page_title: Structuralism (biology)
section_title: Gould and Lewontin's spandrels
hierarchical_section_title: Structuralism (biology) / History / Gould and Lewontin's spandrels
caption_attribution_description: English: Detail view showing abutment and naturalistic detailing of cast iron openwork spandrel - Bow Bridge, Spanning the Lake
context_page_description: Biological or process structuralism, also known as formalism, is a school of biological thought that objects to an exclusively Darwinian or adaptationist explanation of natural selection such as is described in the 20th century's modern synthesis. It proposes instead that evolution is guided differently, basically by more or less physical forces which shape the development of an animal's body, and sometimes implies that these forces supersede selection altogether.
  Structuralists have proposed different mechanisms that might have guided the formation of body plans. Before Darwin, Étienne Geoffroy Saint-Hilaire argued that animals shared homologous parts, and that if one was enlarged, the others would be reduced in compensation. After Darwin, D'Arcy Thompson hinted at vitalism and offered geometric explanations in his classic 1917 book On Growth and Form. Adolf Seilacher suggested mechanical inflation for "pneu" structures in Ediacaran biota fossils such as Dickinsonia. Günter P. Wagner argued for developmental bias, structural constraints on embryonic development.
context_section_description: 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". 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.
caption: An ornamented bridge spandrel. Steven J. Gould and Richard Lewontin argued that the triangular area is a byproduct of the adaptation of structures around it.
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