Spandrel (biology)

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 masonry above an arch, which they considered to be necessary side consequence arising from decisions concerned with the shape of the arch and the circumferential ring of the base of the dome, rather than being deliberately designed for direct utility in themselves. Properties that they singled out were the necessary number of four and their specific three-dimensional shape.

Their suggestive proposal generated a large literature of critique, which Gould characterised (Gould 1997) as being grounded in two ways. Either a terminological claim was offered that the spandrels at Basilica di San Marco were not true spandrels at all, to which he responded (Gould 1997) "The term spandrel may be extended from its particular architectural use for two-dimensional byproducts to the generality of 'spaces left over' a definition that properly includes the San Marco pendentives." Other critics claimed that they are adaptations and not by-products. Critics such as Daniel Dennett 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; he summarised his use of the term 'spandrel' in 1997: "Evolutionary biology needs such an explicit term for features arising as byproducts, rather than adaptations, whatever their subsequent exaptive utility.... Causes of historical origin must always be separated from current utilities; their conflation has seriously hampered the evolutionary analysis of form in the history of life." (Gould 1997:Abstract).

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'.