Editing Allopatric speciation (section)

== Vicariance model ==
[[File:Allopatric Speciation Schematic.svg|right|thumb|upright=1.8|A population becomes separated by a geographic barrier; [[reproductive isolation]] develops, resulting in two separate species.]]
The notion of vicariant evolution was first developed by Venezuelan botanist [[Léon Croizat]] in the mid-twentieth century.<ref>Croizat L (1958).''Panbiogeography or An Introductory Synthesis of Zoogeography, Phytogeography, Geology; with notes on evolution, systematics, ecology, anthropology, etc.''. Caracas: Published by the author, 2755 pp.</ref><ref>Croizat L (1964). ''Space, Time, Form: The Biological Synthesis''. Caracas: Published by the author. p. 676.</ref> The vicariance theory, which showed coherence along with the acceptance of plate tectonics in the 1960s, was developed by Croizat in the early 1950s as an explanation for the similarity of plants and animals found in South America and Africa by deducing that they had originally been a single population before the two continents drifted apart.

Currently, speciation by vicariance is widely regarded as the most common form of speciation;<ref name="eLS2003"/> and is the primary model of allopatric speciation. Vicariance is a process by which the geographical range of an individual [[taxon]], or a whole [[Biota (ecology)|biota]], is split into discontinuous populations ([[disjunct distribution]]s) by the formation of an extrinsic barrier to the exchange of genes: that is, a barrier arising externally to a species. These extrinsic barriers often arise from various [[Geology|geologic]]-caused, [[Topography|topographic]] changes such as: the formation of mountains ([[orogeny]]); the formation of rivers or bodies of water; [[Glacial period|glaciation]]; the formation or elimination of [[land bridge]]s; the [[continental drift|movement of continents]] over time (by [[tectonic plates]]); or island formation, including [[sky island]]s. Vicariant barriers can change the distribution of species populations. Suitable or unsuitable habitat may be come into existence, expand, contract, or disappear as a result of global climate change or even large scale human activities (for example, agricultural, civil engineering developments, and [[habitat fragmentation]]). Such factors can alter a region's geography in substantial ways, resulting in the separation of a species population into isolated subpopulations. The vicariant populations may then undergo [[genotypic]] or [[phenotypic]] divergence as: (a) different [[mutation]]s arise in the [[gene pool]]s of the populations, (b) they become subjected to different selective pressures, and/or (c) they independently undergo [[genetic drift]]. The extrinsic barriers prevent the exchange of genetic information between the two populations, potentially leading to differentiation due to the ecologically different habitats they experience; selective pressure then invariably leads to complete [[reproductive isolation]].<ref name="Speciation"/>{{rp|86}} Furthermore, a species' proclivity to remain in its [[ecological niche]] (see [[phylogenetic niche conservatism]]) through changing environmental conditions may also play a role in isolating populations from one another, driving the evolution of new lineages.<ref>{{Citation |title=Speciation and Ecology Revisited: Phylogenetic Niche Conservatism and the Origin of Species | author=John J. Wiens | journal=Evolution | year=2004 | volume=58 | issue=1 | pages=193–197 | doi=10.1554/03-447 | pmid=15058732 | s2cid=198159058 }}</ref><ref>{{Citation |title=Niche Conservatism: Integrating Evolution, Ecology, and Conservation Biology | author=John J. Wiens | author2= Catherine H. Graham | s2cid=3895737 | journal=Annual Review of Ecology, Evolution, and Systematics | year=2005 | volume=36 | pages=519–539 | doi= 10.1146/annurev.ecolsys.36.102803.095431}}</ref>

Allopatric speciation can be represented as the extreme on a [[gene flow]] continuum. As such, the level of gene flow between populations in allopatry would be <math>m=0</math>, where <math>m</math> equals the rate of gene exchange. In sympatry <math>m=0.5</math> (panmixis), while in [[parapatric speciation]], <math>0 < m < 0.5</math> represents the entire continuum,<ref>{{Citation | title=Fitness landscapes and the origin of species | author=Sergey Gavrilets | date=2004 | pages=13 | publisher=Princeton University Press }}</ref> although some scientists argue<ref name="LanguageSpeciation"/><ref>{{Citation |title=Sympatric speciation in animals: the ugly duckling grows up | author=Sara Via | journal=Trends in Ecology & Evolution | year=2001 | volume=16 | issue=1 | pages=381–390 | doi= 10.1016/S0169-5347(01)02188-7| pmid=11403871 }}</ref> that a classification scheme based solely on geographic mode does not necessarily reflect the complexity of speciation.<ref name="Modes and Mechanisms of Speciation">{{Citation |title=Modes and Mechanisms of Speciation | author=Hannes Schuler| author2=Glen R. Hood| author3=Scott P. Egan | author4=Jeffrey L. Feder | journal=Reviews in Cell Biology and Molecular Medicine | year=2016 | volume=2 | issue=3 | pages=60–93 | doi=10.1002/3527600906 }}</ref> Allopatry is often regarded as the default or "null" model of speciation,<ref name="LanguageSpeciation"/><ref>{{Citation |title=Inverting the null-hypothesis of speciation: a marine snail perspective |author1-link=Kerstin Johannesson| author=Kerstin Johannesson | journal=Evolutionary Ecology | year=2009 | volume=23 |issue=1 | pages=5–16| doi=10.1007/s10682-007-9225-1 |bibcode=2009EvEco..23....5J | s2cid=23644576 }}</ref> but this too is debated.<ref name="Johannesson2010">{{Citation |title=Are we analyzing speciation without prejudice? | author=Kerstin Johannesson | journal=Annals of the New York Academy of Sciences | year=2010 | volume=1206 | issue=1 | pages=143–149 | doi=10.1111/j.1749-6632.2010.05701.x | pmid=20860687 | bibcode=2010NYASA1206..143J | s2cid=41791817 }}</ref>

=== Reproductive isolation ===
{{Main|Reproductive isolation}}
Reproductive isolation acts as the primary mechanism driving genetic divergence in allopatry<ref name="Theory and speciation">{{Citation |title=Theory and speciation | author=Michael Turelli| author2=Nicholas H. Barton| author3=Jerry A. Coyne | journal=Trends in Ecology & Evolution | year=2001 | volume=16 | issue=7 | pages=330–343 | doi=10.1016/s0169-5347(01)02177-2| pmid=11403865}}</ref> and can be amplified by [[Divergent evolution|divergent selection]].<ref name="Rice&Hostert">{{Citation |title=Laboratory Experiments on Speciation: What Have We Learned in 40 Years? | author=William R. Rice | author2=Ellen E. Hostert | journal=Evolution | year=1993 | volume=47 | issue=6 | pages=1637–1653 | doi=10.1111/j.1558-5646.1993.tb01257.x| pmid=28568007 | jstor=2410209 | s2cid=42100751 }}</ref> Pre-zygotic and post-zygotic isolation are often the most cited mechanisms for allopatric speciation, and as such, it is difficult to determine which form evolved first in an allopatric speciation event.<ref name="Theory and speciation"/> Pre-zygotic simply implies the presence of a barrier prior to any act of fertilization (such as an environmental barrier dividing two populations), while post-zygotic implies the prevention of successful inter-population crossing after fertilization (such as the production of an infertile [[Hybrid (biology)|hybrid]]). Since species pairs who diverged in allopatry often exhibit pre- and post-zygotic isolation mechanisms, investigation of the earliest stages in the life cycle of the species can indicate whether or not divergence occurred due to a pre-zygotic or post-zygotic factor. However, establishing the specific mechanism may not be accurate, as a species pair continually diverges over time. For example, if a plant experiences a [[Polyploid|chromosome duplication event]], reproduction will occur, but [[Sterility (physiology)|sterile]] hybrids will result—functioning as a form of post-zygotic isolation. Subsequently, the newly formed species pair may experience pre-zygotic barriers to reproduction as selection, acting on each species independently, will ultimately lead to genetic changes making hybrids impossible. From the researcher's perspective, the current isolating mechanism may not reflect the past isolating mechanism.<ref name="Theory and speciation"/>

=== 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"/>

=== Mathematical models ===
Developed in the context of the genetic basis of reproductive isolation, mathematical scenarios model both pre[[zygote|zygotic]] and postzygotic isolation with respect to the effects of genetic drift, selection, [[sexual selection]], or various combinations of the three. [[Masatoshi Nei]] and colleagues were the first to develop a [[Neutral theory of molecular evolution|neutral]], [[Stochastic process|stochastic]] model of speciation by genetic drift alone. Both selection and drift can lead to postzygotic isolation, supporting the fact that two geographically separated populations can evolve reproductive isolation<ref name="Speciation"/>{{rp|87}}—sometimes occurring rapidly.<ref>{{Citation |title=A Mechanism for Rapid Allopatric Speciation | author=Christopher J. Wills | journal=The American Naturalist | year=1977 | volume=111 | issue=979 | pages=603–605 | doi= 10.1086/283191| bibcode=1977ANat..111..603W | s2cid=84293637 }}</ref> [[Fisherian runaway|Fisherian sexual selection]] can also lead to reproductive isolation if there are minor variations in selective pressures (such as predation risks or habitat differences) among each population.<ref>{{Citation |title=Runaway ornament diversity caused by Fisherian sexual selection | author=Andrew Pomiankowski and Yoh Iwasa | journal=PNAS | year=1998 | volume=95 | issue=9 | pages=5106–5111 | doi=10.1073/pnas.95.9.5106| pmid=9560236 | pmc=20221 | bibcode=1998PNAS...95.5106P | doi-access=free }}</ref> (See the Further reading section below). Mathematical models concerning reproductive isolation-by distance have shown that populations can experience increasing reproductive isolation that correlates directly with physical, geographical distance.<ref>{{Citation |title=Isolation by distance | author=Sewall Wright | journal=Genetics | year=1943 | volume=28 | issue= 2| pages=114–138| doi=10.1093/genetics/28.2.114 | pmid=17247074 | pmc=1209196 }}</ref><ref>{{Citation |title=Isolation by distance in equilibrium and non-equilibrium populations | author=Montgomery Slatkin | journal=Evolution | year=1993 | volume=47 | issue=1 | pages=264–279 | doi= 10.2307/2410134| pmid=28568097 | jstor=2410134 }}</ref> This has been exemplified in models of [[ring species]];<ref name="Modes and Mechanisms of Speciation" /> however, it has been argued that ring species are a special case, representing reproductive isolation-by distance, and demonstrate parapatric speciation instead<ref name="Speciation"/>{{rp|102}}—as parapatric speciation represents speciation occurring along a [[Cline (biology)|cline]].