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Disruptive selection
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==Sympatric speciation== It is believed that disruptive selection is one of the main forces that drive [[sympatric speciation]] in natural populations.<ref name="Smith1966">{{cite journal | date=1966 | first1=J.M. | title=Sympatric speciation | journal=The American Naturalist | volume=100 | issue=916 | doi=10.1086/282457 | jstor=2459301 | last1=Smith | pages=637β950| s2cid=222329634 }}</ref> The pathways that lead from disruptive selection to sympatric speciation seldom are prone to deviation; such speciation is a domino effect that depends on the consistency of each distinct variable. These pathways are the result of disruptive selection in [[intraspecific competition]]; it may cause [[reproductive isolation]], and finally culminate in sympatric speciation. It is important to keep in mind that disruptive selection does not always have to be based on intraspecific competition. It is also important to know that this type of natural selection is similar to the other ones. Where it is not the major factor, intraspecific competition can be discounted in assessing the operative aspects of the course of adaptation. For example, what may drive disruptive selection instead of intraspecific competition might be [[Polymorphism (biology)|polymorphisms]] that lead to reproductive isolation, and thence to speciation.<ref name="Mather1955">{{cite journal | title=Polymorphism as an outcome of disruptive selection | journal=Evolution | jstor=2405357 | last1=Mather |date=March 1955 | volume=9 | issue=1 | pages=51β61 | first1=K. | doi = 10.2307/2405357 }}</ref><ref name="Smith1962">{{cite journal | title=Disruptive selection, polymorphism and sympatric speciation | journal=Nature |date=July 1962 | volume=195 | issue=4836 | pages=60β62 | doi=10.1038/195060a0 | last1=Smith | first1=J.M.| bibcode=1962Natur.195...60M | s2cid=5802520 }}</ref><ref name="Thoday1970">{{cite journal | title=The probability of isolation by disruptive selection | journal=The American Naturalist | year=1970 | volume=104 | issue=937 | pages=219β230 | jstor=2459154 | last1=Thoday | last2=Gibson | first1=J.M. | first2=J.B. | doi=10.1086/282656| s2cid=85333360 }}</ref><ref name="Kondrashov1986">{{cite journal | title=Sympatric speciation: when is it possible? | journal=Biological Journal of the Linnean Society |date=March 1986 | volume=27 | issue=3 | pages=201β223 | doi=10.1111/j.1095-8312.1986.tb01734.x | last1=Kondrashov | last2=Mina | first1=A.S. | first2=M.V.}}</ref><!--the following lack the citation template--><ref name=Smith1966 /><ref name=S1969>{{cite journal | last1 = Sharloo | first1 = W | year = 1969 | title = Stable and disruptive selection on a mutant character in drosophila III polymorphism caused by a developmental switch mechanism | journal = Genetics | volume = 65 | issue = 4| pages = 693β705 | doi = 10.1093/genetics/65.4.693 | pmid = 5518512 | pmc = 1212475 }}</ref><ref name=B2007>{{cite journal | last1 = Bolnick | first1 = D.I. | last2 = Fitzpatrick | first2 = B.M. | year = 2007 | title = Sympatric speciation: models and empirical evidence | doi = 10.1146/annurev.ecolsys.38.091206.095804 | journal = Annual Review of Ecology, Evolution, and Systematics | volume = 38 | pages = 459β487 }}</ref><ref name=S2007>{{cite journal | last1 = Svanback | first1 = R. | last2 = Bolnick | first2 = D.I. | year = 2007 | title = Intraspecific competition drives increased resource use diversity within a natural population | journal = Proc. R. Soc. B | volume = 274 | issue = 1611| pages = 839β844 | doi=10.1098/rspb.2006.0198| pmid = 17251094 | pmc = 2093969 }}</ref> When disruptive selection is based on intraspecific competition, the resulting selection in turn promotes [[ecological niche]] diversification and polymorphisms. If multiple morphs ([[phenotype|phenotypic forms]]) occupy different niches, such separation could be expected to promote reduced competition for resources. Disruptive selection is seen more often in high density populations rather than in low density populations because intraspecific competition tends to be more intense within higher density populations. This is because higher density populations often imply more competition for resources. The resulting competition drives polymorphisms to exploit different niches or changes in niches in order to avoid competition. If one morph has no need for resources used by another morph, then it is likely that neither would experience pressure to compete or interact, thereby supporting the persistence and possibly the intensification of the distinctness of the two morphs within the population.<ref>{{cite journal | last1 = Merrill | first1 = R.M. | display-authors = etal | year = 1968 | title = Disruptive ecological selection on a mating cue | journal = Proceedings of the Royal Society | volume = 10 | issue = 1749| pages = 1β8 | doi = 10.1098/rspb.2012.1968 | pmid = 23075843 | pmc = 3497240 }}</ref><ref>{{cite journal | last1 = Bolnick | first1 = D.I. | year = 2007 | title = Can intraspecific competition drive disruptive selection? An experimental test in natural populations of stickleback | journal = Evolution | volume = 58 | issue = 3| pages = 608β618 | doi = 10.1554/03-326 | pmid = 15119444 | s2cid = 16739680 }}</ref><ref>{{cite journal | last1 = Martin | first1 = R.A. | last2 = Pfennig | first2 = D.W. | year = 2009 | title = Disruptive selection in natural populations: the roles of ecological specialization and resource competition | journal = The American Naturalist | volume = 174 | issue = 2| pages = 268β281 | doi = 10.1086/600090 | pmid=19527118| s2cid = 16154501 }}</ref><ref>{{cite journal | last1 = Alvarez | first1 = E.R. | year = 2006 | title = Sympatric speciation as a byproduct of ecological adaptation in the Galician Littorina saxatilis hybrid zone | journal = Journal of Molluscan Studies | volume = 73 | pages = 1β10 | doi=10.1093/mollus/eyl023| doi-access = free }}</ref><ref>{{cite journal | last1 = Martin | first1 = A. R. | last2 = Pfenning | first2 = D.W. | year = 2012 | title = Widespread disruptive selection in the wild is associated with intense resource competition | doi = 10.1186/1471-2148-12-136 | pmid = 22857143 | journal = BMC Evolutionary Biology | volume = 12 | pages = 1β13 | pmc = 3432600 | doi-access = free }}</ref><ref name=r1984>{{cite journal | last1 = Rice | first1 = W.R. | year = 1984 | title = Disruptive selection on habitat preference and evolution of reproductive isolation: a simulation study | journal = Evolution | volume = 38 | issue = 6| pages = 1251β1260 | doi=10.2307/2408632| pmid = 28563785 | jstor = 2408632 }}</ref> This theory does not necessarily have a lot of supporting evidence in natural populations, but it has been seen many times in experimental situations using existing populations. These experiments further support that, under the right situations (as described above), this theory could prove to be true in nature.<ref name="Kondrashov1986" /><ref name=S2007/> When intraspecific competition is not at work disruptive selection can still lead to sympatric speciation and it does this through maintaining polymorphisms. Once the polymorphisms are maintained in the population, if [[assortative mating]] is taking place, then this is one way that disruptive selection can lead in the direction of sympatric speciation.<ref name=Smith1962/><ref name="Kondrashov1986" /><ref name=S1969/> If different morphs have different mating preferences then assortative mating can occur, especially if the polymorphic trait is a "'''magic trait'''", meaning a trait that is under [[ecological selection]] and in turn has a side effect on reproductive behavior. In a situation where the polymorphic trait is ''not'' a magic trait then there has to be some kind of [[fitness (biology)|fitness]] penalty for those individuals who do not mate assortatively and a mechanism that causes assortative mating has to evolve in the population. For example, if a species of butterflies develops two kinds of wing patterns, crucial to mimicry purposes in their preferred habitat, then mating between two butterflies of different wing patterns leads to an unfavorable [[heterozygote]]. Therefore, butterflies will tend to mate with others of the same wing pattern promoting increased fitness, eventually eliminating the heterozygote altogether. This unfavorable heterozygote generates pressure for a mechanism that cause assortative mating which will then lead to reproductive isolation due to the production of post-mating barriers.<ref>{{cite journal | last1 = Naisbit | first1 = R.E. | display-authors = etal | year = 2001 | title = Disruptive sexual selection against hybrids contributes to speciation between Heliconius cyndo and Heliconius melpomene | journal = Proceedings of the Royal Society of London. Series B: Biological Sciences| volume = 268 | issue = 1478| pages = 1849β1854 | doi=10.1098/rspb.2001.1753| pmid = 11522205 | pmc = 1088818}}</ref><ref>{{cite journal | last1 = Dieckmann | first1 = U. | last2 = Doebeli | first2 = M. | year = 1999 | title = On the origin of species by sympatric speciation | url = http://pure.iiasa.ac.at/id/eprint/5926/1/IR-99-013.pdf| journal = Letters to Nature | volume = 400 | issue = 6742| pages = 353β357 | doi = 10.1038/22521 | pmid = 10432112 | bibcode = 1999Natur.400..354D | s2cid = 4301325 }}</ref><ref>{{cite journal | last1 = Jiggins | first1 = C.D. | display-authors = etal | year = 2001 | title = Reproductive isolation caused by colour pattern mimicry | url = http://doc.rero.ch/record/7889/files/Jiggins_Chris_D._-_Reproductive_isolation_caused_by_coulour_20070615.pdf| journal = Letters to Nature | volume = 411 | issue = 6835| pages = 302β305 | doi = 10.1038/35077075 | pmid = 11357131 | bibcode = 2001Natur.411..302J | s2cid = 2346396 }}</ref> It is actually fairly common to see sympatric speciation when disruptive selection is supporting two morphs, specifically when the phenotypic trait affects fitness rather than [[mate choice]].<ref>{{cite journal | last1 = Kondrashov | first1 = A.S. | last2 = Kondrashov | first2 = F.A. | year = 1999 | title = Interactions among quantitative traits in the course of sympatric speciation | pmid = 10432111| journal = Nature | volume = 400 | issue = 6742| pages = 351β354 | doi=10.1038/22514| bibcode = 1999Natur.400..351K | s2cid = 4425252 }}</ref> In both situations, one where intraspecific competition is at work and the other where it is not, if all these factors are in place, they will lead to reproductive isolation, which can lead to sympatric speciation.<ref name=B2007/><ref name=r1984/><ref>{{cite journal | last1 = Via | first1 = S | year = 1999 | title = Reproductive Isolation between sympatric races of Pea Aphids I. gene flow restriction and habitat choice | journal = Evolution | volume = 53 | issue = 5| pages = 1446β1457 | doi=10.2307/2640891| jstor = 2640891 | pmid = 28565574 }}</ref>
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