Spandrel is a term used in evolutionary biology describing a phenotypic characteristic that is considered to have developed during evolution as a side-effect of a true adaptation, specifically arising from a correlation of growth, rather than arising from natural selection. The term developed from an analogy of causal relationships between forms found in architecture and those found in biology. The term was coined by the Harvard paleontologist Stephen Jay Gould and population geneticist Richard Lewontin in their influential paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" (1979). They drew the analogy with spandrels in Renaissance architecture, which are curved areas of roof above an arch, which they considered to be side consequence arising from the shape of the arch rather than being deliberately designed as an oddly-shaped piece of ceiling.

Critics such as Daniel Dennett—who hold strong selectionist preferences, favoring adaptive explanations over structural ones—argue that the architectural spandrels (specifically called pendentives in three-dimensional space) of San Marco are not the undesigned spaces between design features that Gould and Lewontin suppose, but were deliberately chosen as solutions to an architectural problem, where alternatives, such as corbels or squinches, were selected for their aesthetic value. Critics argue that this misidentification illustrates Lewontin and Gould's underestimation of the pervasiveness of adaptations found in nature. Gould responded that critics ignore that later selective value is a separate issue from origination as necessary consequences of structure.

The linguist Noam Chomsky has argued that the 'language faculty' that plays a central role in his theory of Universal Grammar may have evolved as a spandrel: in this view, human language originated as a by-product of the general recursion faculty of the human mind, which would have evolved without any evolutionary 'reasons'.

References

• Stephen Jay Gould and Richard C. Lewontin. "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme" Proc. Roy. Soc. London B 205 (1979) pp. 581-598
• Gould S. J. (1997). "The exaptive excellence of spandrels as a term and prototype" Proceedings of the National Academy of Sciences USA. 94: 10750-10755.
• Gould S. J. (2002). The Structure of Evolutionary Theory. Belknap Press of Harvard University Press, Cambridge, Mass.
• Phillip Stevens Thurtle. "The G Files: Linking 'The Selfish Gene' And 'The Thinking Reed'"
• Daniel Dennett (1995). Darwin's Dangerous Idea: Evolution and the Meanings of Life. Simon & Schuster. ISBN 0-684-82471-X.
• Marc D. Hauser, Noam Chomsky, and W. Tecumseh Fitch (2002). "The Faculty of Language: What Is It, Who Has It, and How Did It Evolve?" Science 298:1569-1579.
• Robert Mark (1996). "Architecture and Evolution" American Scientist (July-August): 383-389.