allopatric speciation

File:Allopatric Speciation Schematic.svg develops, resulting in two separate species.]]

The notion of vicariant evolution was first developed by Venezuelan botanist Léon Croizat in the mid-twentieth century.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.Croizat L (1964). Space, Time, Form: The Biological Synthesis. Caracas: Published by the author. p. 676. 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; 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, is split into discontinuous populations (disjunct distributions) 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 geologic-caused, topographic changes such as: the formation of mountains (orogeny); the formation of rivers or bodies of water; glaciation; the formation or elimination of land bridges; the movement of continents over time (by tectonic plates); or island formation, including sky islands. 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 mutations arise in the gene pools 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.{{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.{{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 }}{{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}}

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 $m=0$, where $m$ equals the rate of gene exchange. In sympatry $m=0.5$ (panmixis), while in parapatric speciation, represents the entire continuum,{{Citation | title=Fitness landscapes and the origin of species | author=Sergey Gavrilets | date=2004 | pages=13 | publisher=Princeton University Press }} although some scientists argue{{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 }} that a classification scheme based solely on geographic mode does not necessarily reflect the complexity 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 }} Allopatry is often regarded as the default or "null" model of speciation,{{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 }} but this too is debated.{{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 }}

{{Main|Reproductive isolation}}

Reproductive isolation acts as the primary mechanism driving genetic divergence in allopatry{{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}} and can be amplified by divergent selection.{{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 }} 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. 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). 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 chromosome duplication event, reproduction will occur, but 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.

{{Main|Reinforcement (speciation)}}

File:Allopatric Speciation with Reinforcement Schematic.svg

Reinforcement has been a contentious factor in speciation.{{Cite book | last1= Hvala | first1= John A. | last2 =Wood | first2 =Troy E. |title =Speciation: Introduction | journal =eLS | date =2012 | doi =10.1002/9780470015902.a0001709.pub3| isbn= 978-0470016176 }} 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 hybrids.{{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}} Traits of the hybrids drive individuals to discriminate in mate choice, by which pre-zygotic isolation increases between the populations. Some arguments have been put forth that suggest the hybrids themselves can possibly become their own species:{{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}} 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 reproductive character displacement, under which most definitions is the completion of a speciation event.

Research has well established the fact that 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.{{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 }} 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. A major difficulty arises when interpreting reinforcement's role in allopatric speciation, as current 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.

Developed in the context of the genetic basis of reproductive isolation, mathematical scenarios model both prezygotic 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, 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{{rp|87}}—sometimes occurring rapidly.{{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 }} 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.{{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 }} (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.{{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 }}{{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 }} This has been exemplified in models of ring species; however, it has been argued that ring species are a special case, representing reproductive isolation-by distance, and demonstrate parapatric speciation instead{{rp|102}}—as parapatric speciation represents speciation occurring along a cline.

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. Other minor allopatric models have also been developed are discussed below.

File:Peripatric Speciation Schematic.svg

{{Main|Peripatric speciation}}

Peripatric speciation is a mode of speciation in which a new species is formed from an isolated peripheral population.{{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. 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.{{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.{{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 }} 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 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.{{rp|105–106}} Models of peripatry are supported mostly by species distribution patterns in nature. Oceanic islands and archipelagos provide the strongest empirical evidence that peripatric speciation occurs.{{rp|106–110}}

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.{{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 }} 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.{{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 }} Centrifugal speciation has been largely ignored in the scientific literature.{{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 }}{{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}} Nevertheless, a wealth of evidence has been put forth by researchers in support of the model, much of which has not yet been refuted. One example is the possible center of origin in the Indo-West Pacific.

File:Female Cobalt Blue Zebra Cichlid.JPG]]

Microallopatry refers to allopatric speciation occurring on a small geographic scale.{{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 }} Examples of microallopatric speciation in nature have been described. Rico and Turner found intralacustrine allopatric divergence of Pseudotropheus callainos (Maylandia callainos) within Lake Malawi separated only by 35 meters.{{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 }} Gustave Paulay found evidence that species in the subfamily Cryptorhynchinae have microallopatrically speciated on Rapa and its surrounding islets.{{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 }} A sympatrically distributed triplet of diving beetle (Paroster) species living in aquifers of Australia's Yilgarn region have likely speciated microallopatrically within a 3.5 km² area.{{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}} The term was originally proposed by 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".{{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 }} Ben Fitzpatrick and colleagues contend that this original definition, "is misleading because it confuses geographical and ecological concepts".

{{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.{{cite book|last=Nosil|first=P.|title=Ecological Speciation|year=2012|publisher=Oxford University Press|location=Oxford|isbn=978-0199587117|pages=280}} Ecological allopatry is a reverse-ordered form of allopatric speciation in conjunction with reinforcement. 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.{{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 }} 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.{{rp|112}} These are effectively models of reinforcement{{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 }} or "mixed-mode" speciation events.

{{see also|Peripatric speciation#Evidence}}

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| image1 = Areas of Endemism (South America).png

| caption1 = South America's areas of endemism; separated largely by major rivers.

| image2 = Area Cladogram of Charis butterflies (Horizontal).png

| caption2 = A cladogram of species in the Charis cleonus group superimposed over a map of South America showing the biogeographic ranges or each species

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As allopatric speciation is widely accepted as a common mode of speciation, the scientific literature is abundant with studies documenting its existence. The biologist Ernst Mayr was the first to summarize the contemporary literature of the time in 1942 and 1963.{{rp|91}} Many of the examples he set forth remain conclusive; however, modern research supports geographic speciation with molecular phylogenetics{{Citation |title=Detecting the Geographical Pattern of Speciation from Species-Level Phylogenies | author=Timothy G. Barraclough| author2=Alfried P. Vogler | journal=American Naturalist | year=2000 | volume=155 | issue=4 | pages=419–434 | doi= 10.2307/3078926| jstor=3078926| pmid=10753072}}—adding a level of robustness unavailable to early researchers.{{rp|91}} The most recent thorough treatment of allopatric speciation (and speciation research in general) is Jerry Coyne and H. Allen Orr's 2004 publication Speciation. They list six mainstream arguments that lend support to the concept of vicariant speciation:

• Closely related species pairs, more often than not, reside in geographic ranges adjacent to one another, separated by a geographic or climatic barrier.
• Young species pairs (or sister species) often occur in allopatry, even without a known barrier.
• In occurrences where several pairs of related species share a range, they are distributed in abutting patterns, with borders exhibiting zones of hybridization.
• In regions where geographic isolation is doubtful, species do not exhibit sister pairs.
• Correlation of genetic differences between an array of distantly related species that correspond to known current or historical geographic barriers.
• Measures of reproductive isolation increase with the greater geographic distance of separation between two species pairs. (This has been often referred to as reproductive isolation by distance.)

{{See also|Insular biogeography|Elevational diversity gradient}}

Allopatric speciation has resulted in many of the biogeographic and biodiversity patterns found on Earth: on islands,{{Citation | title=Island Biogeography: Ecology, Evolution, and Conservation | edition=2 | author=Robert J. Whittaker | author2=José María Fernández-Palacios | date=2007 | publisher=Oxford University Press }} continents,{{Citation |title=Large-scale processes and the Asian bias in species diversity of temperate plants | author=Hong Qian | author2=Robert E. Ricklefs | journal=Nature | year=2000 | volume=407 | issue= 6801| pages=180–182 | doi= 10.1038/35025052| pmid=11001054 | bibcode=2000Natur.407..180Q | s2cid=4416820 }} and even among mountains.{{Citation |title=Topography-driven isolation, speciation and a global increase of endemism with elevation | author=Manuel J. Steinbauer| author2=Richard Field| author3=John-Arvid Grytnes|author4=Panayiotis Trigas|author5=Claudine Ah-Peng|author6=Fabio Attorre|author7=H. John B. Birks|author8=Paulo A. V. Borges|author9=Pedro Cardoso|author10= Chang-Hung Chou|author11=Michele De Sanctis| author12=Miguel M. de Sequeira| author13=Maria C. Duarte| author14=Rui B. Elias| author15=José María Fernández-Palacios| author16=Rosalina Gabriel| author17=Roy E. Gereau| author18=Rosemary G. Gillespie| author19=Josef Greimler| author20=David E. V. Harter| author21=Tsurng-Juhn Huang| author22=Severin D. H. Irl| author23=Daniel Jeanmonod| author24=Anke Jentsch| author25=Alistair S. Jump| author26=Christoph Kueffer| author27=Sandra Nogué| author28=Rüdiger Otto| author29=Jonathan Price| author30=Maria M. Romeiras| author31=Dominique Strasberg| author32=Tod Stuessy| author33=Jens-Christian Svenning| author34=Ole R. Vetaas| author35=Carl Beierkuhnlein | journal=Global Ecology and Biogeography | year=2016 | volume=25 | issue= 9| pages=1097–1107 | doi=10.1111/geb.12469 | bibcode=2016GloEB..25.1097S| hdl=1893/23221| url=http://eprints.nottingham.ac.uk/34319/1/Submitted%20version%20R3%20March%2016.pdf| hdl-access=free}}

Islands are often home to species endemics—existing only on an island and nowhere else in the world—with nearly all taxa residing on isolated islands sharing common ancestry with a species on the nearest continent.{{Citation | title=Speciation in Birds | author=Trevor Price | date=2008 | pages=1–64 | publisher=Roberts and Company Publishers | isbn=978-0-9747077-8-5 }} Not without challenge, there is typically a correlation between island endemics and diversity;{{Citation |title=Speciation and Endemism under the Model of Island Biogeography | author=Xiao-Yong Chen | author2=Fangliang He | s2cid=24127933 | journal=Ecology | year=2009 | volume=90 | issue=1 | pages=39–45 | doi= 10.1890/08-1520.1| pmid=19294911 | bibcode=2009Ecol...90...39C }} that is, that the greater the diversity (species richness) of an island, the greater the increase in endemism.{{Citation |title=Ecology: Is speciation driven by species diversity? | author=Carlos Daniel Cadena| author2=Robert E. Ricklefs| author3= Iván Jiménez| author4=Eldredge Bermingham | journal=Nature | year=2005 | volume=438 | issue= 7064| pages= E1–E2| doi=10.1038/nature04308 | pmid=16267504| bibcode=2005Natur.438E...1C| s2cid=4418564}} Increased diversity effectively drives speciation.{{Citation |title=Species diversity can drive speciation | author=Brent C. Emerson | author2=Niclas Kolm | journal=Nature | year=2005 | volume=434 | issue= 7036| pages=1015–1017 | doi=10.1038/nature03450 | pmid=15846345 | bibcode=2005Natur.434.1015E | s2cid=3195603 }} Furthermore, the number of endemics on an island is directly correlated with the relative isolation of the island and its area.{{Citation | title=Speciation in Birds | author=Trevor Price | date=2008 | pages=141–155 | publisher=Roberts and Company Publishers | isbn=978-0-9747077-8-5 }} In some cases, speciation on islands has occurred rapidly.{{Citation |title=Analysis of an evolutionary species±area relationship | author=Jonathan B. Losos | author2=Dolph Schluter | journal=Nature | year=2000 | volume=408 | issue=6814 | pages=847–850 | doi= 10.1038/35048558| pmid=11130721 | bibcode=2000Natur.408..847L | s2cid=4400514 }}

Dispersal and in situ speciation are the agents that explain the origins of the organisms in Hawaii.{{Citation |title=Speciation in Hawaiian Angiosperm Lineages: Cause, Consequence, and Mode | author=Jonathan P. Price | author2=Warren L. Wagner | journal=Evolution | year=2004 | volume=58 | issue=10 | pages=2185–2200 | doi= 10.1554/03-498| pmid=15562684 | s2cid=198157925 }} Various geographic modes of speciation have been studied extensively in Hawaiian biota, and in particular, angiosperms appear to have speciated predominately in allopatric and parapatric modes.

Islands are not the only geographic locations that have endemic species. South America has been studied extensively with its areas of endemism representing assemblages of allopatrically distributed species groups. Charis butterflies are a primary example, confined to specific regions corresponding to phylogenies of other species of butterflies, amphibians, birds, marsupials, primates, reptiles, and rodents.{{Citation |title=The Phylogeography of Amazonia Revisited: New Evidence from Riodinid Butterflies | author=Jason P. W. Hall | author2=Donald J. Harvey | journal=Evolution | year=2002 | volume=56 | issue=7 | pages=1489–1497 | doi= 10.1554/0014-3820(2002)056[1489:tpoarn]2.0.co;2| pmid=12206248 | s2cid=198157398 }} The pattern indicates repeated vicariant speciation events among these groups. It is thought that rivers may play a role as the geographic barriers to Charis,{{rp|97}} not unlike the river barrier hypothesis used to explain the high rates of diversity in the Amazon basin—though this hypothesis has been disputed.{{Citation |title=Most species are not limited by an Amazonian river postulated to be a border between endemism areas |first1=Sergio |last1=Santorelli Jr |first2=William E.|last2= Magnusson|first3= Claudia P.|last3= Deus |journal=Scientific Reports |year=2018 |volume=8 |issue=2294 |pages= 2294|doi=10.1038/s41598-018-20596-7 |pmid=29396491 |pmc=5797105 |bibcode=2018NatSR...8.2294S }}{{Citation|title=Moving beyond the riverine barrier vicariant paradigm |author=Luciano N. Naka and Maria W. Pil |journal=Molecular Ecology |year=2020 |volume=29 |issue=12 |pages=2129–2132 |doi=10.1111/mec.15465 |pmid=32392379 |doi-access=free |bibcode=2020MolEc..29.2129N }} Dispersal-mediated allopatric speciation is also thought to be a significant driver of diversification throughout the Neotropics.{{Citation |title=The drivers of tropical speciation | author=Brian Tilston Smith| author2=John E. McCormack| author3=Andrés M. Cuervo| author4=Michael. J. Hickerson| author5=Alexandre Aleixo| author6=Carlos Daniel Cadena| author7=Jorge Pérez-Emán| author8= Curtis W. Burney| author9=Xiaoou Xie| author10=Michael G. Harvey| author11=Brant C. Faircloth| author12=Travis C. Glenn| author13=Elizabeth P. Derryberry| author14=Jesse Prejean| author15=Samantha Fields| author16=Robb T. Brumfield | journal=Nature | year=2014 | volume=515 | issue=7527| pages=406–409 | doi=10.1038/nature13687 | pmid=25209666| bibcode=2014Natur.515..406S| s2cid=1415798| url=https://digitalcommons.lsu.edu/cgi/viewcontent.cgi?article=1519&context=biosci_pubs}}

File:Allopatric speciation caused by topography.svg

Patterns of increased endemism at higher elevations on both islands and continents have been documented on a global level. As topographical elevation increases, species become isolated from one another;{{Citation |title=Are mountain passes higher in the tropics? Janzen's hypothesis revisited | author=C. K. Ghalambor| author2= R. B. Huey| author3=P. R. Martin| author4=J. T. Tewksbury| author5=G. Wang | journal=Integrative and Comparative Biology | year=2014 | volume=46 | issue= 1| pages=5–7 | doi= 10.1093/icb/icj003| pmid=21672718| doi-access=free}} often constricted to graded zones. This isolation on "mountain top islands" creates barriers to gene flow, encouraging allopatric speciation, and generating the formation of endemic species. Mountain building (orogeny) is directly correlated with—and directly affects biodiversity.{{Citation |title=Biodiversity from mountain building | author=Carina Hoorn| author2=Volker Mosbrugger| author3=Andreas Mulch| author4=Alexandre Antonelli | journal=Nature Geoscience | year=2013 | volume=6 | issue= 3| pages=154 | doi= 10.1038/ngeo1742| url=https://pure.uva.nl/ws/files/1737504/135200_Nature_20Geoscience_20Hoorn.pdf| bibcode=2013NatGe...6..154H| doi-access=free}}{{Citation |title=The Role of Mountain Ranges in the Diversification of Birds | author=Jon Fjeldså| author2=Rauri C.K. Bowie| author3=Carsten Rahbek | s2cid=85868089| journal=Annual Review of Ecology, Evolution, and Systematics | year=2012 | volume=43 | pages=249–265 | doi=10.1146/annurev-ecolsys-102710-145113 }} The formation of the Himalayan mountains and the Qinghai–Tibetan Plateau for example have driven the speciation and diversification of numerous plants and animals{{Citation |title=Uplift-driven diversification in the Hengduan Mountains, a temperate biodiversity hotspot | author=Yaowu Xing | author2=Richard H. Ree | journal=PNAS | year=2017 | volume=114 | issue=17 | pages=3444–3451 | doi=10.1073/pnas.1616063114 | pmid=28373546 | pmc=5410793 | bibcode=2017PNAS..114E3444X | doi-access=free }} such as Lepisorus ferns;{{Citation |title=The rise of the Himalaya enforced the diversification of SE Asian ferns by altering the monsoon regimes | author=Li Wang| author2=Harald Schneider| author3=Xian-Chun Zhang| author4= Qiao-Ping Xiang | journal=BMC Plant Biology | year=2012 | volume=12 | issue=210 | pages=1–9 | doi= 10.1186/1471-2229-12-210| pmid=23140168| pmc=3508991| doi-access=free}} glyptosternoid fishes (Sisoridae);{{Citation |title=The uplift of Qinghai-Xizang (Tibet) Plateau and the vicariance speciation of glyptosternoid fishes (Siluriformes: Sisoridae) | author=Shunping He| author2=Wenxuan Cao| author3= Yiyu Chen | journal=Science in China Series C: Life Sciences | year=2001 | volume=44 | issue=6 | pages= 644–651| doi= 10.1007/bf02879359| pmid=18763106| s2cid=22432209| url=http://ir.ihb.ac.cn/handle/152342/9866}} and the Rana chensinensis species complex.{{Citation |title=Speciation in the Rana chensinensis species complex and its relationship to the uplift of the Qinghai–Tibetan Plateau | author=Wei-Wei Zhou| author2=Yang Wen| author3=Jinzhong Fu| author4=Yong-Biao Xu| author5=Jie-Qiong Jin| author6=Li Ding| author7=Mi-Sook Min| author8=Jing Che| author9=Ya-Ping Zhang | journal=Molecular Ecology | year=2012 | volume=21 | issue= 4| pages=960–973 | doi=10.1111/j.1365-294X.2011.05411.x | pmid=22221323| bibcode=2012MolEc..21..960Z| s2cid=37992915}} Uplift has also driven vicariant speciation in Macowania daisies in South Africa's Drakensberg mountains,{{Citation |title=Erosive processes after tectonic uplift stimulate vicariant and adaptive speciation: evolution in an Afrotemperate-endemic paper daisy genus | author=Joanne Bentley| author2=G Anthony Verboom| author3=Nicola G Bergh | journal=BMC Evolutionary Biology | year=2014 | volume=14 | issue=27 | pages=1–16 | doi= 10.1186/1471-2148-14-27| pmid=24524661| pmc=3927823| doi-access=free| bibcode=2014BMCEE..14...27B}} along with Dendrocincla woodcreepers in the South American Andes.{{Citation |title=Andean uplift promotes lowland speciation through vicariance and dispersal in Dendrocincla woodcreepers | author=Jason T. Weir | author2=Momoko Price | journal=Molecular Ecology | year=2011 | volume=20 | issue= 21| pages=4550–4563 | doi=10.1111/j.1365-294X.2011.05294.x | pmid=21981112 | bibcode=2011MolEc..20.4550W | s2cid=33626056 }} The Laramide orogeny during the Late Cretaceous even caused vicariant speciation and radiations of dinosaurs in North America.{{Citation |title=Mountain Building Triggered Late Cretaceous North American Megaherbivore Dinosaur Radiation | author=Terry A. Gates| author2=Albert Prieto-Márquez| author3=Lindsay E. Zanno | journal=PLOS ONE | year=2012 | volume=7 | issue=8 | page= e42135| doi=10.1371/journal.pone.0042135 | pmid=22876302| pmc=3410882| bibcode=2012PLoSO...742135G| doi-access=free}}

Adaptive radiation, like the Galapagos finches observed by Charles Darwin, is often a consequence of rapid allopatric speciation among populations. However, in the case of the finches of the Galapagos, among other island radiations such as the honeycreepers of Hawaii represent cases of limited geographic separation and were likely driven by ecological speciation.

File:Isthmus of Panama (closure) - Speciation of marine organisms (w annot).png (red circle). With the closure, North and South America became connected, allowing the exchange of species (purple). Grey arrows indicate the gradual movement of tectonic plates that resulted in the closure.]]

Geological evidence supports the final closure of the isthmus of Panama approximately 2.7 to 3.5 mya,{{Citation | title=A Multilocus Test of Simultaneous Divergence Across the Isthmus of Panama Using Snapping Shrimp in the Genus Alpheus|author=Carla Hurt| author2=Arthur Anker| author3= Nancy Knowlton | journal=Evolution| year=2008| volume=63| issue=2| pages=514–530| doi=10.1111/j.1558-5646.2008.00566.x|pmid=19154357|s2cid=11820649}} with some evidence suggesting an earlier transient bridge existing between 13 and 15 mya.{{Citation | title=Middle Miocene closure of the Central American Seaway|author1=C. Montes|author2= A. Cardona|author3=C. Jaramillo|author4= A. Pardo|author5=J. C. Silva|author6=V. Valencia|author7=C. Ayala|author8=L. C. Pérez-Angel|author9=L. A. Rodriguez-Parra|author10=V. Ramirez|author11=H. Niño | journal=Science| year=2015| volume=348| issue=6231| pages=226–229| doi=10.1126/science.aaa2815|pmid=25859042| display-authors=etal|bibcode=2015Sci...348..226M|doi-access=free}} Recent evidence increasingly points towards an older and more complex emergence of the Isthmus, with fossil and extant species dispersal (part of the American biotic interchange) occurring in three major pulses, to and from North and South America.{{Citation |title=Biological evidence supports an early and complex emergence of the Isthmus of Panama | author=Christine D. Bacon| author2=Daniele Silvestro| author3=Carlos Jaramillo|author4=Brian Tilston Smith|author5=Prosanta Chakrabarty|author6=Alexandre Antonelli | journal=PNAS | year=2015 | volume=112 | issue=9 | pages=6110–6115 | doi=10.1073/pnas.1423853112 | pmid=25918375| pmc=4434730| bibcode=2015PNAS..112.6110B| doi-access=free}} Further, the changes in terrestrial biotic distributions of both continents such as with Eciton army ants supports an earlier bridge or a series of bridges.{{Citation |title=Army ant invasions reveal phylogeographic processes across the Isthmus of Panama | author=Seàn Brady | journal=Molecular Ecology | year=2017 | volume=26 | issue=3 | pages=703–705 | doi=10.1111/mec.13981 | pmid=28177197 | doi-access=free | bibcode=2017MolEc..26..703B }}{{Citation |title=Early and dynamic colonization of Central America drives speciation in Neotropical army ants | author=Max E. Winston| author2=Daniel J. C. Kronauer| author3=Corrie S. Moreau | journal=Molecular Ecology | year=2017 | volume=26 | issue=3 | pages=859–870 | doi=10.1111/mec.13846 | pmid=27778409| doi-access=free | bibcode=2017MolEc..26..859W}} Regardless of the exact timing of the isthmus closer, biologists can study the species on the Pacific and Caribbean sides in what has been called, "one of the greatest natural experiments in evolution". Additionally, as with most geologic events, the closure was unlikely to have occurred rapidly, but instead dynamically—a gradual shallowing of sea water over millions of years.{{rp|93}}

Studies of snapping shrimp in the genus Alpheus have provided direct evidence of an allopatric speciation event,{{Citation | title=Divergence in Proteins, Mitochondrial DNA, and Reproductive Compatibility Across the Isthmus of Panama| author=Nancy Knowlton| s2cid=31875676| journal=Science| year=1993| volume=260| issue=5114| pages=1629–1632| doi=10.1126/science.8503007| pmid=8503007| bibcode=1993Sci...260.1629K}} as phylogenetic reconstructions support the relationships of 15 pairs of sister species of Alpheus, each pair divided across the isthmus and molecular clock dating supports their separation between 3 and 15 million years ago.{{Citation | title=New dates and new rates for divergence across the Isthmus of Panama|author1=Nancy Knowlton |author2=Lee A. Weigt | journal=Proc. R. Soc. Lond. B| year=1998| volume=265|issue=1412 | pages=2257–2263| doi=10.1098/rspb.1998.0568|pmc=1689526}} Recently diverged species live in shallow mangrove waters while older diverged species live in deeper water, correlating with a gradual closure of the isthmus.{{rp|93}} Support for an allopatric divergence also comes from laboratory experiments on the species pairs showing nearly complete reproductive isolation.{{rp|93}}

Similar patterns of relatedness and distribution across the Pacific and Atlantic sides have been found in other species pairs such as:H. A. Lessios. (1998). The first stage of speciation as seen in organisms separated by the Isthmus of Panama. In Endless forms: species and speciation (ed. D. Howard & S. Berlocher). Oxford University Press

• Diadema antillarum and Diadema mexicanum
• Echinometra lucunter and Echinometra vanbrunti
• Echinometra viridis and E. vanbrunti
• Bathygobius soporator and Bathygobius ramosus
• B. soporator and Bathygobius andrei
• Excirolana braziliensis and variant morphs

Ice ages have played important roles in facilitating speciation among vertebrate species.{{Citation |title=Ice Sheets Promote Speciation in Boreal Birds | author=Jason T. Weir | author2=Dolph Schluter | journal=Proceedings: Biological Sciences | year=2004 | volume=271 | issue=1551 | pages=1881–1887 | doi= 10.1098/rspb.2004.2803| pmid=15347509 | pmc=1691815 }} This concept of refugia has been applied to numerous groups of species and their biogeographic distributions.{{rp|97}}

Glaciation and subsequent retreat caused speciation in many boreal forest birds, such as with North American sapsuckers (Yellow-bellied, Red-naped, and Red-breasted); the warblers in the genus Setophaga (S. townsendii, S. occidentalis, and S. virens), Oreothlypis (O. virginiae, O. ridgwayi, and O. ruficapilla), and Oporornis (O. tolmiei and O. philadelphia now classified in the genus Geothlypis); Fox sparrows (sub species P. (i.) unalaschensis, P. (i.) megarhyncha, and P. (i.) schistacea); Vireo (V. plumbeus, V. cassinii, and V. solitarius); tyrant flycatchers (E. occidentalis and E. difficilis); chickadees (P. rufescens and P. hudsonicus); and thrushes (C. bicknelli and C. minimus).

As a special case of allopatric speciation, peripatric speciation is often invoked for instances of isolation in glaciation refugia as small populations become isolated due to habitat fragmentation such as with North American red (Picea rubens) and black (Picea mariana) spruce{{Citation |title=New evidence from mitochondrial DNA of a progenitor-derivative species relationship between black and red spruce (Pinaceae) | author=Juan P. Jaramillo-Correa | author2=Jean Bousquet | journal=American Journal of Botany | year=2003 | volume=90 | issue=12 | pages=1801–1806 | doi= 10.3732/ajb.90.12.1801| pmid=21653356 }} or the prairie dogs Cynomys mexicanus and C. ludovicianus.{{Citation |title=Peripatric speciation of an endemic species driven by Pleistocene climate change: The case of the Mexican prairie dog (Cynomys mexicanus) | author=Gabriela Castellanos-Morales| author2=Niza Gámez| author3=Reyna A. Castillo-Gámez| author4=Luis E. Eguiarte| journal=Molecular Phylogenetics and Evolution | year=2016 | volume=94 | issue=Pt A| pages=171–181 | doi=10.1016/j.ympev.2015.08.027 | pmid=26343460| bibcode=2016MolPE..94..171C}}

File:Pan (genus) distribution map.png (Pan paniscus). The blue shading indicates the range of the Common chimpanzee (Pan troglodytes). This is an example of allopatric speciation because they are divided by a natural barrier (the Congo River) and have no habitat in common. Other Pan subspecies are shown as well.]]

Numerous species pairs or species groups show abutting distribution patterns, that is, reside in geographically distinct regions next to each other. They often share borders, many of which contain hybrid zones. Some examples of abutting species and superspecies (an informal rank referring to a complex of closely related allopatrically distributed species, also called allospecies{{cite journal |author=Amadon D. |year=1966 |title=The superspecies concept |journal=Systematic Biology |volume=15 |issue=3 |pages=245–249 |doi=10.2307/sysbio/15.3.245}}) include:

• Western, Chihuahuan, and Eastern meadowlarks in North America reside in dry western and wet eastern geographic regions with rare occurrences of hybridization, most of which results in infertile offspring.
• Monarch flycatchers endemic to the Solomon Islands; a complex of several species and subspecies (Bougainville, white-capped, and chestnut-bellied monarchs and their related subspecies).
• North American sapsuckers and members of the genus Setophaga (the hermit warbler, black-throated green warbler, and Townsend's warbler).
• Sixty-six subspecies in the genus Pachycephala residing on the Melanesian islands.{{Citation | title=The Birds of Northern Melanesia | author=Ernst Mayr | author2=Jared Diamond | date=2001 | pages=143 | publisher=Oxford University Press | isbn=978-0-19-514170-2 }}
• Bonobos and chimpanzees.
• Climacteris tree creeper birds in Australia.{{Citation | title=Animal Species and Evolution | author=Ernst Mayr | date=1963 | pages=488–515 | publisher=Harvard University Press | isbn=978-0674037502 }}
• Birds-of-paradise in the mountains of New Guinea (genus Astrapia).
• Red-shafted and yellow-shafted flickers; black-headed grosbeaks and rose-breasted grosbeaks; Baltimore orioles and Bullock's orioles; and the lazuli and indigo buntings.Remington C.L. (1968) Suture-Zones of Hybrid Interaction Between Recently Joined Biotas. In: Dobzhansky T., Hecht M.K., Steere W.C. (eds) Evolutionary Biology. Springer, Boston, MA All of these species pairs connect at zones of hybridization that correspond with major geographic barriers.{{rp|97–99}}
• Dugesia flatworms in Europe, Asia, and the Mediterranean regions.
• Dichromatic toucanets of the genus Selenidera may be a superspecies that arose by the refugia hypothesis in the Amazon basin.{{Citation|title=Speciation in Amazonian Forest Birds |author=Jürgen Haffer |journal=Science |year=1969 |volume=165 |issue= 3889|pages=131–137 |doi= 10.1126/science.165.3889.131|pmid=17834730 |bibcode=1969Sci...165..131H }}

In birds, some areas are prone to high rates of superspecies formation such as the 105 superspecies in Melanesia, comprising 66 percent of all bird species in the region.{{Citation | title=The Birds of Northern Melanesia | author=Ernst Mayr | author2=Jared Diamond | date=2001 | pages=127 | publisher=Oxford University Press | isbn=978-0-19-514170-2 }} Patagonia is home to 17 superspecies of forest birds,{{Citation |title=Forest Birds of Patagonia: Ecological Geography, Speciation, Endemism, and Faunal History | author=François Vuilleumier | journal=Ornithological Monographs | year=1985 | issue=36 | pages=255–304 | doi=10.2307/40168287 | jstor=40168287 }} while North America has 127 superspecies of both land and freshwater birds.Mayr, E., & Short, L. L. (1970). Species taxa of North American birds: a contribution to comparative systematics. Sub-Saharan Africa has 486 passerine birds grouped into 169 superspecies.Hall, B. P., & Moreau, R. E. (1970). An atlas of speciation in African passerine birds. Trustees of the British museum (Natural history). Australia has numerous bird superspecies as well, with 34 percent of all bird species grouped into superspecies.

{{see also|Laboratory experiments of speciation}}

File:Drosophila speciation experiment.svg were raised on harsh maltose and starch mediums respectively. The experiment was replicated with 8 populations; 4 with maltose and 4 with starch. Differences in adaptations were found for each population corresponding to the different mediums.{{Citation |title=Population genetics of Drosophila amylase. IV. Selection in laboratory populations maintained on different carbohydrates | author=J. R. Powell | author2= M. Andjelkovic | journal=Genetics | year=1983 | volume=103 | issue= 4| pages=675–689 | doi=10.1093/genetics/103.4.675 | pmid=6189764 | pmc=1202048 }} Later investigation found that the populations evolved behavioral isolation as a pleiotropic by-product from this adaptive divergence. This form of pre-zygotic isolation is a prerequisite for speciation to occur.]]

Experiments on allopatric speciation are often complex and do not simply divide a species population into two. This is due to a host of defining parameters: measuring reproductive isolation, sample sizes (the number of matings conducted in reproductive isolation tests), bottlenecks, length of experiments, number of generations allowed,{{Citation |title=Laboratory environments are not conducive for allopatric speciation | author=Ann-Britt Florin | author2=Anders Ödeen | journal=Journal of Evolutionary Biology | year=2002 | volume=15 | pages=10–19 | doi=10.1046/j.1420-9101.2002.00356.x| s2cid=85410953 | doi-access=free }} or insufficient genetic diversity.{{Citation |title=Speciation by Natural and Sexual Selection: Models and Experiments | author=Mark Kirkpatrick | author2=Virginie Ravigné | journal=The American Naturalist | year=2002 | volume=159 | issue=3 | pages=S22 | doi=10.2307/3078919| jstor=3078919 }} Various isolation indices have been developed to measure reproductive isolation (and are often employed in laboratory speciation studies) such as here (index $Y${{Citation | title=Discrete Multivariate Analysis: Theory and Practice | author=Bishop, Y. M.|author1-link= Yvonne Bishop |author2=Fienberg, S. E.| author3=Holland, P. W. | date=1975 | publisher=MIT Press: Cambridge, MA. }} and index $I${{Citation |title=Sexual isolation studies in the species complex Drosophila virilis | author=H. D. Stalker | journal=Genetics | year=1942 | volume=27 | issue= 2| pages=238–257 | doi=10.1093/genetics/27.2.238 | pmid=17247038 | pmc=1209156 }}):

$Y=\; \{\backslash sqrt\{(AD/BC)\}-1\; \backslash over\; \backslash sqrt\{(AD/BC)+1\}\}$

$I=\; \{A+D-B-C\; \backslash over\; A+D+B+C\}$

Here, $A$ and $D$ represent the number of matings in heterogameticity where $B$ and $C$ represent homogametic matings. $A$ and $B$ is one population and $D$ and $C$ is the second population. A negative value of $Y$ denotes negative assortive mating, a positive value denotes positive assortive mating (i. e. expressing reproductive isolation), and a null value (of zero) means the populations are experiencing random mating.

The experimental evidence has solidly established the fact that reproductive isolation evolves as a by-product of selection.{{rp|90}} Reproductive isolation has been shown to arise from pleiotropy (i.e. indirect selection acting on genes that code for more than one trait)—what has been referred to as genetic hitchhiking. Limitations and controversies exist relating to whether laboratory experiments can accurately reflect the long-scale process of allopatric speciation that occurs in nature. Experiments often fall beneath 100 generations, far less than expected, as rates of speciation in nature are thought to be much larger.{{rp|87}} Furthermore, rates specifically concerning the evolution of reproductive isolation in Drosophila are significantly higher than what is practiced in laboratory settings.{{Citation |title="Patterns of Speciation in Drosophila" Revisited | author=Jerry A. Coyne |author1-link=Jerry Coyne | author2=H. Allen Orr |author2-link=H. Allen Orr | journal=Evolution | year=1997 | volume=51 | issue=1 | pages=295–303 | doi= 10.1111/j.1558-5646.1997.tb02412.x| pmid=28568795 | s2cid=40390753 | doi-access=free }} Using index Y presented previously, a survey of 25 allopatric speciation experiments (included in the table below) found that reproductive isolation was not as strong as typically maintained and that laboratory environments have not been well-suited for modeling allopatric speciation. Nevertheless, numerous experiments have shown pre-zygotic and post-zygotic isolation in vicariance, some in less than 100 generations.{{rp|87}}

Below is a non-exhaustive table of the laboratory experiments conducted on allopatric speciation. The first column indicates the species used in the referenced study, where the "Trait" column refers to the specific characteristic selected for or against in that species. The "Generations" column refers to the number of generations in each experiment performed. If more than one experiment was formed generations are separated by semicolons or dashes (given as a range). Some studies provide a duration in which the experiment was conducted. The "Selection type" column indicates if the study modeled vicariant or peripatric speciation (this may not be explicitly). Direct selection refers to selection imposed to promote reproductive isolation whereas indirect selection implies isolation occurring as a pleiotropic byproduct of natural selection; whereas divergent selection implies deliberate selection of each allopatric population in opposite directions (e.g. one line with more bristles and the other line with less). Some studies performed experiments modeling or controlling for genetic drift. Reproductive isolation occurred pre-zygotically, post-zygotically, both, or not at all. It is important to note that many of the studies conducted contain multiple experiments within—a resolution of which this table does not reflect.

{{main|History of speciation}}

Early speciation research typically reflected geographic distributions and were thus termed geographic, semi-geographic, and non-geographic. Geographic speciation corresponds to today's usage of the term allopatric speciation, and in 1868, Moritz Wagner was the first to propose the concept{{Citation |title=The Origin of Species Through Isolation | author=David Starr Jordan | journal=Science | year=1905 | volume=22 | issue=566 | pages=545–562 | doi=10.1126/science.22.566.545| pmid=17832412 | bibcode=1905Sci....22..545S | url=https://zenodo.org/record/1447948 }} of which he used the term Separationstheorie.{{Citation |title=Why was Darwin's view of species rejected by twentieth century biologists? | author=James Mallet | journal=Biology & Philosophy | year=2010 | volume=25 | issue=4 | pages=497–527 | doi=10.1007/s10539-010-9213-7 | s2cid=38621736 | url=http://nrs.harvard.edu/urn-3:HUL.InstRepos:37971068 }} His idea was later interpreted by Ernst Mayr as a form of founder effect speciation as it focused primarily on small geographically isolated populations.

Edward Bagnall Poulton, an evolutionary biologist and a strong proponent of the importance of natural selection, highlighted the role of geographic isolation in promoting speciation, in the process coining the term "sympatric speciation" in 1903.Mayr, Ernst 1942. Systematics and origin of species. Columbia University Press, New York. p148

Controversy exists as to whether Charles Darwin recognized a true geographical-based model of speciation in his publication of the Origin of Species. In chapter 11, "Geographical Distribution", Darwin discusses geographic barriers to migration, stating for example that "barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions [of the world]".{{cite book |last1=Darwin |first1=Charles |title=On the Origin of Species |date=1859 |publisher=Murray |page=347 |url=http://darwin-online.org.uk/content/frameset?itemID=F373&viewtype=text&pageseq=1}} F. J. Sulloway contends that Darwin's position on speciation was "misleading" at the least{{cite journal | author = Sulloway FJ | year = 1979 | title = Geographic isolation in Darwin's thinking: the vicissitudes of a crucial idea | journal = Studies in the History of Biology | volume = 3 | pages = 23–65 | pmid = 11610987 }} and may have later misinformed Wagner and David Starr Jordan into believing that Darwin viewed sympatric speciation as the most important mode of speciation.{{rp|83}} Nevertheless, Darwin never fully accepted Wagner's concept of geographical speciation.

Image:Ernst Mayr PLoS.jpg

David Starr Jordan played a significant role in promoting allopatric speciation in the early 20th century, providing a wealth of evidence from nature to support the theory.{{rp|86}}{{Citation |title=The Law of Geminate Species | author=David Starr Jordan | journal=American Naturalist | year=1908 | volume=42 | issue=494 | pages=73–80 | doi=10.1086/278905| doi-access=free | bibcode=1908ANat...42...73J }} Much later, the biologist Ernst Mayr was the first to encapsulate the then contemporary literature in his 1942 publication Systematics and the Origin of Species, from the Viewpoint of a Zoologist and in his subsequent 1963 publication Animal Species and Evolution. Like Jordan's works, they relied on direct observations of nature, documenting the occurrence of allopatric speciation, of which is widely accepted today.{{rp|83–84}} Prior to this research, Theodosius Dobzhansky published Genetics and the Origin of Species in 1937 where he formulated the genetic framework for how speciation could occur.{{rp|2}}

Other scientists noted the existence of allopatrically distributed pairs of species in nature such as Joel Asaph Allen (who coined the term "Jordan's Law", whereby closely related, geographically isolated species are often found divided by a physical barrier{{rp|91}}) and Robert Greenleaf Leavitt;{{Citation |title=Mutations and the Geographic Distribution of Nearly Related Species in Plants and Animals | author=Joel Asaph Allen | journal=American Naturalist | year=1907 | volume=41 | issue=490 | pages=653–655 | doi=10.1086/278852| doi-access=free | bibcode=1907ANat...41..653A }} however, it is thought that Wagner, Karl Jordan, and David Starr Jordan played a large role in the formation of allopatric speciation as an evolutionary concept;{{Citation | title=The Growth of Biological Thought | author=Ernst Mayr | date=1982 | pages=561–566 | publisher=Harvard University Press | isbn=978-0674364462 }} where Mayr and Dobzhansky contributed to the formation of the modern evolutionary synthesis.

The late 20th century saw the development of mathematical models of allopatric speciation, leading to the clear theoretical plausibility that geographic isolation can result in the reproductive isolation of two populations.{{rp|87}}

Since the 1940s, allopatric speciation has been accepted.{{Citation |title=The speciation revolution | author=James Mallet | s2cid=36627140 | journal=Journal of Evolutionary Biology | year=2001 | volume=14 | issue= 6| pages=887–888 | doi= 10.1046/j.1420-9101.2001.00342.x| doi-access=free }} Today, it is widely regarded as the most common form of speciation taking place in nature.{{rp|84}} However, this is not without controversy, as both parapatric and sympatric speciation are both considered tenable modes of speciation that occur in nature. Some researchers even consider there to be a bias in reporting of positive allopatric speciation events, and in one study reviewing 73 speciation papers published in 2009, only 30 percent that suggested allopatric speciation as the primary explanation for the patterns observed considered other modes of speciation as possible.

Contemporary research relies largely on multiple lines of evidence to determine the mode of a speciation event; that is, determining patterns of geographic distribution in conjunction with phylogenetic relatedness based on molecular techniques.{{rp|123–124}} This method was effectively introduced by John D. Lynch in 1986 and numerous researchers have employed it and similar methods, yielding enlightening results.{{Citation |title=Integrating Phylogenetics and Environmental Niche Models to Explore Speciation Mechanisms in Dendrobatid Frogs | author=Catherine H. Graham| author2=Santiago R. Ron Juan C. Santos| author3=Christopher J. Schneider| author4=Craig Moritz | journal=Evolution | year=2004 | volume=58 | issue=8 | pages=1781–1793 | doi=10.1554/03-274 | pmid=15446430| s2cid=198157565}} Correlation of geographic distribution with phylogenetic data also spawned a sub-field of biogeography called vicariance biogeography{{rp|92}} developed by Joel Cracraft, James Brown, Mark V. Lomolino, among other biologists specializing in ecology and biogeography. Similarly, full analytical approaches have been proposed and applied to determine which speciation mode a species underwent in the past using various approaches or combinations thereof: species-level phylogenies, range overlaps, symmetry in range sizes between sister species pairs, and species movements within geographic ranges. Molecular clock dating methods are also often employed to accurately gauge divergence times that reflect the fossil or geological record{{rp|93}} (such as with the snapping shrimp separated by the closure of the Isthmus of Panama or speciation events within the genus Cyclamen{{Citation |title=Cyclamen: time, sea and speciation biogeography using a temporally calibrated phylogeny | author=C. Yesson| author2=N.H. Toomey|author3=A. Culham | journal=Journal of Biogeography | year=2009 | volume=36 | issue=7| pages=1234–1252 | doi= 10.1111/j.1365-2699.2008.01971.x| bibcode=2009JBiog..36.1234Y| s2cid=85573501}}). Other techniques used today have employed measures of gene flow between populations, ecological niche modelling (such as in the case of the Myrtle and Audubon's warblers{{Citation |title=The Geography of Speciation: Case Studies from Birds | author=Robert M. Zink | journal=Evolution: Education and Outreach | year=2012 | volume=5 | issue=4 | pages=541–546 | doi=10.1007/s12052-012-0411-4 | doi-access=free }} or the environmentally-mediated speciation taking place among dendrobatid frogs in Ecuador), and statistical testing of monophyletic groups.{{Citation |title=Modes of speciation in birds: a test of Lynch's method | author=R. T. Chesser | author2=R. M. Zink | journal=Evolution | year=1994 | volume=48 | issue= 2| pages=490–497 | doi= 10.2307/2410107| pmid=28568302 | jstor=2410107 }} Biotechnological advances have allowed for large scale, multi-locus genome comparisons (such as with the possible allopatric speciation event that occurred between ancestral humans and chimpanzees{{Citation |title=Patterns of autosomal divergence between the human and chimpanzee genomes support an allopatric model of speciation | author=Matthew T. Webster | journal=Gene | year=2009 | volume=443 | issue= 1–2| pages=70–75 | doi=10.1016/j.gene.2009.05.006 | pmid=19463924 }}), linking species' evolutionary history with ecology and clarifying phylogenetic patterns.{{Citation |title=Assessing models of speciation under different biogeographic scenarios; an empirical study using multi-locus and RNA-seq analyses | author=Taylor Edwards| author2=Marc Tollis| author3=PingHsun Hsieh| author4=Ryan N. Gutenkunst| author5=Zhen Liu|author6=Kenro Kusumi|author7= Melanie Culver|author8=Robert W. Murphy | journal=Ecology and Evolution | year=2016 | volume=6 | issue=2 | pages=379–396 | doi=10.1002/ece3.1865 | pmid=26843925| pmc=4729248| bibcode=2016EcoEv...6..379E}}

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Mathematical models of reproductive isolation

• {{Citation |title=The evolution of postzygotic isolation: Accumulating Dobzhansky-Muller incompatibilities | author=H. Allen Orr | author2=Michael Turelli | journal=Evolution | year=2001 | volume=55 | issue=6 | pages=1085–1094 | doi=10.1554/0014-3820(2001)055[1085:teopia]2.0.co;2| pmid=11475044 | arxiv=0904.3308 | s2cid=198153495 }}
• {{Citation |title=Waiting for Speciation: The Effect of Population Subdivision on the Time to Speciation | author=H. Allen Orr | author2=Lynne H. Orr | journal=Evolution | year=1996 | volume=50 | issue=5 | pages=1742–1749 | doi=10.2307/2410732| pmid=28565607 | jstor=2410732 }}
• {{Citation |title=The Population Genetics of Speciation: The Evolution of Hybrid Incompatibilities | author=H. Allen Orr|author-link=H. Allen Orr | journal=Genetics | year=1995 | volume=139 | issue=4| pages=1805–1813 | doi=10.1093/genetics/139.4.1805| pmid=7789779| pmc=1206504}}
• {{Citation |title=Models of Evolution of Reproductive Isolation | author=Masatoshi Nei| author2= Takeo Maruyama|author3=Chung-i Wu | journal=Genetics | year=1983 | volume=103 | issue=3| pages=557–579 | doi=10.1093/genetics/103.3.557| pmid=6840540| pmc=1202040}}
• {{Citation |title=Mathematical Models of Speciation and Genetic Distance | author=Masatoshi Nei | journal=Population Genetics and Ecology | year=1976 | pages=723–766 }}

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Category:Biogeography

Category:Ecology

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Category:Speciation