Editing Living fossil (section)

== Evolution and living fossils ==
The term ''living fossil'' is usually reserved for species or larger clades that are exceptional for their lack of morphological diversity and their exceptional conservatism, and several hypotheses could explain morphological stasis on a geologically long time-scale. Early analyses of evolutionary rates emphasized the persistence of a taxon rather than rates of evolutionary change.<ref>{{cite book |last1=Simpson |first1=George |title=The Major Features of Evolution |publisher=Columbia University Press |url=https://archive.org/details/majorfeaturesofe0000simp |url-access=registration |date=1953 |location=New York}}</ref> Contemporary studies instead analyze rates and modes of phenotypic evolution, but most have focused on clades that are thought to be adaptive radiations rather than on those thought to be living fossils. Thus, very little is presently known about the evolutionary mechanisms that produce living fossils or how common they might be. Some recent studies have documented exceptionally low rates of ecological and phenotypic evolution despite rapid speciation.<ref>{{cite journal |last1=Kozack |first1=K. |last2=Weisrock |first2=D. W. |last3=Larson |first3=A. |title=Rapid lineage accumulation in a non-adaptive radiation: phylogenetic analysis of diversification rates in eastern North American woodland salamanders (Plethodontidae: Plethodon) |journal=Proceedings of the Royal Society B: Biological Sciences |date=2006 |volume=273 |issue=1586 |pages=539–546 |doi=10.1098/rspb.2005.3326 |pmid=16537124 |pmc=1560065}}</ref> This has been termed a "non-adaptive radiation" referring to diversification not accompanied by adaptation into various significantly different niches.<ref>{{cite journal |last=Gittenberger |first=E. |title=What about non-adaptive radiation? |journal=Biological Journal of the Linnean Society |date=1991 |volume=43 |issue=4 |pages=263–272 |doi=10.1111/j.1095-8312.1991.tb00598.x}}</ref> Such radiations are explanation for groups that are morphologically conservative. Persistent adaptation within an adaptive zone is a common explanation for morphological stasis.<ref>{{cite journal |last1=Estes |first1=Suzanne |last2=Arnold |first2=Stevan |title=Resolving the paradox of stasis: Models with stabilizing selection explain evolutionary divergence on all timescales |journal=The American Naturalist |date=2007 |volume=169 |issue=2 |pages=227–244 |doi=10.1086/510633 |pmid=17211806 |s2cid=18734233}}</ref> The subject of very low evolutionary rates, however, has received much less attention in the recent literature than that of high rates.

Living fossils are not expected to exhibit exceptionally low rates of molecular evolution, and some studies have shown that they do not.<ref>{{cite web |title=Diversification in Ancient Tadpole Shrimps Challenges the Term 'Living Fossil' |work=Science Daily |url=https://www.sciencedaily.com/releases/2013/04/130402091641.htm |access-date=2 April 2013 |date=2 April 2013}}</ref><ref name=Falsity>{{Cite web |author=Ed Yong |date=Apr 2, 2013 |title=The Falsity of Living Fossils |url=http://www.the-scientist.com/?articles.view/articleNo/34927/title/The-Falsity-of-Living-Fossils/ |website=The Scientist |access-date=2015-12-03 |url-access=registration}}</ref> For example, on tadpole shrimp (''[[Triops]]''), one article notes, "Our work shows that organisms with conservative body plans are constantly radiating, and presumably, adapting to novel conditions... I would favor retiring the term 'living fossil' altogether, as it is generally misleading."<ref name=Falsity/> Some scientists instead prefer a new term stabilomorph, being defined as "an effect of a specific formula of adaptative strategy among organisms whose taxonomic status does not exceed genus-level. A high effectiveness of adaptation significantly reduces the need for differentiated phenotypic variants in response to environmental changes and provides for long-term evolutionary success."<ref>{{Cite journal |last1=Kin |first1=Adrian |last2=Błażejowski |first2=Błażej |date=2014-10-02 |title=The Horseshoe Crab of the Genus Limulus: Living Fossil or Stabilomorph? |journal=PLOS ONE |language=en |volume=9 |issue=10 |at=e108036 |doi=10.1371/journal.pone.0108036 |issn=1932-6203 |pmc=4183490 |pmid=25275563 |bibcode=2014PLoSO...9j8036K |doi-access=free}}</ref>

The question posed by several recent studies pointed out that the morphological conservatism of coelacanths is not supported by paleontological data.<ref>{{cite journal | vauthors = Friedman M, Coates MI, Anderson P | year = 2007 | title = First discovery of a primitive coelacanth fin fills a major gap in the evolution of lobed fins and limbs | journal = Evolution & Development | volume = 9 | issue = 4| pages = 329–37 | doi = 10.1111/j.1525-142X.2007.00169.x | pmid=17651357| s2cid = 23069133 }}</ref><ref>{{cite journal | vauthors = Friedman M, Coates MI | year = 2006 | title = A newly recognized fossil coelacanth highlights the early morphological diversification of the clade | journal = Proc. R. Soc. B | volume = 273 | issue = 1583| pages = 245–250 | doi = 10.1098/rspb.2005.3316 | pmid=16555794 | pmc=1560029}}</ref> In addition, it was shown recently that studies concluding that a slow rate of molecular evolution is linked to morphological conservatism in coelacanths are biased by the ''[[A priori and a posteriori|a priori]]'' hypothesis that these species are 'living fossils'.<ref name="Casane"/> Accordingly, the genome stasis hypothesis is challenged by the recent finding that the genome of the two extant coelacanth species ''L. chalumnae'' and ''L. menadoensis'' contain multiple species-specific insertions, indicating transposable element recent activity and contribution to post-speciation genome divergence.<ref name="Naville">{{cite journal | vauthors = Naville M, Chalopin D, Casane D, Laurenti P, Volff JN |date=July–August 2015 | title = The coelacanth: Can a "living fossil" have active transposable elements in its genome? | journal = Mobile Genetic Elements | volume = 5| issue = 4| pages = 55–9| doi = 10.1080/2159256X.2015.1052184 | pmid=26442185| pmc=4588170}}</ref> Such studies, however, challenge only a genome stasis hypothesis, not the hypothesis of exceptionally low rates of phenotypic evolution.