Editing Speciation (section)

== Modes ==
[[File:Speciation modes.svg|right|thumb|upright=2|Comparison of [[allopatric speciation|allopatric]], [[peripatric speciation|peripatric]], [[parapatric speciation|parapatric]] and [[sympatric speciation]]]]

All forms of natural speciation have taken place over the course of [[evolution]]; however, debate persists as to the relative importance of each mechanism in driving [[biodiversity]].<ref>{{cite journal |last=Baker |first=Jason M. |date=June 2005 |title=Adaptive speciation: The role of natural selection in mechanisms of geographic and non-geographic speciation |journal=Studies in History and Philosophy of Science Part C: Studies in History and Philosophy of Biological and Biomedical Sciences |volume=36 |issue=2 |pages=303–326 |doi=10.1016/j.shpsc.2005.03.005 |pmid=19260194|s2cid=3555049 |url=http://philsci-archive.pitt.edu/2331/1/baker-speciation-2005.pdf }}</ref>

One example of natural speciation is the diversity of the [[three-spined stickleback]], a [[Marine (ocean)|marine]] fish that, after the [[last glacial period]], has undergone speciation into new [[Fresh water|freshwater]] colonies in isolated lakes and streams. Over an estimated 10,000 generations, the sticklebacks show structural differences that are greater than those seen between different [[Genus|genera]] of fish including variations in fins, changes in the number or size of their bony plates, variable jaw structure, and color differences.<ref name="sciamstickleback">{{cite journal |last=Kingsley |first=David M. |date=January 2009 |title=Diversity Revealed: From Atoms to Traits |journal=[[Scientific American]] |volume=300 |issue=1 |pages=52–59 |doi=10.1038/scientificamerican0109-52|pmid=19186749 }}</ref>

=== Allopatric ===
{{Main|Allopatric speciation}}

During allopatric (from the ancient Greek ''allos'', "other" + ''patrā'', "fatherland") speciation, a population splits into two geographically isolated populations (for example, by [[habitat fragmentation]] due to geographical change such as [[mountain formation]]). The isolated populations then undergo genotypic or [[Phenotype|phenotypic]] divergence as: (a) they become subjected to dissimilar [[Selection (biology)|selective]] pressures; (b) different [[mutation]]s arise in the two populations. When the populations come back into contact, they have evolved such that they are reproductively isolated and are no longer capable of exchanging [[gene]]s. [[Small population size#Genetic consequences|Island genetics]] is the term associated with the tendency of small, isolated genetic pools to produce unusual traits. Examples include [[insular dwarfism]] and the radical changes among certain famous island chains, for example on [[Komodo (island)|Komodo]]. The [[Galápagos Islands]] are particularly famous for their influence on Charles Darwin. During his five weeks there he heard that [[Galápagos tortoise]]s could be identified by island, and noticed that [[finch]]es differed from one island to another, but it was only nine months later that he speculated that such facts could show that species were changeable. When he returned to [[England]], his speculation on evolution deepened after experts informed him that these were separate species, not just varieties, and famously that other differing Galápagos birds were all species of finches. Though the finches were less important for Darwin, more recent research has shown the birds now known as [[Darwin's finches]] to be a classic case of adaptive [[evolutionary radiation]].<ref>{{cite journal |last=Sulloway |first=Frank J. |author-link=Frank Sulloway |date=September 30, 1982 |title=The ''Beagle'' collections of Darwin's finches (Geospizinae) |url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=A86&pageseq=1 |journal=Bulletin of the British Museum (Natural History), Zoology |volume=43 |issue=2 |pages=49–58}}</ref>

=== Peripatric ===
{{main|Peripatric speciation}}

In peripatric speciation, a subform of allopatric speciation, new species are formed in isolated, smaller peripheral populations that are prevented from exchanging genes with the main population. It is related to the concept of a [[founder effect]], since small populations often undergo [[Population bottleneck|bottleneck]]s. Genetic drift is often proposed to play a significant role in peripatric speciation.{{sfn|Coyne|Orr|2004|p=105}}<ref name="Lawson2015">{{cite journal |title=Divergence at the edges: peripatric isolation in the montane spiny throated reed frog complex |author1=Lawson, Lucinda P. |author2=Bates, John M. |author3=Menegon, Michele |author4=Loader, Simon P. |journal=BMC Evolutionary Biology |year=2015 |volume=15 |issue=128 |pages=128 |doi=10.1186/s12862-015-0384-3 |pmid=26126573 |pmc=4487588 |doi-access=free |bibcode=2015BMCEE..15..128L }}</ref>

Case studies include Mayr's investigation of bird fauna;<ref name="Mayr1992">{{harvnb|Mayr|1992|pp=21–53}}</ref> the Australian bird ''[[Petroica multicolor]]'';<ref>{{Cite book|last=Tokeshi|first=M.|year=1999 |title=Species coexistence: ecological and evolutionary perspectives|location=Oxford|publisher=Blackwell Science|isbn=0632061464|oclc=47011551}}</ref> and reproductive isolation in populations of ''[[Drosophila]]'' subject to population bottlenecking.{{citation needed|date=May 2018}}

=== Parapatric ===
{{Main|Parapatric speciation}}

In parapatric speciation, there is only partial separation of the zones of two diverging populations afforded by geography; individuals of each species may come in contact or cross habitats from time to time, but reduced fitness of the [[Zygosity|heterozygote]] leads to selection for behaviours or mechanisms that prevent their [[Hybrid (biology)|interbreeding]]. Parapatric speciation is modelled on continuous variation within a "single", connected habitat acting as a source of natural selection rather than the effects of isolation of habitats produced in peripatric and allopatric speciation.<ref>{{Cite web|url=https://www.nature.com/scitable/knowledge/library/speciation-the-origin-of-new-species-26230527/|title=Speciation: The Origin of New Species {{!}} Learn Science at Scitable|website=Nature |language=en|access-date=2020-02-16}}</ref>

Parapatric speciation may be associated with differential landscape-dependent [[Selection (biology)|selection]]. Even if there is a [[gene flow]] between two populations, strong differential selection may impede assimilation and different species may eventually develop.<ref>{{harvnb|Endler|1977}}</ref> Habitat differences may be more important in the development of reproductive isolation than the isolation time. Caucasian rock lizards ''[[Darevskia]] rudis'', ''D. valentini'' and ''D. portschinskii'' all [[hybrid (biology)|hybrid]]ize with each other in their [[hybrid zone]]; however, hybridization is stronger between ''D. portschinskii'' and ''D. rudis'', which separated earlier but live in similar habitats than between ''D. valentini'' and two other species, which separated later but live in climatically different habitats.<ref>{{cite journal |last1=Tarkhnishvili |first1=David |last2=Murtskhvaladze |first2=Marine |last3=Gavashelishvili |first3=Alexander |date=August 2013 |title=Speciation in Caucasian lizards: climatic dissimilarity of the habitats is more important than isolation time |journal=[[Biological Journal of the Linnean Society]] |volume=109 |issue=4 |pages=876–892 |doi=10.1111/bij.12092|doi-access=free }}</ref>

Ecologists refer to{{clarify|date=May 2018<!--what is being claimed here?-->}} parapatric and peripatric speciation in terms of [[ecological niche]]s. A niche must be available in order for a new species to be successful. [[Ring species]] such as ''[[gull|Larus]]'' gulls have been claimed to illustrate speciation in progress, though the situation may be more complex.<ref name=Liebers>{{cite journal |last1=Liebers |first1=Dorit |last2=Knijff |first2=Peter de |last3=Helbig |first3=Andreas J. |title=The herring gull complex is not a ring species |journal=Proc Biol Sci |date=2004 |volume=271 |issue=1542 |pages=893–901 |doi=10.1098/rspb.2004.2679 |pmc=1691675 |pmid=15255043}}<!--nor Herring Gull--></ref> The grass ''[[Anthoxanthum odoratum]]'' may be starting parapatric speciation in areas of mine contamination.<ref>{{cite web |title=Parapatric speciation |url=http://evolution.berkeley.edu/evolibrary/article/_0/speciationmodes_04 |publisher=University of California Berkeley |access-date=3 April 2017}}</ref>

=== Sympatric ===
{{Main|Sympatric speciation}}
[[File:Pundamilia (Haplochromis) nyererei male.jpg|thumb|left|[[Cichlid]]s such as ''[[Haplochromis nyererei]]'' diversified by [[sympatric speciation]] in the [[Rift Valley lakes]].]]

Sympatric speciation is the formation of two or more descendant species from a single ancestral species all occupying the same geographic location.

Often-cited examples of sympatric speciation are found in insects that become dependent on different [[Host (biology)|host]] plants in the same area.<ref name="rhagoletis1">{{cite journal |last1=Feder |first1=Jeffrey L. |author2=Xianfa Xie |last3=Rull |first3=Juan |last4=Velez |first4=Sebastian |last5=Forbes |first5=Andrew |last6=Leung |first6=Brian |last7=Dambroski |first7=Hattie |last8=Filchak |first8=Kenneth E. |last9=Aluja |first9=Martin |display-authors=3 |date=May 3, 2005 |title=Mayr, Dobzhansky, and Bush and the complexities of sympatric speciation in ''Rhagoletis'' |journal=PNAS |volume=102 |issue=Suppl 1 |pages=6573–6580 |doi=10.1073/pnas.0502099102 |pmc=1131876 |pmid=15851672 |bibcode=2005PNAS..102.6573F |doi-access=free }}</ref><ref name=hostplant1>{{cite journal |last1=Berlocher |first1=Stewart H. |last2=Feder |first2=Jeffrey L. |date=January 2002 |title=Sympatric Speciation in Phytophagous Insects: Moving Beyond Controversy? |journal=Annual Review of Entomology |volume=47 |pages=773–815 |doi=10.1146/annurev.ento.47.091201.145312 |pmid=11729091|s2cid=9677456 }}</ref>

The best known example of sympatric speciation is that of the [[cichlid]]s of [[East Africa]] inhabiting the [[Rift Valley lakes]], particularly [[Lake Victoria]], [[Lake Malawi]] and [[Lake Tanganyika]]. There are over 800 described species, and according to estimates, there could be well over 1,600 species in the region. Their evolution is cited as an example of both [[natural selection|natural]] and [[sexual selection]].<ref>{{cite journal |last1=Machado |first1=Heather E. |last2=Pollen |first2=Alexander A. |last3=Hofmann |first3=Hans A. |last4=Renn |first4=Suzy C. P. |display-authors=3 |date=December 2009 |title=Interspecific profiling of gene expression informed by comparative genomic hybridization: A review and a novel approach in African cichlid fishes |journal=[[Integrative and Comparative Biology]] |volume=49 |issue=6 |pages=644–659 |doi=10.1093/icb/icp080 |pmid=21665847|doi-access=free }}</ref><ref>{{cite journal |last1=Fan |first1=Shaohua |last2=Elmer |first2=Kathryn R. |last3=Meyer |first3=Axel |author-link3=Axel Meyer |date=February 5, 2012 |title=Genomics of adaptation and speciation in cichlid fishes: recent advances and analyses in African and Neotropical lineages |journal=[[Philosophical Transactions of the Royal Society B]]  |volume=367 |issue=1587 |pages=385–394 |doi=10.1098/rstb.2011.0247 |pmc=3233715 |pmid=22201168}}</ref> A 2008 study suggests that sympatric speciation has occurred in [[Tennessee cave salamander]]s.<ref>{{cite journal |last1=Niemiller |first1=Matthew L. |last2=Fitzpatrick |first2=Benjamin M. |last3=Miller |first3=Brian T. |date=May 2008 |title=Recent divergence with gene flow in Tennessee cave salamanders (Plethodontidae: ''Gyrinophilus'') inferred from gene genealogies |journal=[[Molecular Ecology]] |volume=17 |issue=9 |pages=2258–2275 |doi=10.1111/j.1365-294X.2008.03750.x |pmid=18410292|s2cid=20761880 |doi-access=free |bibcode=2008MolEc..17.2258N }}</ref> Sympatric speciation driven by ecological factors may also account for the extraordinary diversity of crustaceans living in the depths of Siberia's [[Lake Baikal]].<ref>{{cite journal |last1=Martens |first1=Koen |title=Speciation in ancient lakes |journal=Trends in Ecology & Evolution |date=May 1997 |volume=12 |issue=5 |pages=177–182 |doi=10.1016/S0169-5347(97)01039-2|pmid=21238028 |bibcode=1997TEcoE..12..177M }}</ref>

Budding speciation has been proposed as a particular form of sympatric speciation, whereby small groups of individuals become progressively more isolated from the ancestral stock by breeding preferentially with one another. This type of speciation would be driven by the conjunction of various advantages of inbreeding such as the expression of advantageous recessive phenotypes, reducing the recombination load, and reducing the cost of sex.<ref>{{cite journal |last1=Joly |first1=E. |title=The existence of species rests on a metastable equilibrium between inbreeding and outbreeding. An essay on the close relationship between speciation, inbreeding and recessive mutations |journal=Biology Direct |date=9 December 2011 |volume=6 |page=62 |pmid=22152499 |doi=10.1186/1745-6150-6-62|pmc=3275546 |doi-access=free }}</ref>

[[File:Rhagoletis pomonella.jpg|thumb|''[[Rhagoletis pomonella]]'', the hawthorn fly, appears to be in the process of sympatric speciation.]]

The hawthorn fly (''[[Rhagoletis pomonella]]''), also known as the apple maggot fly, appears to be undergoing sympatric speciation.<ref>{{cite journal |last1=Feder |first1=Jeffrey L. |last2=Roethele |first2=Joseph B. |last3=Filchak |first3=Kenneth |last4=Niedbalski |first4=Julie |last5=Romero-Severson |first5=Jeanne |display-authors=3 |date=March 2003 |title=Evidence for inversion polymorphism related to sympatric host race formation in the apple maggot fly, ''Rhagoletis pomonella'' |url=http://www.genetics.org/content/163/3/939.long |journal=[[Genetics (journal)|Genetics]]  |volume=163 |issue=3 |pages=939–953 |doi=10.1093/genetics/163.3.939 |pmc=1462491 |pmid=12663534 |access-date=2015-09-07}}</ref> Different populations of hawthorn fly feed on different fruits. A distinct population emerged in North America in the 19th century some time after [[apple]]s, a non-native species, were introduced. This apple-feeding population normally feeds only on apples and not on the historically preferred fruit of [[Crataegus|hawthorns]]. The current hawthorn feeding population does not normally feed on apples. Some evidence, such as that six out of thirteen [[Alloenzyme|allozyme]] loci are different, that hawthorn flies mature later in the season and take longer to mature than apple flies; and that there is little evidence of interbreeding (researchers have documented a 4–6% hybridization rate) suggests that sympatric speciation is occurring.<ref>{{cite journal |last1=Berlocher |first1=Stewart H. |last2=Bush |first2=Guy L. |date=June 1982 |title=An electrophoretic analysis of Rhagoletis (Diptera: Tephritidae) phylogeny |journal=[[Systematic Zoology]] |volume=31 |issue=2 |pages=136–155 |doi=10.2307/2413033 |jstor=2413033}}</ref>