Editing Punctuated equilibrium (section)

==Theoretical mechanisms==
===Punctuational change===

When Eldredge and Gould published their 1972 paper, [[allopatric speciation]] was considered the "standard" model of speciation.<ref name=pe1972 /> This model was popularized by Ernst Mayr in his 1954 paper "Change of genetic environment and evolution,"<ref name=Mayr1954 /> and his classic volume ''Animal Species and Evolution'' (1963).<ref name="Mayr1963">[[Ernst Mayr|Mayr, Ernst]] (1963). ''Animal Species and Evolution''. Cambridge, MA: [[Harvard University Press]].</ref>

Allopatric speciation suggests that species with large central populations are stabilized by their large volume and the process of [[gene flow]]. New and even [[Mutation#Beneficial mutations|beneficial mutations]] are diluted by the population's large size and are unable to reach fixation, due to such factors as constantly changing environments.<ref name="Mayr1963" /> If this is the case, then the transformation of whole lineages should be rare, as the fossil record indicates. Smaller populations on the other hand, which are isolated from the parental stock, are decoupled from the [[wikt:Homogeneous|homogenizing]] effects of gene flow. In addition, pressure from [[natural selection]] is especially intense, as peripheral isolated populations exist at the outer edges of [[ecological tolerance]]. If most evolution happens in these rare instances of allopatric speciation then evidence of gradual evolution in the fossil record should be rare. This [[hypothesis]] was alluded to by Mayr in the closing paragraph of his 1954 paper:

{{Blockquote|Rapidly evolving peripherally isolated populations may be the place of origin of many evolutionary novelties. Their isolation and comparatively small size may explain phenomena of rapid evolution and lack of documentation in the fossil record, hitherto puzzling to the palaeontologist.<ref name=Mayr1954 />}}

Although punctuated equilibrium generally applies to sexually reproducing organisms,<ref>[[Niles Eldredge|Eldredge, Niles]] and [[Stephen Jay Gould|S. J. Gould]] (1997). [http://www.stephenjaygould.org/library/science_on-punctuated-equilibria.html "On punctuated equilibria (letter)."] ''Science'' 276 (5311): 337-341.</ref> some biologists have applied the model to non-sexual species like [[virus]]es,<ref>Nichol, S.T, Joan Rowe, and Walter M. Fitch (1993). [http://www.pnas.org/content/90/22/10424.full.pdf "Punctuated equilibrium and positive Darwinian evolution in vesicular stomatitis virus."] ''Proceedings of the National Academy of Sciences'' 90 (Nov.): 10424-28.</ref><ref>Elena S.F., V.S. Cooper, and R. Lenski (1996). [https://web.archive.org/web/20030514101700/http://myxo.css.msu.edu/lenski/pdf/1996,%20Science,%20Elena%20et%20al.pdf "Punctuated Evolution Caused by Selection of Rare Beneficial Mutations."] ''Science'' 272 (June 21): 1802-1804.</ref> which cannot be stabilized by conventional gene flow. As time went on biologists like Gould moved away from wedding punctuated equilibrium to allopatric speciation, particularly as evidence accumulated in support of other modes of speciation.<ref name="GouldNewScientist"/> Gould, for example, was particularly attracted to [[Douglas J. Futuyma|Douglas Futuyma's]] work on the importance of reproductive isolating mechanisms.<ref>{{cite journal |last=Futuyma |first=Douglas |date=1987 |title=On the role of species in anagenesis |journal=[[The American Naturalist|American Naturalist]] |doi=10.1086/284724 |volume=130 |issue=3 |pages=465–473|bibcode=1987ANat..130..465F |s2cid=83546424 }}</ref>

===Stasis===
Many hypotheses have been proposed to explain the [[wikt:putative|putative]] causes of stasis. Gould was initially attracted to [[I. Michael Lerner]]'s theories of developmental and genetic homeostasis. However this hypothesis was rejected over time,<ref>[[Stephen Jay Gould|S. J. Gould]] 2002. ''[[The Structure of Evolutionary Theory]]''. Cambridge, Massachusetts: Harvard University Press, p. [https://books.google.com/books?id=nhIl7e61WOUC&pg=PA39 39].</ref> as evidence accumulated against it.<ref name=Futuyma86>Futuyma, Douglas (2005). ''Evolution''. Sunderland, MA: Sinauer Associates, p. 86.</ref> Other plausible mechanisms which have been suggested include: habitat tracking,<ref>{{cite book |last1=Eldredge |first1=Niles |author1-link=Niles Eldredge |last2=Gould |first2=S. J. |title=Evolutionary Biology |chapter=Morphological Transformation, the Fossil Record, and the Mechanisms of Evolution: A Debate |author2-link=Stephen Jay Gould |date=1974 |volume=7 |pages=305–306 |doi=10.1007/978-1-4615-6944-2_8 |isbn=978-1-4615-6946-6 }}</ref><ref>Niles Eldredge (1989). ''Time Frames''. Princeton University Press, pp. 139-141.</ref> [[stabilizing selection]],<ref name="Lieberman">{{cite journal |last1=Lieberman |first1=B. S. |last2=Dudgeon |first2=S. |date=1996 |title=An evaluation of stabilizing selection as a mechanism for stasis |journal=Palaeogeogr. Palaeoclimatol. Palaeoecol. |doi=10.1016/S0031-0182(96)00097-1 |volume=127 |issue=1–4 |pages=229–238|bibcode=1996PPP...127..229L |doi-access=free }}</ref> the Stenseth-Maynard Smith stability hypothesis,<ref>{{cite journal |last1=Stenseth |first1=N. C. |last2=Maynard Smith |first2=John |author-link=John Maynard Smith |date=1984 |title=Coevolution in ecosystems: Red Queen evolution or stasis? |journal=Evolution |doi=10.2307/2408397 |volume=38 |issue=4 |pages=870–880|jstor=2408397 |pmid=28555824 }}</ref> constraints imposed by the nature of subdivided populations,<ref name="Lieberman" /> normalizing clade selection,<ref>[[George C. Williams (biologist)|Williams, G. C.]] (1992). ''Natural Selection: Domains, Levels and Challenges''. New York: Oxford University Press, p. 132.</ref> and [[koinophilia]].<ref name= "Koeslag, 1990">{{cite journal |last1=Koeslag |first1=J. H. |year=1990 |title=Koinophilia groups sexual creatures into species, promotes stasis, and stabilizes social behaviour |journal=Journal of Theoretical Biology |volume=144 |issue=1 |pages=15–35 |doi=10.1016/s0022-5193(05)80297-8 |pmid=2200930 |bibcode=1990JThBi.144...15K }}</ref><ref name= "Koeslag, 1995">Koeslag, J.H. (1995). On the engine of speciation. [http://www.sciencedirect.com/science?_ob=MImg&_imagekey=B6WMD-45S96TH-5-1&_cdi=6932&_user=613892&_orig=search&_coverDate=12%2F21%2F1995&_sk=998229995&view=c&wchp=dGLbVzb-zSkzS&md5=5ca90c43829c1ac4503e69973e9de576&ie=/sdarticle.pdf ''J. theor. Biol.'' '''177''', 401-409]</ref>

Evidence for stasis has also been corroborated from the genetics of [[sibling species]], species which are morphologically indistinguishable, but whose proteins have diverged sufficiently to suggest they have been separated for millions of years.<ref>{{cite book |last=Maynard Smith |first=John |author-link=John Maynard Smith |year=1989 |title=Did Darwin Get it Right? |location=New York |publisher=Chapman and Hall |page=126}}</ref> Fossil evidence of reproductively isolated extant species of sympatric Olive Shells (''[[Amalda]]'' sp.) also confirm morphological stasis in multiple lineages over three million years.<ref>{{Cite journal |last1=Gemmell |first1=Michael R. |last2=Trewick |first2=Steven A. |last3=Hills |first3=Simon F. K. |last4=Morgan-Richards |first4=Mary |date=2019 |title=Phylogenetic topology and timing of New Zealand olive shells are consistent with punctuated equilibrium |journal=Journal of Zoological Systematics and Evolutionary Research |volume=58 |issue=1 |pages=209–220 |doi=10.1111/jzs.12342 |s2cid=213493738 |doi-access=free }}</ref><ref>{{Cite journal |last=Michaux |first=B. |date=1989 |title=Morphological variation of species through time |journal=Biological Journal of the Linnean Society |volume=38 |issue=3 |pages=239–255 |doi=10.1111/j.1095-8312.1989.tb01577.x}}</ref>

According to Gould, "stasis may emerge as the theory's most important contribution to evolutionary science."<ref>[[Stephen Jay Gould|S. J. Gould]] 2002. ''[[The Structure of Evolutionary Theory]]''. Cambridge MA: Harvard University Press, p. [https://books.google.com/books?id=nhIl7e61WOUC&pg=PA1155 872.]</ref> Philosopher [[Kim Sterelny]] in clarifying the meaning of stasis adds, "In claiming that species typically undergo no further evolutionary change once speciation is complete, they are not claiming that there is no change at all between one generation and the next. Lineages do change. But the change between generations does not accumulate. Instead, over time, the species wobbles about its [[Phenotype|phenotypic]] mean. [[Jonathan Weiner]]'s ''[[The Beak of the Finch]]'' describes this very process."<ref>Sterelny, Kim (2007). ''[[Dawkins vs. Gould|Dawkins vs. Gould: Survival of the Fittest]]''. Cambridge, U.K.: Icon Books, p. 96.</ref>

===Hierarchical evolution===
Punctuated equilibrium has also been cited as contributing to the hypothesis that species are [[Unit of selection|Darwinian individuals]], and not just [[class (biology)|classes]], thereby providing a stronger framework for a [[Unit of selection#Species selection and selection at higher taxonomic levels|hierarchical theory]] of evolution.<ref name="CoordinatedStasis">{{cite journal |last1=Brett |first1=Carlton E. |last2=Ivany |first2=Linda C. |last3=Schopf |first3=Kenneth M. |date=1996 |title=Coordinated stasis: An overview |journal=Palaeogeography, Palaeoclimatology, Palaeoecology |doi=10.1016/S0031-0182(96)00085-5 |volume=127 |issue=1–4 |pages=1–20|bibcode=1996PPP...127....1B |doi-access=free }}</ref>