Structuralism (biology)

File:Alternatives to Darwinism.svg have been offered since the 19th century to explain how evolution took place, given that many scientists initially objected to natural selection. Many of these theories, including structural or developmental constraints, led (solid blue arrows) to some form of directed evolution (orthogenesis), with or without invoking divine control (dotted blue arrows). These theories were largely swept aside by the modern synthesis of genetics and natural selection in the early 20th century (dashed orange arrows).{{cite book |last=Bowler |first=Peter J. |author-link=Peter J. Bowler |title=The Eclipse of Darwinism: anti-Darwinian evolutionary theories in the decades around 1900 |publisher=Johns Hopkins University Press |year=1989 |orig-year=1983 |isbn=978-0-8018-4391-4 |pages=261–262, 280–281}}]]

{{anchor|Law of compensation}}

{{see|Alternatives to Darwinism}}

{{Main|Cuvier–Geoffroy debate}}

In 1830, Étienne Geoffroy Saint-Hilaire argued a structuralist case against the functionalist (teleological) position of Georges Cuvier. Geoffroy believed that homologies of structure between animals indicated that they shared an ideal pattern; these did not imply evolution but a unity of plan, a law of nature.{{efn|In this, Geoffroy's homologies were like Aristotle's forms.}} He further believed that if one part was more developed within a structure, the other parts would necessarily be reduced in compensation, as nature always used the same materials: if more of them were used for one feature, less was available for the others.{{cite web |last=Racine |first=Valerie |title=Essay: The Cuvier-Geoffroy Debate |url=https://embryo.asu.edu/pages/essay-cuvier-geoffroy-debate |publisher=The Embryo Project Encyclopedia, Arizona State University |date=7 October 2013 |access-date=10 December 2016}}

{{main|On Growth and Form}}

In his "eccentric, beautiful"{{cite book |last=Leroi |first=Armand Marie |author-link=Armand Marie Leroi |title=The Lagoon: How Aristotle Invented Science |title-link=Aristotle's Lagoon |publisher=Bloomsbury |date=2014 |isbn=978-1-4088-3622-4 |page=13}} 1917 book On Growth and Form, D'Arcy Wentworth Thompson revisited the old idea of "universal laws of form" to explain the observed forms of living organisms.{{cite book |last=Ruse |first=Michael |editor1-last=Henning |editor1-first=Brian G. |editor2-last=Scarfe |editor2-first=Adam |title=Beyond Mechanism: Putting Life Back Into Biology |date=2013 |publisher=Lexington Books |page=419 |chapter-url=https://books.google.com/books?id=3VtosxAtq-EC |chapter=17. From Organicism to Mechanism-and Halfway Back? |isbn=9780739174371 }} The science writer Philip Ball states that Thompson "presents mathematical principles as a shaping agency that may supersede natural selection, showing how the structures of the living world often echo those in inorganic nature", and notes his "frustration at the 'Just So' explanations of morphology offered by Darwinians." Instead, Ball writes, Thompson elaborates on how not heredity but physical forces govern biological form.{{cite journal |last=Ball |first=Philip |title=In retrospect: On Growth and Form |journal=Nature |date=7 February 2013|volume=494 |issue=7435 |pages=32–33 |doi=10.1038/494032a |bibcode=2013Natur.494...32B |s2cid=205076253 |doi-access=free }} The philosopher of biology Michael Ruse similarly wrote that Thompson "had little time for natural selection", certainly preferring "mechanical explanations" and possibly straying into vitalism.

{{anchor|Pneu structure}}

File:DickinsoniaCostata.jpg fossil described as a "pneu" structure with chambers inflated like a quilted air mattress. In Adolf Seilacher's structuralist view, the structure is determined mechanically by the need to distribute the tension across the surface, rather than having been guided by natural selection.]]

Like Thompson, the palaeontologist Adolf Seilacher emphasised fabricational constraints on form. He interpreted fossils such as Dickinsonia in the Ediacaran biota as "pneu" structures determined by mechanical inflation like a quilted air mattress, rather than having been driven by natural selection.{{cite book |last1=Seilacher |first1=Adolf |title=Constructional Morphology and Evolution |editor1-last=Schmidt-Kittler|editor1-first=Norbert |editor2-last=Vogel |editor2-first=Klaus |publisher=Springer |isbn=978-3-642-76158-4 |date=1991 |pages=251–271|doi=10.1007/978-3-642-76156-0_17 |chapter=Self-Organizing Mechanisms in Morphogenesis and Evolution }}{{cite journal |last1=Seilacher |first1=Adolf |author-link=Adolf Seilacher |title=Vendozoa: Organismic construction in the Proterozoic biosphere |journal=Lethaia |date=July 1989 |volume=22 |issue=3 |pages=229–239 |doi=10.1111/j.1502-3931.1989.tb01332.x|bibcode=1989Letha..22..229S }}

In his 2014 book Homology, Genes, and Evolutionary Innovation, the evolutionary biologist Günter P. Wagner argues for "the study of novelty as distinct from adaptation." He defines novelty as occurring when some part of the body develops an individual and quasi-independent existence, in other words as a distinct and recognisable structure, which he implies might occur before natural selection begins to adapt the structure for some function.Wagner, Günter P., Homology, Genes, and Evolutionary Innovation. Princeton University Press. 2014. {{ISBN|978-0691156460}}. Pages 7–38, 125{{cite book |last1=Simpson |first1=Carl |last2=Erwin |first2=Douglas H. (quoted) |title=Homology, Genes, and Evolutionary Innovation Günter P. Wagner |date=13 April 2014 |url=http://press.princeton.edu/titles/10283.html |publisher=Princeton University Press |isbn=9780691156460 |access-date=9 December 2016}} He forms a structuralist picture of evolutionary developmental biology, using empirical evidence, arguing that homology and biological novelty are key aspects requiring explanation, and that developmental bias (i.e. structural constraints on embryonic development) is a key explanation for these.{{cite journal |last1=Brown |first1=Rachael L. |title=Why development matters |journal=Biology & Philosophy |date=November 2015 |volume=30 |issue=6 |pages=889–899 |doi=10.1007/s10539-015-9488-9|s2cid=82602032 }}{{cite journal | last1=Muller | first1=G. B. | last2=Wagner | first2=G. P. | title=Novelty in Evolution: Restructuring the Concept | journal=Annual Review of Ecology and Systematics | volume=22 | issue=1 | year=1991 | issn=0066-4162 | doi=10.1146/annurev.es.22.110191.001305 | pages=229–256 | bibcode=1991AnRES..22..229M |url=http://homepage.univie.ac.at/brian.metscher/MuellerWagner91_InnovConcept.pdf |archive-url=https://web.archive.org/web/20170926235821/http://homepage.univie.ac.at/brian.metscher/MuellerWagner91_InnovConcept.pdf |archive-date=26 September 2017 |url-status=live}}

File:Fluid Mosaic.svg spontaneously self-organize. They disagree on how important self-organization is in other areas of biology.]]

The mathematical biologist Stuart Kauffman suggested in 1993 that self-organization may play a role alongside natural selection in three areas of evolutionary biology, namely population dynamics, molecular evolution, and morphogenesis. With respect to molecular biology, Kauffman has been criticised for ignoring the role of energy in driving biochemical reactions in cells, which can fairly be called self-catalysing but which do not simply self-organise.{{cite journal |last1=Fox |first1=Ronald F. |title=Review of Stuart Kauffman, The Origins of Order: Self-Organization and Selection in Evolution |journal=Biophys. J. |date=December 1993 |volume=65 |issue=6 |pages=2698–2699 |pmc=1226010 |doi=10.1016/s0006-3495(93)81321-3 |bibcode=1993BpJ....65.2698F}}

The biochemist Michael Denton has argued a structuralist case for self-organization. In a 2013 paper, he claimed that "the basic forms of the natural world—the Types—are immanent in nature, and determined by a set of special natural biological laws, the so called 'laws of form'." He asserts that these "recurring patterns and forms" are "genuine universals".{{efn|Universals are central to the ancient Greek theory, Platonic realism.{{cite SEP |url-id=plato-metaphysics |title=Plato's Middle Period Metaphysics and Epistemology |last=Silverman |first=Allan}}}} Form is in this view not shaped by natural selection, but by "self-organizing properties of particular categories of matter" and by "cosmic fine-tuning of the laws of nature".{{cite journal |last1=Denton |first1=Michael J. |title=The Types: A Persistent Structuralist Challenge to Darwinian Pan-Selectionism |journal=BIO-Complexity |date=August 2013 |volume=2013 |issue=3 |doi=10.5048/BIO-C.2013.3 |url=http://bio-complexity.org/ojs/index.php/main/article/view/BIO-C.2013.3|url-access=subscription }} Denton has been criticised by the biochemist Laurence A. Moran as anti-Darwinian and favouring creationism.{{cite web |last1=Moran |first1=Laurence A. |title=What is "Structuralism"? |url=http://sandwalk.blogspot.co.uk/2016/02/what-is-structuralism.html |website=Sandwalk (blog of a recognised expert) |access-date=9 December 2016|date=2016-02-02 }}

File:Central Park New York City New York 33.jpg. Steven J. Gould and Richard Lewontin argued that the triangular area is a byproduct of the adaptation of structures around it.]]

{{main|Spandrel (biology)}}

In 1979, influenced by Seilacher among others, the paleontologist Stephen J. Gould and the population geneticist Richard Lewontin wrote what Wagner called "the most influential structuralist manifesto", "The Spandrels of San Marco and the Panglossian Paradigm".{{cite journal |title=The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme |journal=Proc. R. Soc. Lond. B |year=1979 |volume=205 |issue=1161 |pages=581–598 |author1=Stephen Jay Gould |author2=Richard Lewontin |author-link1=Stephen Jay Gould | author-link2=Richard Lewontin |ref=Gould1979 |doi=10.1098/rspb.1979.0086|bibcode=1979RSPSB.205..581G |pmid=42062 |s2cid=2129408 }} They pointed out that biological features (like architectural spandrels) did not necessarily have adaptation as their direct cause. Instead, architects couldn't help creating small triangular areas between arches and pillars, as arches need (evolve) to be curved, and pillars need to be vertical. The resulting spandrels are exaptations, consequences of other evolutionary changes. Evolution, they argued, did not select for a protruding human chin: instead, reducing the length of the tooth row left the jaw protruding.

Extreme structuralists like Gerd B. Müller and Stuart A. Newman, inheriting the viewpoint of D'Arcy Thompson, have proposed that physical laws of structure, not genetics, govern major diversifications such as the Cambrian explosion, followed later by co-opted genetic mechanisms.{{cite journal |last1=Erwin |first1=Douglas H. |author1-link=Douglas Erwin |title=Evolutionary uniformitarianism |journal=Developmental Biology |date=September 2011 |volume=357 |issue=1 |pages=27–34 |doi=10.1016/j.ydbio.2011.01.020 |pmid=21276788 |doi-access=free |bibcode=2011DevBi.357...27E }}{{cite journal | last1=Müller | first1=Gerd B. |author1-link=Gerd B. Müller | last2=Newman | first2=Stuart A. | year=2005 | title=The innovation triad: an EvoDevo agenda | journal=J. Exp. Zool. (Mol. Dev. Evol.) | volume=304B | issue=6| pages=487–503 | pmid=16299770 | doi=10.1002/jez.b.21081| bibcode=2005JEZB..304..487M | citeseerx=10.1.1.501.1440 }} They argued further that there was a "pre-Mendelian" phase of the evolution of animals, involving physical forces, before genes took over.{{cite journal |last1=Newman |first1=Stuart A. |last2=Forgacs |first2=Gabor |last3=Müller |first3=Gerd B. |author3-link=Gerd B. Müller |title=Before programs: The physical origination of multicellular forms |journal=The International Journal of Developmental Biology |volume=50 |issue=2–3 |year=2006 |pages=289–299 |doi=10.1387/ijdb.052049sn|pmid=16479496 |doi-access=free }} Darwinian biologists freely admit that physical factors such as surface tension can cause self-assembly, but insist that genes play a crucial role. They note for example that deep homologies between widely separated groups of organisms, such as the signalling pathways and transcription factors of choanoflagellates and metazoans, demonstrate that genes have been involved throughout evolutionary history.{{cite journal |last1=King |first1=Nicole |title=The Unicellular Ancestry of Animal Development |journal=Developmental Cell |volume=7 |issue=3 |year=2004 |pages=313–325 |doi=10.1016/j.devcel.2004.08.010|pmid=15363407 |doi-access=free }}

File:Giant Puffer fish skin pattern.JPG like those on the skin of the Giant pufferfish can be created by spatial oscillations of chemical signals.]]

{{main|Morphogenetic field}}

What Wagner calls "a fringe movement in evolutionary biology", the form of structuralism exemplified by Brian Goodwin,{{cite book |last=Goodwin |first=Brian |author-link=Brian Goodwin |title=Beyond the Darwinian Paradigm: Understanding Biological Forms |work=Evolution: The First Four Billion Years |editor1=Ruse, Michael |editor1-link=Michael Ruse |editor2=Travis, Joseph |publisher=Harvard University Press |date=2009 |isbn=978-0674062214}} effectively denies that natural selection is important, or at least that biological complexity could be reduced to natural selection. This led to conflict with Darwinists such as Richard Dawkins.[https://www.theguardian.com/theguardian/2009/aug/09/brian-goodwin-obituary Brian Goodwin obituary - The Guardian, 9 August 2009] Goodwin related the old concept of a morphogenetic field to the spatial distribution of chemical signals in a developing embryo.{{cite journal | last1=Dickinson | first1=W. Joseph | year=1998 | title=Form and Transformation: Generative and Relational Principles in Biology. by Gerry Webster; Brian Goodwin | journal=The Quarterly Review of Biology | volume=73 | issue=1| pages=62–63 | doi=10.1086/420070}} He demonstrated with a mathematical model that a variety of patterns could be formed by choosing parameter values to set up either static geometric patterns or dynamic oscillations,{{cite journal |last1=Price |first1=Catherine S. C. |last2=Goodwin |first2=Brian |title=Structurally Unsound |journal=Evolution |volume=49 |issue=6 |year=1995 |pages=1298 |doi=10.2307/2410461|jstor=2410461 }}{{cite journal | last1=Wake | first1=David B. | author-link=David B. Wake | year=1996 | title=How the Leopard Changed Its Spots: The Evolution of Complexity by Brian Goodwin | journal=American Scientist | volume=84 | issue=3| pages=300–301 |jstor=29775684 }} implying that the signalling system involved was somehow an alternative to natural selection. Dawkins commented "He thinks he's anti-Darwinian, although he can't be, because he has no alternative explanation."{{cite web |last=Dawkins|first=Richard |author-link=Richard Dawkins |title=Chapter 3 "A Survival Machine" |url=https://www.edge.org/conversation/richard_dawkins-chapter-3-a-survival-machine |publisher=Edge |access-date=11 February 2018 |date=1 May 1996}}

File:CollapsedtreeLabels-simplified, Animals emphasized.svgs, but that animals (emphasized) form only a small portion of the tree of life.]]

While agreeing that pattern formation mechanisms such as those described by Goodwin exist, the biologists Richard Dawkins, Stephen J. Gould, Lynn Margulis, and Steve Jones have criticised Goodwin for suggesting that chemical signalling forms an alternative to natural selection.

Moran, a "skeptical biochemist", comments that 'structuralism' is a "new buzzword ... guaranteed to impress the creationist crowd because nobody understands what it means but it sounds very 'sciency' and philosophical." The philosopher of science Paul E. Griffiths writes that structuralists "view this structuring of the space of biological possibility as part of the fundamental physical structure of nature. But the phenomena of phylogenetic inertia and developmental constraint do not support this interpretation. These phenomena show that the evolutionary pathways available to an organism are a function of the developmental structure of the organism."{{cite journal |last1=Griffiths |first1=Paul E. |author-link=Paul E. Griffiths |title=Darwinism, Process Structuralism, and Natural Kinds |journal=Philosophy of Science |volume=63 |year=1996 |issue=5 |pages=S1–S9 |doi=10.1086/289930 |jstor=188505|s2cid=146674266 }}

Moran summarizes: "There's nothing in science that supports the views of the structuralists. We have perfectly good explanations for why bumblebees are different than mushrooms and why all vertebrates have vertebrae and not exoskeletons. There's no evidence to support the idea that if you replay the tape of life it will come out looking anything like what we see today. You can be confident that when you visit another planet you will not find vertebrates."

The evolutionary developmental biologist Lewis Held wrote that "The notion that aspects of anatomy can be explained by physical forces (like expansion cracking) was advocated ~ 100 years earlier in D'Arcy Thompson's 1917 On Growth and Form and in Theodore Cook's 1914 book The Curves of Life.{{efn|The artist Theodore Cook's [https://archive.org/details/cu31924028937179 The Curves of Life], Constable, 1914 to some extent anticipated D'Arcy Thompson, exploring spirals in art and nature.}} Over the intervening century, various traits have been proposed to arise mechanically rather than genetically: brain convolutions, cartilage condensations, flower corrugations, tooth cusps, and fish otoliths. To this kooky list we can now add the crooked smile of the crocodile, or at least the cracked skin that surrounds it."{{efn |Held's final point was that the cracks in the crocodile's skin are genuinely explained by cracking, unlike all the other examples he lists.}}{{cite book |last=Held |first=Lewis Irving |title=How the snake lost its legs: curious tales from the frontier of evo-devo |title-link=How the Snake Lost its Legs |publisher=Cambridge University Press |location=Cambridge, United Kingdom New York |year=2014 |isbn=978-1-107-62139-8 |page=121}}

• Alternatives to Darwinism
• Eclipse of Darwinism
• Extended evolutionary synthesis
• Orthogenesis

{{notelist}}

{{reflist|30em}}

{{Evolution}}

Category:Non-Darwinian evolution

Category:Philosophy of biology

Category:Evolutionary biology