Editing Evolutionary developmental biology (section)

==The origins of novelty==
{{Further|History of evolutionary thought#21st century}}

Among the more surprising and, perhaps, counterintuitive (from a [[Modern synthesis (20th century)|neo-Darwinian]] viewpoint) results of recent research in evolutionary developmental biology is that the diversity of [[body plan]]s and [[Morphology (biology)|morphology]] in organisms across many [[phylum|phyla]] are not necessarily reflected in diversity at the level of the sequences of genes, including those of the developmental genetic toolkit and other genes involved in development. Indeed, as John Gerhart and Marc Kirschner have noted, there is an apparent paradox: "where we most expect to find variation, we find conservation, a lack of change".<ref>{{Cite book |last=Gerhart |first=John |title=Cells, Embryos and Evolution |last2=Kirschner |first2=Marc |publisher=Blackwell Science |year=1997 |isbn=978-0-86542-574-3}}</ref> So, if the observed morphological novelty between different [[clade]]s does not come from changes in gene sequences (such as by [[mutation (biology)|mutation]]), where does it come from? Novelty may arise by mutation-driven changes in [[gene regulation]].<ref name="Carroll_2008">{{Cite journal |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |date=2008 |title=Evo-Devo and an Expanding Evolutionary Synthesis: A Genetic Theory of Morphological Evolution |journal=Cell |volume=134 |issue=1 |pages=25–36 |doi=10.1016/j.cell.2008.06.030 |pmid=18614008 |s2cid=2513041 |doi-access=free}}</ref><ref>{{Cite book |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |title=From DNA to Diversity: Molecular Genetics and the Evolution of Animal Design&nbsp;— Second Edition |last2=Grenier |first2=Jennifer K. |last3=Weatherbee |first3=Scott D. |publisher=Blackwell Publishing |year=2005 |isbn=978-1-4051-1950-4}}</ref><ref>{{Cite journal |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |year=2000 |title=Endless forms: the evolution of gene regulation and morphological diversity |journal=[[Cell (journal)|Cell]] |volume=101 |issue=6 |pages=577–80 |doi=10.1016/S0092-8674(00)80868-5 |pmid=10892643 |s2cid=12375030 |doi-access=free}}</ref><ref name="Moczek_2015">{{Cite journal |last=Moczek, Armin P. |display-authors=etal |date=2015 |title=The Significance and Scope of Evolutionary Developmental Biology: A Vision for the 21st Century |url=http://www.extavourlab.com/pdfs/papers/2015_Moczek_EvolDev.pdf |journal=Evolution & Development |volume=17 |issue=3 |pages=198–219 |doi=10.1111/ede.12125 |pmid=25963198 |s2cid=9652129 |archive-url=https://web.archive.org/web/20160417114046/http://www.extavourlab.com/pdfs/papers/2015_Moczek_EvolDev.pdf |archive-date=17 April 2016 |access-date=27 November 2015 |hdl-access=free |hdl=2027.42/111768}}</ref>

===Variations in the toolkit===
{{Multiple image 
| total_width = 300px
| image1      = Heliconius erato Richard Bartz.jpg 
| caption1    = ''[[Heliconius erato]]''
| image2      = Heliconius melpomene 2b Richard Bartz.jpg 
| caption2    = ''[[Heliconius melpomene]]''
| footer      = Different species of ''Heliconius'' butterfly have [[parallel evolution|independently evolved]] similar patterns, apparently both [[facilitated variation|facilitated and constrained]] by the available [[developmental-genetic toolkit]] genes controlling wing [[pattern formation]]. 
}}

Variations in the toolkit may have produced a large part of the morphological evolution of animals. The toolkit can drive evolution in two ways. A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's [[large ground-finch]] was enlarged by the ''[[Bone morphogenetic protein|BMP]]'' gene,<ref>{{Cite journal |last=Abzhanov |first=A. |last2=Protas, M. |last3=Grant, B.R. |last4=Grant, P.R. |last5=Tabin, C.J. |year=2004 |title=Bmp4 and Morphological Variation of Beaks in Darwin's Finches |journal=Science |volume=305 |issue=5689 |pages=1462–1465 |bibcode=2004Sci...305.1462A |doi=10.1126/science.1098095 |pmid=15353802 |s2cid=17226774}}</ref> or when snakes lost their legs as ''distal-less'' became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs.<ref>{{Cite journal |last=Cohn, M.J. |last2=Tickle, C. |year=1999 |title=Developmental basis of limblessness and axial patterning in snakes |journal=Nature |volume=399 |issue=6735 |pages=474–479 |bibcode=1999Natur.399..474C |doi=10.1038/20944 |pmid=10365960 |s2cid=4309833}}</ref> Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, ''distal-less'', which controls such diverse structures as the mandible in vertebrates,<ref name="Beverdam2002">{{Cite journal |last=Beverdam |first=A. |last2=Merlo, G.R. |last3=Paleari, L. |last4=Mantero, S. |last5=Genova, F. |last6=Barbieri, O. |last7=Janvier, P. |last8=Levi, G. |date=August 2002 |title=Jaw Transformation With Gain of Symmetry After DLX5/DLX6 Inactivation: Mirror of the Past? |url=https://iris.unito.it/bitstream/2318/87307/1/Beverdam_2002.pdf |journal=Genesis |volume=34 |issue=4 |pages=221–227 |doi=10.1002/gene.10156 |pmid=12434331 |s2cid=19592597 |hdl-access=free |hdl=2318/87307}}</ref><ref name="Depew2002">{{Cite journal |last=Depew, M.J. |last2=Lufkin, T. |last3=Rubenstein, J.L. |date=October 2002 |title=Specification of jaw subdivisions by DLX genes |journal=Science |volume=298 |issue=5592 |pages=381–385 |doi=10.1126/science.1075703 |pmid=12193642 |s2cid=10274300 |doi-access=free}}</ref> legs and antennae in the fruit fly,<ref>{{Cite journal |last=Panganiban |first=Grace |last2=Rubenstein |first2=John L. R. |date=2002 |title=Developmental functions of the Distal-less/Dlx homeobox genes |url=http://dev.biologists.org/content/129/19/4371 |journal=Development |volume=129 |issue=19 |pages=4371–4386 |doi=10.1242/dev.129.19.4371 |pmid=12223397}}</ref> and [[eyespot (mimicry)|eyespot pattern]] in [[butterfly]] [[wing]]s.<ref>{{Cite journal |last=Beldade, P. |last2=Brakefield, P.M. |last3=Long, A.D. |year=2002 |title=Contribution of Distal-less to quantitative variation in butterfly eyespots |journal=Nature |volume=415 |issue=6869 |pages=315–318 |doi=10.1038/415315a |pmid=11797007 |s2cid=4430563}}</ref> Given that small changes in toolbox genes can cause significant changes in body structures, they have often enabled the same function [[Convergent evolution|convergently]] or [[Parallel Evolution|in parallel]]. ''distal-less'' generates wing patterns in the butterflies ''[[Heliconius erato]]'' and ''[[Heliconius melpomene]]'', which are [[Müllerian mimicry|Müllerian mimics]]. In so-called [[facilitated variation]],<ref>{{Cite journal |last=Gerhart |first=John |last2=Kirschner |first2=Marc |year=2007 |title=The theory of facilitated variation |journal=Proceedings of the National Academy of Sciences |volume=104 |issue=suppl1 |pages=8582–8589 |bibcode=2007PNAS..104.8582G |doi=10.1073/pnas.0701035104 |pmc=1876433 |pmid=17494755 |doi-access=free}}</ref> their wing patterns arose in different evolutionary events, but are controlled by the same genes.<ref>{{Cite journal |last=Baxter, S.W. |last2=Papa, R. |last3=Chamberlain, N. |last4=Humphray, S.J. |last5=Joron, M. |last6=Morrison, C. |last7=ffrench-Constant, R.H. |last8=McMillan, W.O. |last9=Jiggins, C.D. |year=2008 |title=Convergent Evolution in the Genetic Basis of Mullerian Mimicry in Heliconius Butterflies |journal=Genetics |volume=180 |issue=3 |pages=1567–1577 |doi=10.1534/genetics.107.082982 |pmc=2581958 |pmid=18791259}}</ref> Developmental changes can contribute directly to [[speciation]].<ref>{{Cite journal |last=Pennisi, E. |author-link=Elizabeth Pennisi |year=2002 |title=Evolutionary Biology:Evo-Devo Enthusiasts Get Down to Details |journal=Science |volume=298 |issue=5595 |pages=953–955 |doi=10.1126/science.298.5595.953 |pmid=12411686 |s2cid=154023266}}</ref>

===Consolidation of epigenetic changes===
{{Main|Genetic assimilation|Transgenerational epigenetic inheritance}}
{{Further|Extended evolutionary synthesis}}

Evolutionary innovation may sometimes begin [[Lamarckism|in Lamarckian style]] with [[Epigenetics|epigenetic]] alterations of gene regulation or [[morphogenesis|phenotype generation]], subsequently [[genetic assimilation|consolidated by changes at the gene level]]. Epigenetic changes include modification of DNA by reversible methylation,<ref>{{Cite book |last=Jablonka |first=Eva |author-link=Eva Jablonka |title=Epigenetic Inheritance and Evolution: The Lamarckian Dimension |last2=Lamb |first2=Marion |publisher=Oxford University Press |year=1995 |isbn=978-0-19-854063-2 |location=Oxford, New York}}</ref> as well as nonprogrammed remoulding of the organism by physical and other environmental effects due to the inherent [[phenotypic plasticity|plasticity]] of developmental mechanisms.<ref name="West-Eberhard 2003">{{Cite book |last=West-Eberhard |first=Mary Jane |author-link=Mary Jane West-Eberhard |title=Developmental plasticity and evolution |publisher=Oxford University Press |year=2003 |isbn=978-0-19-512235-0 |location=New York}}</ref> The biologists [[Stuart Newman|Stuart A. Newman]] and [[Gerd Müller (theoretical biologist)|Gerd B. Müller]] have suggested that organisms early in the history of multicellular life were more susceptible to this second category of epigenetic determination than are modern organisms, providing a basis for early [[macroevolution]]ary changes.<ref>{{Cite book |title=Origination of Organismal Form: Beyond the Gene in Developmental and Evolutionary Biology |title-link=Origination of Organismal Form |publisher=MIT Press |year=2003 |editor-last=Müller, Gerd B. |editor-link=Gerd B. Müller |editor-last2=Newman, Stuart A.}}</ref>

===Developmental bias===
{{Main|Developmental bias}}

[[File:Chilipoda- Geophilomorpha (3309242471).jpg|thumb|Among the [[centipede]]s, all members of the [[Geophilomorpha]] are constrained by a developmental bias to have an odd number of segments, whether as few as 27 or as many as 191.]]

Development in specific lineages can be biased either positively, towards a given trajectory or phenotype,{{efn|Positive bias is sometimes called developmental drive.<ref name=Arthur2001/>}} or negatively, away from producing certain types of change; either may be absolute (the change is always or never produced) or relative. Evidence for any such direction in evolution is however hard to acquire and can also result from developmental constraints that limit diversification.<ref name="Drost 2017 69–75"/> For example, in the [[gastropod]]s, the snail-type shell is always built as a tube that grows both in length and in diameter; selection has created a wide variety of shell shapes such as flat spirals, [[cowrie]]s and tall turret spirals within these constraints. Among the [[centipede]]s, the [[Lithobiomorpha]] always have 15 trunk segments as adults, probably the result of a developmental bias towards an odd number of trunk segments. Another centipede order, the [[Geophilomorpha]], the number of segments varies in different species between 27 and 191, but the number is always odd, making this an absolute constraint; almost all the odd numbers in that range are occupied by one or another species.<ref name="Arthur2001">{{Cite journal |last=Arthur |first=W. |date=July 2001 |title=Developmental drive: an important determinant of the direction of phenotypic evolution |journal=Evolution & Development |volume=3 |issue=4 |pages=271–278 |doi=10.1046/j.1525-142x.2001.003004271.x |pmid=11478524 |s2cid=41698287}}</ref><ref name="Arthur2002Centipedes">{{Cite journal |last=Arthur |first=W. |date=October 2002 |title=The interaction between developmental bias and natural selection: from centipede segments to a general hypothesis |journal=Heredity |volume=89 |issue=4 |pages=239–246 |doi=10.1038/sj.hdy.6800139 |pmid=12242638 |doi-access=free}}</ref><ref name="ChipmanArthurAkam2004">{{Cite journal |last=Chipman |first=Ariel D. |last2=Arthur |first2=Wallace |last3=Akam |first3=Michael |date=July 2004 |title=A Double Segment Periodicity Underlies Segment Generation in Centipede Development |journal=Current Biology |volume=14 |issue=14 |pages=1250–1255 |doi=10.1016/j.cub.2004.07.026 |pmid=15268854 |s2cid=2371623 |doi-access=free}}</ref>