}</ref> Traits of the hybrids drive individuals to discriminate in [[mate choice]], by which pre-zygotic isolation increases between the populations.<ref name="Modes and Mechanisms of Speciation"/> Some arguments have been put forth that suggest the hybrids themselves can possibly become their own species:<ref name=Arnold>{{cite book|last=Arnold |first=M.L. |title=Natural Hybridization and Evolution |year=1996 |publisher=Oxford University Press |location=New York |isbn=978-0-19-509975-1 |pages=232}}</ref> known as [[hybrid speciation]]. Reinforcement can play a role in all geographic modes (and other non-geographic modes) of speciation as long as gene flow is present and viable hybrids can be formed. The production of inviable hybrids is a form of [[Character displacement|reproductive character displacement]], under which most definitions is the completion of a speciation event.<ref name="Modes and Mechanisms of Speciation"/>

Research has well established the fact that [[Biological specificity|interspecific]] mate discrimination occurs to a greater extent between sympatric populations than it does in purely allopatric populations; however, other factors have been proposed to account for the observed patterns.<ref>{{Citation |title=Reinforcement and other consequences of sympatry | author=Mohamed A. F. Noor | journal=Heredity | year=1999 | volume=83 | issue= 5| pages=503–508 | doi= 10.1038/sj.hdy.6886320| pmid=10620021 | doi-access=free }}</ref> Reinforcement in allopatry has been shown to occur in nature ([[evidence for speciation by reinforcement]]), albeit with less frequency than a classic allopatric speciation event.<ref name="Theory and speciation"/> A major difficulty arises when interpreting reinforcement's role in allopatric speciation, as current [[phylogenetics|phylogenetic]] patterns may suggest past gene flow. This masks possible initial divergence in allopatry and can indicate a "mixed-mode" speciation event—exhibiting both allopatric and sympatric speciation processes.<ref name="Johannesson2010"/>