Editing Sympatric speciation

{{Short description|Evolution of a new species from an ancestor in the same location}}
[[File:Sympatric Speciation Schematic.svg|thumb|upright=1.8|In sympatric speciation, reproductive isolation evolves within a population without the aid of geographic barriers.]]
{{Evolutionary biology}}

'''Sympatric speciation''' is the [[evolution]] of a new species from a surviving [[Common descent|ancestral species]] while both continue to inhabit the same geographic region. In evolutionary biology and [[biogeography]], ''sympatric'' and ''[[sympatry]]'' are terms referring to organisms whose [[Range (biology)|ranges]] overlap so that they occur together at least in some places. If these organisms are closely related (e.g. [[sister species]]), such a distribution may be the result of sympatric [[speciation]]. [[Etymological]]ly, sympatry is derived {{ety|el|''[[wikt:συν-|συν]]'' (sun-)|together||''[[wikt:πατρίς|πατρίς]]'' (patrís)|fatherland}}.<ref name=Poulton1903/> The term was coined by [[Edward Bagnall Poulton]] in 1904, who explains the derivation.<ref name=Poulton1903>{{cite journal | last1 = Poulton | first1 = E. B. | author-link = Edward Bagnall Poulton | year = 1904 | title = What is a species? | journal = Proceedings of the Entomological Society of London | volume = 1903 | pages = 77–116 }}</ref>

Sympatric speciation is one of three traditional geographic modes of speciation.<ref name="Futuyma 2001">Futuyma, D. J. 2001. ''Evolution'' (2nd edition). Sinauer Associates, Inc.{{page needed|date=November 2013}}</ref><ref name="Fitzpatrick et al. 2008">{{cite journal |doi=10.1111/j.1420-9101.2008.01611.x |title=What, if anything, is sympatric speciation? |year=2008 |last1=Fitzpatrick |first1=B. M. |last2=Fordyce |first2=J. A. |last3=Gavrilets |first3=S. |journal=Journal of Evolutionary Biology |volume=21 |issue=6 |pages=1452–9 |pmid=18823452|s2cid=8721116 |doi-access=free }}</ref> [[Allopatric speciation]] is the evolution of species caused by the geographic isolation of two or more populations of a species. In this case, divergence is facilitated by the absence of gene flow. [[Parapatric speciation]] is the evolution of geographically adjacent populations into distinct species. In this case, divergence occurs despite limited interbreeding where the two diverging groups come into contact. In sympatric speciation, there is no geographic constraint to interbreeding. These categories are special cases of a continuum from zero (sympatric) to complete (allopatric) spatial segregation of diverging groups.<ref name="Fitzpatrick et al. 2008" />

In multicellular [[Eukaryotes|eukaryotic]] organisms, sympatric speciation is a plausible process that is known to occur, but the frequency with which it occurs is not known.<ref name="Bolnick and Fitzpatrick 2007">{{cite journal |doi=10.1146/annurev.ecolsys.38.091206.095804 |title=Sympatric Speciation: Models and Empirical Evidence |year=2007 |last1=Bolnick |first1=Daniel I. |last2=Fitzpatrick |first2=Benjamin M. |journal=Annual Review of Ecology, Evolution, and Systematics |volume=38 |pages=459–87}}</ref>
In bacteria, however, the analogous process (defined as "the origin of new bacterial species that occupy definable [[ecological niche]]s") might be more common because bacteria are less constrained by the homogenizing effects of sexual reproduction and are prone to comparatively dramatic and rapid genetic change through [[horizontal gene transfer]].<ref name="King, Stansfield, Mulligan">{{cite book | last =King, Stansfield, Mulligan | title =Dictionary of Genetics | publisher =Oxford University Press| edition = 7th | year =2006 }}{{page needed|date=November 2013}}</ref>

==Evidence==
Sympatric speciation events are quite common in plants, which are prone to acquiring multiple [[Homologous chromosome|homologous]] sets of [[chromosomes]], resulting in [[polyploidy]]. The polyploid offspring occupy the same environment as the parent plants (hence sympatry), but are reproductively isolated.

A number of models have been proposed for alternative modes of sympatric speciation. The most popular, which invokes the [[disruptive selection]] model, was first put forward by [[John Maynard Smith]] in 1966.<ref>{{cite journal|title= Sympatric Speciation |author= John Maynard Smith |journal= American Naturalist |volume=100 | issue = 916 |year=1966 |pages=637–50 |jstor=2459301 |doi=10.1086/282457|bibcode= 1966ANat..100..637S |s2cid= 222329634 }}</ref> Maynard Smith suggested that [[homozygote|homozygous]] individuals may, under particular environmental conditions, have a greater fitness than those with [[allele]]s [[heterozygote|heterozygous]] for a certain trait. Under the mechanism of [[natural selection]], therefore, homozygosity would be favoured over heterozygosity, eventually leading to speciation. Sympatric divergence could also result from the [[sexual conflict]].<ref>[[Thierry Lodé]] ''La guerre des sexes chez les animaux'' Eds O Jacob, Paris, 2006{{page needed|date=November 2013}}</ref>

Disruption may also occur in multiple-gene traits. The [[Darwin's finches|medium ground finch]] (''Geospiza fortis'') is showing gene pool divergence in a population on [[Santa Cruz Island (Galapagos)|Santa Cruz Island]]. Beak morphology conforms to two different size ideals, while intermediate individuals are selected against. Some characteristics (termed [[magic trait]]s) such as beak morphology may drive speciation because they also affect mating signals. In this case, different beak [[phenotype]]s may result in different [[bird call]]s, providing a barrier to exchange between the gene pools.<ref>{{cite journal |doi=10.1098/rspb.2007.0224 |title=Reproductive isolation of sympatric morphs in a population of Darwin's finches |year=2007 |last1=Huber |first1=S. K |last2=Leon |first2=L. F. D. |last3=Hendry |first3=A. P |last4=Bermingham |first4=E. |last5=Podos |first5=J. |journal=Proceedings of the Royal Society B: Biological Sciences |volume=274 |issue=1619 |pages=1709–14 |pmid=17504742 |pmc=2493575}}</ref>

A somewhat analogous system has been reported in horseshoe bats, in which echolocation call frequency appears to be a magic trait. In these bats, the constant frequency component of the call not only determines prey size but may also function in aspects of social communication. Work from one species, the [[large-eared horseshoe bat]] (''Rhinolophus philippinensis''), shows that abrupt changes in call frequency among sympatric morphs is correlated with reproductive isolation.<ref>{{cite journal |doi=10.1038/nature02487 |title=Harmonic-hopping in Wallacea's bats |year=2004 |last1=Kingston |first1=Tigga |last2=Rossiter |first2=Stephen J. |journal=Nature |volume=429 |issue=6992 |pages=654–7 |pmid=15190351|bibcode=2004Natur.429..654K |s2cid=4422561 }}</ref> A further well-studied circumstance of sympatric speciation is when insects feed on more than one species of [[Host (biology)|host plant]]. In this case insects become specialized as they struggle to overcome the various plants' [[Antipredator adaptation|defense mechanisms]]. (Drès and Mallet, 2002)<ref>Begon, Townsend, Harper: ''Ecology – From individuals to ecosystems'', 4th ed., p.10</ref>

''Rhagoletis pomonella'', the [[apple maggot]], may be currently undergoing sympatric or, more precisely, heteropatric (see [[heteropatry]]) speciation. The apple feeding race of this species appears to have spontaneously emerged from the [[Crataegus|hawthorn]] feeding race in the 1800–1850 AD time frame, after apples were first introduced into [[North America]]. The apple feeding race does not now normally feed on [[Crataegus|hawthorn]]s, and the hawthorn feeding race does not now normally feed on apples. This may be an early step towards the emergence of a new species.<ref>{{cite journal |last1=Feder |first1=Jeffrey L. |last2=Chilcote |first2=Charles A. |last3=Bush |first3=Guy L. |year=1988 |title=Genetic differentiation between sympatric host races of the apple maggot fly ''Rhagoletis pomonella'' |url=https://www.nature.com/articles/336061a0 |journal=Nature |volume=336 |issue=6194 |pages=61–64 |bibcode=1988Natur.336...61F |doi=10.1038/336061a0 |s2cid=4318103 |hdl-access=free |hdl=2027.42/62806}}</ref><ref>{{cite journal |last1=McPheron |first1=Bruce A. |last2=Smith |first2=D. Courtney |last3=Berlocher |first3=Stewart H. |year=1988 |title=Genetic differences between host races of ''Rhagoletis pomonella'' |journal=Nature |volume=336 |issue=6194 |pages=64–66 |bibcode=1988Natur.336...64M |doi=10.1038/336064a0 |s2cid=4264026}}</ref><ref>{{cite journal |last1=Smith |first1=D. Courtney |year=1988 |title=Heritable divergence of ''Rhagoletis pomonella'' host races by seasonal asynchrony |journal=Nature |volume=336 |issue=6194 |pages=66–67 |bibcode=1988Natur.336...66S |doi=10.1038/336066a0 |s2cid=4371982}}</ref> 

Some parasitic ants may have evolved via sympatric speciation.<ref>{{cite journal|last1=Rabeling|first1=Christian|last2=Schultz|first2=Ted R.|last3=Pierce|first3=Naomi E.|last4=Bacci Jr|first4=Maurício|title=A Social Parasite Evolved Reproductive Isolation from Its Fungus-Growing Ant Host in Sympatry|journal=[[Current Biology]]|date=August 2014|doi=10.1016/j.cub.2014.07.048|pmid=25155509|volume=24|issue=17|pages=2047–2052|doi-access=free|bibcode=2014CBio...24.2047R }}</ref> Isolated and relatively homogeneous habitats such as crater lakes and islands are among the best geographical settings in which to demonstrate sympatric speciation. For example, [[Amphilophus|Nicaragua crater lake cichlid fishes]] include nine described species and dozens of undescribed species that have evolved by sympatric speciation.<ref>{{cite journal |doi=10.1016/j.ympev.2010.05.015 |title=Not a simple case – A first comprehensive phylogenetic hypothesis for the Midas cichlid complex in Nicaragua (Teleostei: Cichlidae: Amphilophus) |year=2010 |last1=Geiger |first1=Matthias F. |last2=McCrary |first2=Jeffrey K. |last3=Schliewen |first3=Ulrich K. |journal=Molecular Phylogenetics and Evolution |volume=56 |issue=3 |pages=1011–24 |pmid=20580847|bibcode=2010MolPE..56.1011G }}</ref><ref>{{cite journal |doi=10.1038/nature04325 |title=Sympatric speciation in Nicaraguan crater lake cichlid fish |year=2006 |last1=Barluenga |first1=Marta |last2=Stölting |first2=Kai N. |last3=Salzburger |first3=Walter |last4=Muschick |first4=Moritz |last5=Meyer |first5=Axel |journal=Nature |volume=439 |issue=7077 |pages=719–23 |pmid=16467837|bibcode=2006Natur.439..719B |s2cid=3165729 |url=https://kops.uni-konstanz.de/bitstream/123456789/6577/1/sympatric_speciation_in_nicaraguan_crater_lake_cichlid_fish_2006.pdf }}</ref> ''[[Monostroma]] latissimum'', a marine green algae, also shows sympatric speciation in southwest Japanese islands. Although [[panmictic]], the molecular phylogenetics using nuclear introns revealed staggering diversification of population.<ref>{{cite journal | last1 = Bast | first1 = F. | last2 = Kubota | first2 = S. | last3 = Okuda | first3 = K. | year = 2014 | title = Phylogeographic Assessment of Panmictic Monostroma Species from Kuroshio Coast, Japan Reveals Sympatric Speciation | journal = Journal of Applied Phycology | volume =  27| issue = 4| pages =  1725–1735| doi = 10.1007/s10811-014-0452-x | s2cid = 17236629 }}</ref>

African [[cichlid]]s also offer some evidence for sympatric speciation. They show a large amount of diversity in the [[African Great Lakes]]. Many studies point to sexual selection as a way of maintaining reproductive isolation. Female choice with regards to male coloration is one of the more studied modes of sexual selection in African cichlids. Female choice is present in cichlids because the female does much of the work in raising the offspring, while the male has little energy input in the offspring. She exerts sensory bias when picking males by choosing those that have colors similar to her or those that are the most colorful.<ref>{{cite journal | last1 = Allender | first1 = C.J. | last2 = Seehausen | first2 = O. | last3 = Knight | first3 = M.E. | last4 = Turner | first4 = G.F. | last5 = Macleen | first5 = N. | year = 2003 | title = Divergent selection during speciation of the Lake Malawi cichlid fishes inferred from parallel radiations in nuptial coloration | journal = PNAS | volume = 100 | issue = 24| pages = 14074–14079 | doi=10.1073/pnas.2332665100 | pmid=14614144 | pmc=283548| bibcode = 2003PNAS..10014074A | doi-access = free }}</ref><ref>{{cite journal | last1 = Egger | first1 = B. | last2 = Mattersdorfer | first2 = K. | last3 = Sefc | first3 = K.M. | year = 2009 | title = Variable discrimination and asymmetric preferences in laboratory tests of reproductive isolation between cichlid colour morphs | journal = Journal of Evolutionary Biology | volume = 23 | issue = 2| pages = 433–439 | doi=10.1111/j.1420-9101.2009.01906.x| pmid = 20002244 | s2cid = 6533055 }}</ref><ref>{{cite journal | last1 = Selz | first1 = O.M. | last2 = Pierotti | first2 = M.E.R. | last3 = Mann | first3 = M.E. | last4 = Schmid | first4 = C. | last5 = Seehausen | first5 = O. | year = 2014 | title = Female preference for male color is necessary and sufficient for assortative mating in 2 cichlid sister species | journal = Behavioral Ecology | volume = 25 | issue = 3| pages = 612–626 | doi=10.1093/beheco/aru024| doi-access = free }}</ref> This helps maintain sympatric speciation within the lakes. Cichlids also use acoustic reproductive communication. The male cichlid quivers as a ritualistic display for the female which produces a certain number of pulses and pulse period. Female choice for good genes and sensory bias is one of the deciding factors in this case, selecting for calls that are within her species and that give the best fitness advantage to increase the survivability of the offspring.<ref>{{cite journal | last1 = Amorim | first1 = M.C.P. | last2 = Simóes | first2 = J.M. | last3 = Fonseca | first3 = P.J. | last4 = Turners | first4 = G.F. | year = 2008 | title = Species differences in courtship acoustic signals among five Lake Malawi cichlid species (Pseudotropheus spp.) | journal = J. Fish Biol. | volume = 72 | issue = 6| pages = 1355–1368 | doi=10.1111/j.1095-8649.2008.01802.x| bibcode = 2008JFBio..72.1355A | hdl = 10400.12/1391 | hdl-access = free }}</ref><ref>{{cite journal | last1 = Maruska | first1 = K.P. | last2 = Ung | first2 = U.S. | last3 = Fernald | first3 = R.D. | year = 2012 | title = The African Cichlid Fish Astatotilapia burtoni Uses Acoustic Communication for Reproduction: Sound Production, Hearing, and Behavioral Significance | journal = PLOS ONE | volume = 7 | issue = 5| page = e37612 | doi=10.1371/journal.pone.0037612 | pmid=22624055 | pmc=3356291| bibcode = 2012PLoSO...737612M | doi-access = free }}</ref> Male-male competition is a form of intrasexual selection and also has an effect on speciation in African cichlids. Ritualistic fighting among males establishes which males are going to be more successful in mating. This is important in sympatric speciation because species with similar males may be competing for the same females. There may be a fitness advantage for one phenotype that could allow one species to invade another.<ref>{{cite journal | last1 = Seehausen | first1 = O. | last2 = Schulter | first2 = D. | year = 2004 | title = Male-male competition and nuptial-colour displacement as a diversifying force in Lake Victoria cichlid fishes | journal = Proc. R. Soc. Lond. | volume = 241 | issue = 1546| pages = 1345–1353 | doi = 10.1098/rspb.2004.2737 | pmid = 15306332 | pmc = 1691729 }}</ref><ref>{{cite journal | last1 = Dijkstra | first1 = P.D. | last2 = Seehausen | first2 = O. | last3 = Groothuis | first3 = T.G.G. | year = 2005 | title = Direct male-male competition can facilitate invasion of new colour types in Lake Victoria cichlids | journal = Behav. Ecol. Sociobiol. | volume = 58 | issue = 2| pages = 136–143 | doi=10.1007/s00265-005-0919-5| bibcode = 2005BEcoS..58..136D | s2cid = 22364262 | url = https://boris.unibe.ch/49522/ }}</ref> Studies show this effect in species that are genetically similar, have the capability to interbreed, and show phenotypic color variation. Ecological character displacement is another means for sympatric speciation. Within each lake there are different niches that a species could occupy. For example, different diets and depth of the water could help to maintain isolation between species in the same lake.

[[Allochronic speciation|Allochrony]] offers some empirical evidence that sympatric speciation has taken place, as many examples exist of recently diverged ([[sister taxon|sister taxa]]) allochronic species. A case of ongoing sympatric divergence due to allochrony might be found in the marine insect [[Clunio marinus]].<ref>{{Cite journal|last1=Kaiser|first1=Tobias S.|last2=Haeseler|first2=Arndt von|last3=Tessmar-Raible|first3=Kristin|last4=Heckel|first4=David G.|date=2021|title=Timing strains of the marine insect Clunio marinus diverged and persist with gene flow|journal=Molecular Ecology|language=en|volume=30|issue=5|pages=1264–1280|doi=10.1111/mec.15791|pmid=33410230|issn=1365-294X|doi-access=free|bibcode=2021MolEc..30.1264K |hdl=21.11116/0000-0007-AC55-8|hdl-access=free}}</ref>

A rare example of sympatric speciation in animals is the divergence of "resident" and "transient" [[orca]] forms in the northeast Pacific.<ref>{{cite journal |doi=10.1093/jhered/89.2.121 |title=Low genetic variation among killer whales (''Orcinus orca'') in the eastern North Pacific and genetic differentiation between foraging specialists |year=1998 |last1=Hoelzel |first1=A. R. |last2=Dahlheim |first2=M. |last3=Stern |first3=S. J. |journal=Journal of Heredity |volume=89 |issue=2 |pages=121–8 |pmid=9542159|doi-access=free }}</ref> Resident and transient orcas inhabit the same waters, but avoid each other and do not interbreed. The two forms hunt different prey species and have different diets, vocal behaviour, and social structures. Some divergences between species could also result from contrasts in microhabitats. A population bottleneck occurred around 200,000 years ago greatly reducing the population size at the time as well as the variance of genes which allowed several ecotypes to emerge afterwards.<ref>{{cite journal|doi=10.1101/gr.102954.109 | volume=20 | issue=7 | title=Complete mitochondrial genome phylogeographic analysis of killer whales (Orcinus orca) indicates multiple species | journal=Genome Research | pages=908–916 | pmid=20413674 | pmc=2892092 | year=2010 | last1 = Morin | first1 = PA | last2 = Archer | first2 = FI | last3 = Foote | first3 = AD | last4 = Vilstrup | first4 = J | last5 = Allen | first5 = EE | last6 = Wade | first6 = P | last7 = Durban | first7 = J | last8 = Parsons | first8 = K | last9 = Pitman | first9 = R | last10 = Li | first10 = L | last11 = Bouffard | first11 = P | last12 = Abel Nielsen | first12 = SC | last13 = Rasmussen | first13 = M | last14 = Willerslev | first14 = E | last15 = Gilbert | first15 = MT | last16 = Harkins | first16 = T}}</ref>

The [[European polecat]] (''Mustela putorius'') exhibited a rare dark phenotype similar to the European [[mink]] (''Mustela lutreola'') phenotype, which is directly influenced by peculiarities of forest brooks.<ref>{{cite journal |doi=10.1046/j.1420-9101.2001.00275.x |title=Genetic divergence without spatial isolation in polecat Mustela putorius populations |year=2001 |last1=Lodé |first1=T. |journal=Journal of Evolutionary Biology |volume=14 |issue=2 |pages=228–36|s2cid=83203438 |doi-access=free }}</ref>

==Controversy==

For some time it was difficult to prove that sympatric speciation was possible, because it was impossible to observe it happening.<ref name="Fitzpatrick et al. 2008" /> It was believed by many, and championed by [[Ernst Mayr]], that the theory of evolution by [[natural selection]] could not explain how two species could emerge from one if the subspecies were able to interbreed.<ref name="Mayr 1947">{{cite journal |first1=Ernst |last1=Mayr |date=December 1947 |title=Ecological Factors in Speciation |journal=Evolution |volume=1 |issue=4 |pages=263–88 |jstor=2405327 |doi=10.2307/2405327}}</ref> Since Mayr's heyday in the 1940s and 50s, mechanisms have been proposed that explain how speciation might occur in the face of interbreeding, also known as [[gene flow]].<ref name="Kondrashov & Kondrashov 1999">{{cite journal |doi=10.1038/22514 |year=1999 |last1=Kondrashov |first1=Fyodor A. |last2=Kondrashov |first2=Alexey S. |journal=Nature |volume=400 |issue=6742 |pages=351–4 |pmid=10432111 |title=Interactions among quantitative traits in the course of sympatric speciation|bibcode=1999Natur.400..351K |s2cid=4425252 }}</ref> And even more recently concrete examples of sympatric divergence have been empirically studied.<ref name="Savolainen et al 2006">{{cite journal |doi=10.1038/nature04566 |title=Sympatric speciation in palms on an oceanic island |year=2006|author1-link=Vincent Savolainen |last1=Savolainen |first1=Vincent |last2=Anstett |first2=Marie-Charlotte |last3=Lexer |first3=Christian |last4=Hutton |first4=Ian |last5=Clarkson |first5=James J. |last6=Norup |first6=Maria V. |last7=Powell |first7=Martyn P. |last8=Springate |first8=David |last9=Salamin |first9=Nicolas |last10=Baker |first10=William J. |journal=Nature |volume=441 |issue=7090 |pages=210–3 |pmid=16467788|bibcode=2006Natur.441..210S |s2cid=867216 }}</ref><ref>{{cite journal |last1=Jaenike |first1=John |last2=Dyer |first2=Kelly A |last3=Cornish |first3=Chad |last4=Minhas |first4=Miranda S |last5=Noor |first5=Mohamed |title=Asymmetrical Reinforcement and Wolbachia Infection in Drosophila |journal=PLOS Biology |date=10 October 2006 |volume=4 |issue=10 |pages=e325 |doi=10.1371/journal.pbio.0040325 |pmid=17032063 |pmc=1592313 |doi-access=free }}</ref>  The debate now turns to how often sympatric speciation may actually occur in nature and how much of life's diversity it may be responsible for.

===History===
{{further|Modern synthesis (20th century)}}

The German evolutionary biologist Ernst Mayr argued in the 1940s that speciation cannot occur without geographic, and thus reproductive, isolation.<ref name="Mayr 1947"/> He stated that gene flow is the inevitable result of sympatry, which is known to squelch genetic differentiation between populations. Thus, a physical barrier must be present, he believed, at least temporarily, in order for a new biological species to arise.<ref name="Mallet et al 2009">{{cite journal |doi=10.1111/j.1420-9101.2009.01816.x |title=Space, sympatry and speciation |year=2009 |last1=Mallet |first1=J. |last2=Meyer |first2=A. |last3=Nosil |first3=P. |last4=Feder |first4=J. L. |journal=Journal of Evolutionary Biology |volume=22 |issue=11 |pages=2332–41 |pmid=19732264|s2cid=9722101 |url=https://kops.uni-konstanz.de/bitstream/123456789/8588/1/space_sympatry_and_speciation.pdf |doi-access=free }}</ref> This hypothesis is the source of much controversy around the possibility of sympatric speciation. Mayr's hypothesis was popular and consequently quite influential, but is now widely disputed.<ref name="Jiggins 2006">{{cite journal |doi=10.1016/j.cub.2006.03.077 |title=Sympatric Speciation: Why the Controversy? |year=2006 |last1=Jiggins |first1=Chris D. |journal=Current Biology |volume=16 |issue=9 |pages=R333–4 |pmid=16682343|s2cid=16947323 |doi-access=free |bibcode=2006CBio...16.R333J }}</ref>

The first to propose what is now the most pervasive hypothesis on how sympatric speciation may occur was [[John Maynard Smith]], in 1966. He came up with the idea of disruptive selection. He figured that if two ecological niches are occupied by a single species, diverging selection between the two niches could eventually cause [[reproductive isolation]].<ref>{{cite journal |first=J. Maynard |last=Smith |year=1966 |title=Sympatric Speciation |journal=The American Naturalist |volume=100 |issue=916 |pages=637–50 |jstor=2459301 |doi=10.1086/282457|bibcode=1966ANat..100..637S |s2cid=222329634 }}</ref> By adapting to have the highest possible fitness in the distinct niches, two species may emerge from one even if they remain in the same area, and even if they are mating randomly.<ref name="Kondrashov & Kondrashov 1999"/>

===Defining sympatry===
Investigating the possibility of sympatric speciation requires a definition thereof, especially in the 21st century, when mathematical modeling is used to investigate or to predict evolutionary phenomena.<ref name="Jiggins 2006"/> Much of the controversy concerning sympatric speciation may lie solely on an argument over what sympatric divergence actually is. The use of different definitions by researchers is a great impediment to empirical progress on the matter. The dichotomy between sympatric and allopatric speciation is no longer accepted by the scientific community. It is more useful to think of a continuum, on which there are limitless levels of geographic and reproductive overlap between species. On one extreme is allopatry, in which the overlap is zero (no gene flow), and on the other extreme is sympatry, in which the ranges overlap completely (maximal gene flow).

The varying definitions of sympatric speciation fall generally into two categories: definitions based on biogeography, or on population genetics. As a strictly geographical concept, sympatric speciation is defined as one species diverging into two while the ranges of both nascent species overlap entirely – this definition is not specific enough about the original population to be useful in modeling.<ref name="Fitzpatrick et al. 2008"/>

Definitions based on population genetics are not necessarily spatial or geographical in nature, and can sometimes be more restrictive. These definitions deal with the demographics of a population, including allele frequencies, selection, population size, the probability of gene flow based on sex ratio, life cycles, etc. The main discrepancy between the two types of definitions tends to be the necessity for "panmixia". Population genetics definitions of sympatry require that mating be dispersed randomly – or that it be equally likely for an individual to mate with either subspecies, in one area as another, or on a new host as a nascent one: this is also known as panmixia.<ref name="Fitzpatrick et al. 2008"/> Population genetics definitions, also known as non-spatial definitions, thus require the real possibility for random mating, and do not always agree with spatial definitions on what is and what is not sympatry.

For example, micro-allopatry, also known as macro-sympatry, is a condition where there are two populations whose ranges overlap completely, but contact between the species is prevented because they occupy completely different ecological niches (such as diurnal vs. nocturnal). This can often be caused by host-specific parasitism, which causes dispersal to look like a mosaic across the landscape. Micro-allopatry is included as sympatry according to spatial definitions, but, as it does not satisfy panmixia, it is not considered sympatry according to population genetics definitions.<ref name="Fitzpatrick et al. 2008"/>

Mallet et al. (2002) claims that the new non-spatial definition is lacking in an ability to settle the debate about whether sympatric speciation regularly occurs in nature. They suggest using a spatial definition, but one that includes the role of dispersal, also known as cruising range, so as to represent more accurately the possibility for gene flow. They assert that this definition should be useful in modeling. They also state that under this definition, sympatric speciation seems plausible.<ref name="Mallet et al 2009"/>

===Current state of the controversy===
Evolutionary theory as well as mathematical models have predicted some plausible mechanisms for the divergence of species without a physical barrier.<ref name="Kondrashov & Kondrashov 1999"/> In addition there have now been several studies that have identified speciation that has occurred, or is occurring with gene flow (see section above: evidence). Molecular studies have been able to show that, in some cases where there is no chance for allopatry, species continue to diverge. One such example is a pair of species of isolated desert palms. Two distinct, but closely related species exist on the same island, but they occupy two distinct soil types found on the island, each with a drastically different pH balance.<ref name="Savolainen et al 2006"/> Because they are palms they send pollen through the air they could freely interbreed, except that speciation has already occurred, so that they do not produce viable hybrids. This is hard evidence for the fact that, in at least some cases, fully sympatric species really do experience diverging selection due to competition, in this case for a spot in the soil.

This, and the other few concrete examples that have been found, are just that; they're few, so they tell us little about how often sympatry actually results in speciation in a more typical context. The burden now lies on providing evidence for sympatric divergence occurring in non-isolated habitats. It is not known how much of the earth's diversity it could be responsible for. Some still say that panmixia should slow divergence, and thus sympatric speciation should be possible but rare (1). Meanwhile, others claim that much of the earth's diversity could be due to speciation without geographic isolation.<ref name="Nosil 2008">{{cite journal |doi=10.1111/j.1365-294X.2008.03715.x |title=Speciation with gene flow could be common |year=2008 |last1=Nosil |first1=Patrik |journal=Molecular Ecology |volume=17 |issue=9 |pages=2103–6 |pmid=18410295|s2cid=164288751 |doi-access=free |bibcode=2008MolEc..17.2103N }}</ref> The difficulty in supporting a sympatric speciation hypothesis has always been that an allopatric scenario could always be invented, and those can be hard to rule out – but with modern molecular genetic techniques can be used to support the theory.<ref name="Nosil 2008"/>

In 2015 [[Cichlid]] fish from a tiny volcanic crater lake in Africa were observed in the act of sympatric speciation using DNA sequencing methods. A study found a complex combination of ecological separation and [[mate choice]] preference had allowed two [[Ecomorphology|ecomorphs]] to genetically separate even in the presence of some genetic exchange.<ref>{{cite news |url=http://phys.org/news/2015-12-darwin-puddle-species-emerge-geographic.html |title='Darwin's puddle' shows how new species can emerge without geographic separation |date=18 December 2015 |work=Phyorg.com }}</ref><ref>{{cite journal |title=Genomic islands of speciation separate cichlid ecomorphs in an East African crater lake |last1=Malinsky |first1=M. |display-authors=etal |date=2015 |journal=Science |doi=10.1126/science.aac9927 |pmid= 26680190|volume=350 |issue=6267 |pages=1493–1498 |pmc=4700518|bibcode=2015Sci...350.1493M }}</ref>

== Heteropatric speciation ==

Heteropatric [[speciation]] is a special case of sympatric speciation that occurs when different [[ecotypes]] or [[Race (biology)|race]]s of the same species geographically coexist but exploit different niches in the same patchy or heterogeneous environment. It is thus is a refinement of sympatric speciation, with a behavioral, rather than geographical barrier to the flow of genes among diverging groups within a population. Behavioral separation as a mechanism for promoting sympatric speciation in a [[heterogeneous]] (or patchwork landscape) was highlighted in [[John Maynard Smith]]'s seminal paper on sympatric speciation.<ref>{{cite journal | last1 = Smith | first1 = J. Maynard | author-link = John Maynard Smith | year = 1966 | title = Sympatric speciation | journal = The American Naturalist | volume = 110 | issue = 916| pages = 637–650 | doi = 10.1086/282457 | bibcode = 1966ANat..100..637S | s2cid = 222329634 }}</ref> In recognition of the importance of this behavioral versus geographic distinction, Wayne Getz and Veijo Kaitala introduced the term [[heteropatry]] in their extension of Maynard Smiths' analysis<ref>{{cite journal | last1 = Getz | first1 = W. M. | last2 = Kaitala | first2 = V. | year = 1989 | title = Ecogenetic models, competition, and heteropatry | journal = Theoretical Population Biology | volume = 36 | issue = 1 | pages = 34–58 | doi = 10.1016/0040-5809(89)90022-1 | bibcode = 1989TPBio..36...34G }}</ref> of conditions that facilitate sympatric speciation.

Although some evolutionary biologists still regard sympatric speciation as highly contentious, both theoretical<ref>{{cite journal | last1 = Bolnick | first1 = D. I. | year = 2006 | title = Multispecies outcomes in a common model of sympatric speciation | journal = Journal of Theoretical Biology | volume = 241 | issue = 4| pages = 734–744 | doi = 10.1016/j.jtbi.2006.01.009 | pmid = 16483610 | bibcode = 2006JThBi.241..734B }}</ref> and empirical<ref>{{cite journal | last1 = Forbes | first1 = A. A. | last2 = Fisher | first2 = J. | last3 = Feder | first3 = J. L. | year = 2005 | title = Habitat avoidance: overlooking an important aspect of host-specific mating and sympatric speciation | journal = Evolution | volume = 59 | issue = 7| pages = 1552–1559 | doi = 10.1111/j.0014-3820.2005.tb01804.x | pmid = 16153040 | s2cid = 221730795 | doi-access = free }}</ref> studies support it as a likely explanation of the diversity of life in particular ecosystems. Arguments implicate competition and niche separation of sympatric ecological variants that evolve through [[assortative mating]] into separate races and then species. Assortative mating most easily occurs if mating is linked to niche preference, as occurs in the [[apple maggot]] ''Rhagoletis pomonella'', where individual flies from different races use volatile odors to discriminate between [[Crataegus|hawthorn]] and [[apple]] and look for mates on [[Birth|natal]] fruit. The term heteropatry semantically resolves the issue of sympatric speciation by reducing it to a scaling issue in terms of the way the landscape is used by individuals versus populations. From a population perspective, the process looks sympatric, but from an individual's perspective, the process looks allopatric, once the time spent flying over or moving quickly through intervening non-preferred niches is taken into account.{{cn|date=February 2018}}

== See also ==
{{Portal|Environment|Ecology|Earth sciences|Evolutionary biology}}
* [[Adaptive radiation]]
* [[Cladistics]]
* [[Ecotype]]
* [[History of speciation]]
* [[Hybrid speciation]]
* [[Phylogenetics]]
* [[Polymorphism (biology)]]
* [[Polyploidy]]
* [[Reinforcement (speciation)|Reinforcement]]
* [[Laboratory experiments of speciation]]
* [[Taxonomy (biology)|Taxonomy]]

== References ==
{{reflist|30em}}

== External links ==
{{Wiktionary|sympatric}}
* [https://web.archive.org/web/20100617232226/http://evolution.berkeley.edu/evosite/evo101/VC1eSympatric.shtml Berkeley evolution 101]

{{speciation|show=yes}}
{{evolution|show=no}}

{{DEFAULTSORT:Sympatric Speciation}}
[[Category:Ecology]]
[[Category:Evolutionary biology]]
[[Category:Speciation]]
[[Category:Taxonomy (biology)]]