Editing Evolutionary developmental biology (section)

===Evolutionary morphology===
{{Further|Morphology (biology)|Body plan}}

[[File:Comparison of Three Invertebrate Chordates.svg|thumb|left|A. [[Lancelet]] (a chordate), B. Larval [[tunicate]], C. Adult tunicate. [[Alexander Kovalevsky|Kowalevsky]] saw that the [[notochord]] (1) and gill slits (5) are shared by tunicates and vertebrates.]]

From the early 19th century through most of the 20th century, [[embryology]] faced a mystery. Animals were seen to develop into adults of widely differing [[body plan]], often through similar stages, from the egg, but zoologists knew almost nothing about how [[embryogenesis|embryonic development]] was controlled at the [[molecular biology|molecular level]], and therefore equally little about how [[developmental biology|developmental processes]] had evolved.<ref name="CarrollNatHist">{{Cite web |last=Carroll |first=Sean B. |author-link=Sean B. Carroll |title=The Origins of Form |url=http://www.naturalhistorymag.com/features/061488/the-origins-of-form |access-date=9 October 2016 |website=Natural History |quote=Biologists could say, with confidence, that forms change, and that natural selection is an important force for change. Yet they could say nothing about how that change is accomplished. How bodies or body parts change, or how new structures arise, remained complete mysteries.}}</ref> [[Charles Darwin]] argued that a shared embryonic structure implied a common ancestor. For example, Darwin cited in his 1859 book ''[[On the Origin of Species]]'' the [[shrimp]]-like [[larva]] of the [[barnacle]], whose [[Sessility (motility)|sessile]] adults looked nothing like other [[arthropods]]; [[Carl Linnaeus|Linnaeus]] and [[Cuvier]] had classified them as [[mollusc]]s.<ref name="Gilbert2003">{{Cite journal |last=Gilbert |first=Scott F. |author-link=Scott F. Gilbert |date=2003 |title=The morphogenesis of evolutionary developmental biology |url=http://www.chd.ucsd.edu/_files/fall2008/Gilbert.2003.IJDB.pdf |journal=International Journal of Developmental Biology |volume=47 |issue=7–8 |pages=467–477 |pmid=14756322}}</ref><ref>{{Cite book |last=Darwin |first=Charles |author-link=Charles Darwin |url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F373&pageseq=457 |title=On the Origin of Species |publisher=John Murray |year=1859 |isbn=978-0-8014-1319-3 |location=London |pages=439–440 |quote=Cirripedes afford a good instance of this: even the illustrious Cuvier did not perceive that a barnacle was, as it certainly is, a crustacean; but a glance at the larva shows this to be the case in an unmistakeable manner.}}</ref> Darwin also noted [[Alexander Kovalevsky|Alexander Kowalevsky]]'s finding that the [[tunicate]], too, was not a mollusc, but in its larval stage had a [[notochord]] and pharyngeal slits which developed from the same germ layers as the equivalent structures in [[vertebrate]]s, and should therefore be grouped with them as [[chordates]].<ref name=Gilbert2003/><ref>{{Cite web |last=Richmond |first=Marsha |date=January 2007 |title=Darwin's Study of the Cirripedia |url=http://darwin-online.org.uk/EditorialIntroductions/Richmond_cirripedia.html |access-date=9 October 2016 |publisher=Darwin Online}}</ref>

19th century zoology thus converted [[embryology]] into an evolutionary science, connecting [[phylogeny]] with [[homology (biology)|homologies]] between the germ layers of embryos. Zoologists including [[Fritz Müller]] proposed the use of embryology to discover [[phylogeny|phylogenetic relationships]] between taxa. Müller demonstrated that [[crustaceans]] shared the [[Crustacean larvae#Nauplius|Nauplius]] larva, identifying several parasitic species that had not been recognized as crustaceans. Müller also recognized that [[natural selection]] must act on larvae, just as it does on adults, giving the lie to recapitulation, which would require larval forms to be shielded from natural selection.<ref name=Gilbert2003/> Two of Haeckel's other ideas about the evolution of development have fared better than recapitulation: he argued in the 1870s that changes in the timing ([[heterochrony]]) and changes in the positioning within the body ([[heterotopy]]) of aspects of embryonic development would drive evolution by changing the shape of a descendant's body compared to an ancestor's. It took a century before these ideas were shown to be correct.<ref name="ReferenceA">{{Cite journal |last=Hall |first=B. K. |date=2003 |title=Evo-Devo: evolutionary developmental mechanisms |journal=International Journal of Developmental Biology |volume=47 |issue=7–8 |pages=491–495 |pmid=14756324}}</ref><ref>{{Cite book |last=Ridley |first=Mark |author-link=Mark Ridley (zoologist) |url=http://www.blackwellpublishing.com/ridley/ |title=Evolution |publisher=Wiley-Blackwell |year=2003 |isbn=978-1-4051-0345-9}}</ref>{{sfn|Gould|1977|pp=221–222}}

[[File:Giant Pufferfish skin pattern detail.jpg|thumb|upright=0.8|Turing's 1952 paper explained mathematically how patterns such as stripes and spots, as in the [[giant pufferfish]], may arise, without molecular evidence.<ref name="Turing 1952"/>]]

In 1917, [[D'Arcy Thompson]] wrote [[On Growth and Form|a book on the shapes of animals]], showing with simple [[Mathematical biology|mathematics]] how small changes to [[parameter]]s, such as the angles of a [[gastropod]]'s spiral shell, can radically alter [[Morphology (biology)|an animal's form]], though he preferred a mechanical to evolutionary explanation.<ref name="Ball">{{Cite journal |last=Ball |first=Philip |author-link=Philip Ball |date=7 February 2013 |title=In retrospect: On Growth and Form |journal=Nature |volume=494 |issue=32–33 |pages=32–33 |bibcode=2013Natur.494...32B |doi=10.1038/494032a |s2cid=205076253 |doi-access=free}}</ref><ref name="Shalizi">{{Cite web |last=Shalizi |first=Cosma |title=Review: The Self-Made Tapestry by Philip Ball |url=http://bactra.org/reviews/self-made-tapestry/ |access-date=14 October 2016 |publisher=University of Michigan}}</ref> But without molecular evidence, progress stalled.<ref name=Gilbert2003/>

In 1952, [[Alan Turing]] published his paper "[[The Chemical Basis of Morphogenesis]]", on the development of patterns in animals' bodies. He suggested that [[morphogenesis]] could be explained by a [[reaction–diffusion system]], a system of reacting chemicals able to diffuse through the body.<ref name="Turing 1952">{{Cite journal |last=Turing |first=Alan M. |author-link=Alan Turing |date=14 August 1952 |title=The Chemical Basis of Morphogenesis |journal=Philosophical Transactions of the Royal Society of London B |volume=237 |pages=37–72 |bibcode=1952RSPTB.237...37T |doi=10.1098/rstb.1952.0012 |s2cid=120437796 |number=641}}</ref> He modelled catalysed chemical reactions using [[partial differential equations]], showing that patterns emerged when the chemical reaction produced both a [[Catalysis|catalyst]] (A) and an [[enzyme inhibitor|inhibitor]] (B) that slowed down production of A. If A and B then diffused at different rates, A dominated in some places, and B in others. The Russian biochemist [[Boris Pavlovich Belousov|Boris Belousov]] had run experiments with similar results, but was unable to publish them because scientists thought at that time that creating visible order violated the [[second law of thermodynamics]].<ref>{{Cite book |last=Gribbin |first=John |author-link=John Gribbin |title=Deep Simplicity |publisher=Random House |year=2004 |page=126}}</ref>