Editing Allopatric speciation (section)

== Other models ==
Various alternative models have been developed concerning allopatric speciation. Special cases of vicariant speciation have been studied in great detail, one of which is peripatric speciation, whereby a small subset of a species population becomes isolated geographically; and centrifugal speciation, an alternative model of peripatric speciation concerning expansion and contraction of a species' range.<ref name="eLS2003"/> Other minor allopatric models have also been developed are discussed below.

=== Peripatric ===
[[File:Peripatric Speciation Schematic.svg|thumb|upright=1.5|In peripatric speciation, a small, isolated population on the periphery of a central population evolves reproductive isolation due to the reduction or elimination of gene flow between the two.]] 
{{Main|Peripatric speciation}}
Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population.<ref name="Speciation"/>{{rp|105}} If a small population of a species becomes isolated (''e.g.'' a population of birds on an oceanic island), selection can act on the population independent of the parent population. Given both geographic separation and enough time, speciation can result as a byproduct.<ref name="Theory and speciation"/> It can be distinguished from allopatric speciation by three important features: 1) the size of the isolated population, 2) the strong selection imposed by the dispersal and colonization into novel environments, and 3) the potential effects of [[genetic drift]] on small populations.<ref name="Speciation"/>{{rp|105}} However, it can often be difficult for researchers to determine if peripatric speciation occurred as vicariant explanations can be invoked due to the fact that both models posit the absence of gene flow between the populations.<ref name="Lawson">{{Citation |title=Divergence at the edges: peripatric isolation in the montane spiny throated reed frog complex | author=Lucinda P. Lawson |display-authors=etal | journal=BMC Evolutionary Biology | year=2015 | volume=15 | issue=128 | pages=128 | doi=10.1186/s12862-015-0384-3 | pmid=26126573 | pmc=4487588 | doi-access=free | bibcode=2015BMCEE..15..128L }}</ref> The size of the isolated population is important because individuals colonizing a new habitat likely contain only a small sample of the genetic variation of the original population. This promotes divergence due to strong selective pressures, leading to the rapid [[Fixation (population genetics)|fixation]] of an [[allele]] within the descendant population. This gives rise to the potential for genetic incompatibilities to evolve. These incompatibilities cause reproductive isolation, giving rise to rapid speciation events.<ref name="Speciation"/>{{rp|105–106}} Models of peripatry are supported mostly by species distribution patterns in nature. Oceanic islands and [[archipelago]]s provide the strongest empirical evidence that peripatric speciation occurs.<ref name="Speciation"/>{{rp|106–110}}

=== Centrifugal ===
[[Peripatric speciation#Centrifugal speciation|Centrifugal speciation]] is a variant, alternative model of peripatric speciation. This model contrasts with peripatric speciation by virtue of the origin of the genetic novelty that leads to reproductive isolation.<ref name="Gavrilets">{{Citation |title=Patterns of Parapatric Speciation | author=Sergey Gavrilets |display-authors=etal | journal=Evolution | year=2000 | volume=54 | issue=4 | pages=1126–1134 | doi=10.1554/0014-3820(2000)054[1126:pops]2.0.co;2| pmid=11005282 | citeseerx=10.1.1.42.6514 | s2cid=198153997 }}</ref> When a population of a species experiences a period of geographic range expansion and contraction, it may leave small, fragmented, peripherally isolated populations behind. These isolated populations will contain samples of the genetic variation from the larger parent population. This variation leads to a higher likelihood of ecological niche specialization and the evolution of reproductive isolation.<ref name="eLS2003"/><ref>{{Citation |title=Centrifugal speciation | author=W. L. Brown Jr. | journal=Quarterly Review of Biology | year=1957 | volume=32 | issue=3 | pages=247–277 | doi=10.1086/401875| s2cid=225071133 }}</ref> Centrifugal speciation has been largely ignored in the scientific literature.<ref name="Briggs">{{Citation |title=Centrifugal speciation and centres of origin | author=John C. Briggs | journal=Journal of Biogeography | year=2000 | volume=27 | issue=5 | pages=1183–1188 | doi=10.1046/j.1365-2699.2000.00459.x| bibcode=2000JBiog..27.1183B | s2cid=86734208 }}</ref><ref name="Gavrilets" /><ref>{{Citation |title=Modes of Peripheral Isolate Formation and Speciation | author=Jennifer K. Frey | s2cid=32546573 | journal=Systematic Biology | year=1993 | volume=42 | issue=3 | pages=373–381 | doi=10.1093/sysbio/42.3.373}}</ref> Nevertheless, a wealth of evidence has been put forth by researchers in support of the model, much of which has not yet been refuted.<ref name="eLS2003"/> One example is the possible [[center of origin]] in the [[Indo-Pacific|Indo-West Pacific]].<ref name="Briggs" />

=== Microallopatric ===
[[File:Female Cobalt Blue Zebra Cichlid.JPG|right|thumb|upright|A female [[Maylandia callainos|cobalt blue zebra cichlid]]]]
Microallopatry refers to allopatric speciation occurring on a small geographic scale.<ref name="BMFetal">{{Citation |title=What, if anything, is sympatric speciation? | author=B. M. Fitzpatrick| author2=A. A. Fordyce| author3= S. Gavrilets | journal=Journal of Evolutionary Biology | year=2008 | volume=21 | issue=6| pages=1452–1459 | doi=10.1111/j.1420-9101.2008.01611.x | pmid=18823452| s2cid=8721116| doi-access=free }}</ref> Examples of microallopatric speciation in nature have been described. Rico and Turner found [[intralacustrine]] allopatric divergence of ''[[Maylandia callainos|Pseudotropheus callainos]]'' (''Maylandia callainos'') within [[Lake Malawi]] separated only by 35 meters.<ref>{{Citation |title=Extreme microallopatric divergence in a cichlid species from Lake Malawi | author=C. Rico | author2=G. F. Turner | journal=Molecular Ecology | year=2002 | volume=11 | issue=8 | pages=1585–1590 | doi=10.1046/j.1365-294X.2002.01537.x | pmid=12144678 | bibcode=2002MolEc..11.1585R | hdl=10261/59425 | s2cid=16543963 | hdl-access=free }}</ref> [[Gustave Paulay]] found evidence that species in the subfamily [[Cryptorhynchinae]] have microallopatrically speciated on [[Rapa Iti|Rapa]] and its surrounding [[islet]]s.<ref>{{Citation |title=Adaptive radiation on an isolated oceanic island: the Cryptorhynchinae (Curculionidae)of Rapa revisited | author=Gustav Paulay | journal=Biological Journal of the Linnean Society | year=1985 | volume=26 | issue=2 | pages=95–187 | doi=10.1111/j.1095-8312.1985.tb01554.x | doi-access=free }}</ref> A sympatrically distributed triplet of [[Dytiscidae|diving beetle]] (''[[Paroster]]'') species [[Stygofauna|living in aquifers]] of Australia's [[Yilgarn craton|Yilgarn]] region have likely speciated microallopatrically within a 3.5&nbsp;km<sup>2</sup> area.<ref>{{Citation |title=Fine-scale comparative phylogeography of a sympatric sister species triplet of subterranean diving beetles from a single calcrete aquifer in Western Australia | author=M. T. Guzik| author2=S. J. B. Cooper| author3= W. F. Humphreys| author4= A. D. Austin | journal=Molecular Ecology | year=2009 | volume=18 | issue=17| pages=3683–3698 | doi=10.1111/j.1365-294X.2009.04296.x | pmid=19674311| bibcode=2009MolEc..18.3683G| s2cid=25821896}}</ref> The term was originally proposed by [[Hobart Muir Smith|Hobart M. Smith]] to describe a level of geographic resolution. A sympatric population may exist in low resolution, whereas viewed with a higher resolution (''i.e.'' on a small, localized scale within the population) it is "microallopatric".<ref>{{Citation |title=More Evolutionary Terms | author=Hobart M. Smith | journal=Systematic Biology | year=1965 | volume=14 | issue=1 | pages=57–58 | doi=10.2307/2411904| jstor=2411904 | doi-access=free }}</ref> Ben Fitzpatrick and colleagues contend that this original definition, "is misleading because it confuses geographical and ecological concepts".<ref name="BMFetal"/>

=== Modes with secondary contact ===
{{See also|Ecological speciation}}
Ecological speciation can occur allopatrically, sympatrically, or parapatrically; the only requirement being that it occurs as a result of adaptation to different ecological or micro-ecological conditions.<ref>{{cite book|last=Nosil|first=P.|title=Ecological Speciation|year=2012|publisher=Oxford University Press|location=Oxford|isbn=978-0199587117|pages=280}}</ref> Ecological allopatry is a reverse-ordered form of allopatric speciation in conjunction with [[Reinforcement (speciation)|reinforcement]].<ref name="Johannesson2010"/> First, divergent selection separates a non-allopatric population emerging from pre-zygotic barriers, from which genetic differences evolve due to the obstruction of complete gene flow.<ref>{{Citation |title=Natural selection in action during speciation | author=Sara Via | journal=PNAS | year=2009 | volume=106 | issue= Suppl 1| pages=9939–9946 | doi=10.1073/pnas.0901397106| pmid=19528641 | pmc=2702801 | bibcode=2009PNAS..106.9939V | doi-access=free }}</ref> The terms allo-parapatric and allo-sympatric have been used to describe speciation scenarios where divergence occurs in allopatry but speciation occurs only upon secondary contact.<ref name="Speciation"/>{{rp|112}} These are effectively models of reinforcement<ref>{{Citation |title=Sympatric speciation in animals: new wine in old bottles | author=Guy L. Bush | journal=Trends in Ecology & Evolution | year=1994 | volume=9 | issue=8 | pages=285–288 | doi=10.1016/0169-5347(94)90031-0| pmid=21236856 | bibcode=1994TEcoE...9..285B }}</ref> or "mixed-mode" speciation events.<ref name="Johannesson2010"/>