Editing Speciation (section)

== Genetics ==

Few speciation genes have been found. They usually involve the reinforcement process of late stages of speciation. In 2008, a speciation gene causing reproductive isolation was reported.<ref>{{cite journal |last1=Phadnis |first1=Nitin |last2=Orr |first2=H. Allen |author-link2=H. Allen Orr |date=January 16, 2009 |title=A Single Gene Causes Both Male Sterility and Segregation Distortion in ''Drosophila'' Hybrids |journal=Science |volume=323 |issue=5912 |pages=376–379 |doi=10.1126/science.1163934 |pmc=2628965 |pmid=19074311|bibcode=2009Sci...323..376P }}</ref> It causes hybrid sterility between related subspecies. The order of speciation of three groups from a common ancestor may be unclear or unknown; a collection of three such species is referred to as a "trichotomy".{{Citation needed|date=November 2023}}

=== Speciation via polyploidy ===
[[File:Polyploidization.svg|right|thumb|Speciation via polyploidy: A [[diploid]] cell undergoes failed [[meiosis]], producing diploid [[gamete]]s, which self-fertilize to produce a tetraploid [[zygote]]. In plants, this can effectively be a new species, reproductively isolated from its parents, and able to reproduce.]]
{{main|Polyploid}}

[[Polyploid]]y is a mechanism that has caused many rapid speciation events in [[sympatry]] because offspring of, for example, tetraploid x diploid matings often result in triploid sterile progeny.<ref>{{cite journal |last1=Ramsey |first1=Justin |last2=Schemske |first2=Douglas W. |date=November 1998 |title=Pathways, Mechanisms, and Rates of Polyploid Formation in Flowering Plants |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=29 |pages=467–501 |doi=10.1146/annurev.ecolsys.29.1.467|bibcode=1998AnRES..29..467R |s2cid=31637733 |url=https://pdfs.semanticscholar.org/4696/87a97276b274bba6469e7979dfc99a23e4cc.pdf |archive-url=https://web.archive.org/web/20200608074533/https://pdfs.semanticscholar.org/4696/87a97276b274bba6469e7979dfc99a23e4cc.pdf |url-status=dead |archive-date=2020-06-08 }}</ref> However, among plants, not all polyploids are reproductively isolated from their parents, and gene flow may still occur, such as through triploid hybrid x diploid matings that produce tetraploids, or matings between [[meiosis|meiotically unreduced]] gametes from diploids and gametes from tetraploids (see also [[hybrid speciation]]).{{Citation needed|date=November 2023}}

It has been suggested that many of the existing plant and most animal species have undergone an event of polyploidization in their evolutionary history.<ref>{{cite journal |last1=Otto |first1=Sarah P. |author-link1=Sarah Otto |last2=Whitton |first2=Jeannette |date=December 2000 |title=Polyploid Incidence and Evolution |journal=[[Annual Review of Genetics]] |volume=34 |pages=401–437 |doi=10.1146/annurev.genet.34.1.401 |pmid=11092833|url=http://www.zoology.ubc.ca/~otto/Reprints/OttoWhitton2000.pdf |citeseerx=10.1.1.323.1059 }}</ref><ref name="Comai_2005">{{cite journal |last1=Comai |first1=Luca |author-link1=Luca Comai |date=November 2005 |title=The advantages and disadvantages of being polyploid |journal=[[Nature Reviews Genetics]] |volume=6 |issue=11 |pages=836–846 |doi=10.1038/nrg1711 |pmid=16304599|s2cid=3329282 }}</ref> Reproduction of successful polyploid species is sometimes asexual, by [[parthenogenesis]] or [[apomixis]], as for unknown reasons many asexual organisms are polyploid. Rare instances of polyploid mammals are known, but most often result in prenatal death. <ref>{{Cite journal |last1=Mezzasalma |first1=Marcello |last2=Brunelli |first2=Elvira |last3=Odierna |first3=Gaetano |last4=Guarino |first4=Fabio Maria |date=2023-03-12 |title=Evolutionary and Genomic Diversity of True Polyploidy in Tetrapods |journal=Animals |volume=13 |issue=6 |pages=1033 |doi=10.3390/ani13061033 |doi-access=free |issn=2076-2615 |pmid=36978574|pmc=10044425 }}</ref>

=== Hybrid speciation ===
{{Main|Hybrid speciation}}

Hybridization between two different species sometimes leads to a distinct [[phenotype]]. This phenotype can also be fitter than the parental lineage and as such natural selection may then favor these individuals. Eventually, if reproductive isolation is achieved, it may lead to a separate species. However, reproductive isolation between hybrids and their parents is particularly difficult to achieve and thus hybrid speciation is considered an extremely rare event. The [[Mariana mallard]] is thought to have arisen from hybrid speciation.{{Citation needed|date=November 2023}}

Hybridization is an important means of speciation in plants, since [[polyploid]]y (having more than two copies of each [[chromosome]]) is tolerated in plants more readily than in animals.<ref name=Wendel>{{cite journal |last=Wendel |first=Jonathan F. |date=January 2000 |title=Genome evolution in polyploids |journal=Plant Molecular Biology |volume=42 |issue=1 |pages=225–249 |doi=10.1023/A:1006392424384 |pmid=10688139|s2cid=14856314 }}</ref><ref name=Semon>{{cite journal |last1=Sémon |first1=Marie |last2=Wolfe |first2=Kenneth H. |date=December 2007 |title=Consequences of genome duplication |journal=[[Current Opinion (Elsevier)|Current Opinion in Genetics & Development]] |volume=17 |issue=6 |pages=505–512 |doi=10.1016/j.gde.2007.09.007 |pmid=18006297}}</ref> Polyploidy is important in hybrids as it allows reproduction, with the two different sets of chromosomes each being able to pair with an identical partner during meiosis.<ref name="Comai_2005"/> Polyploids also have more genetic diversity, which allows them to avoid [[inbreeding depression]] in small populations.<ref>{{cite journal |last1=Soltis |first1=Pamela S. |author-link1=Pamela S. Soltis |last2=Soltis |first2=Douglas E. |date=June 20, 2000 |title=The role of genetic and genomic attributes in the success of polyploids |journal=PNAS |volume=97 |issue=13 |pages=7051–7057 |doi=10.1073/pnas.97.13.7051 |pmc=34383 |pmid=10860970 |bibcode=2000PNAS...97.7051S|doi-access=free }}</ref>

Hybridization without change in chromosome number is called homoploid hybrid speciation. It is considered very rare but has been shown in ''[[Heliconius]]'' [[butterfly|butterflies]]<ref name="Mavarez2006">{{cite journal |last1=Mavarez |first1=Jesús |last2=Salazar |first2=Camilo A. |last3=Bermingham |first3=Eldredge |last4=Salcedo |first4=Christian |last5=Jiggins |first5=Chris D. |last6=Linares |first6=Mauricio |display-authors=3 |date=June 15, 2006 |title=Speciation by hybridization in ''Heliconius'' butterflies |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7095 |pages=868–871 |doi=10.1038/nature04738 |pmid=16778888 |bibcode=2006Natur.441..868M|s2cid=2457445 }}</ref> and [[sunflower]]s. Polyploid speciation, which involves changes in chromosome number, is a more common phenomenon, especially in plant species. {{Citation needed|date=November 2023}}

=== Gene transposition  ===
{{Further|Transposable element}}

[[Theodosius Dobzhansky]], who studied fruit flies in the early days of genetic research in 1930s, speculated that parts of chromosomes that switch from one location to another might cause a species to split into two different species. He mapped out how it might be possible for sections of chromosomes to relocate themselves in a genome. Those mobile sections can cause sterility in inter-species hybrids, which can act as a speciation pressure. In theory, his idea was sound, but scientists long debated whether it actually happened in nature. Eventually a competing theory involving the gradual accumulation of mutations was shown to occur in nature so often that geneticists largely dismissed the moving gene hypothesis.<ref>{{cite press release |last=Sherwood |first=Jonathan |date=September 8, 2006 |title=Genetic Surprise Confirms Neglected 70-Year-Old Evolutionary Theory |url=http://www.rochester.edu/news/show.php?id=2603 |publisher=[[University of Rochester]] |access-date=2015-09-10}}</ref> However, 2006 research shows that jumping of a gene from one chromosome to another can contribute to the birth of new species.<ref>{{cite journal |last1=Masly |first1=John P. |last2=Jones |first2=Corbin D. |last3=Mohamed |first3=A. F. Noor |author-link3=Mohamed Noor |last4=Locke |first4=John |last5=Orr |first5=H. Allen |display-authors=3 |date=September 8, 2006 |title=Gene Transposition as a Cause of Hybrid Sterility in ''Drosophila'' |journal=Science |volume=313 |issue=5792 |pages=1448–1450 |doi=10.1126/science.1128721 |pmid=16960009|bibcode=2006Sci...313.1448M |s2cid=23462115 }}</ref> This validates the reproductive isolation mechanism, a key component of speciation.<ref>{{cite journal |last=Minkel |first=J. R. |date=September 8, 2006 |title=Wandering Fly Gene Supports New Model of Speciation |url=http://www.scientificamerican.com/article/wandering-fly-gene-suppor/ |journal=Scientific American |access-date=2015-09-11}}</ref>