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Spandrel is a term used in evolutionary biology to describe a phenotypic characteristic that is a byproduct of the evolution of some other character, rather than a direct product of adaptive selection. The term was coined by the Harvardmarker 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). In this paper the authors employed the analogy of spandrels in Renaissance architecture: curved areas of masonry between arches supporting a dome that arise as a consequence of decisions about the shape of the arches and the base of the dome, rather than being designed for the artistic purposes for which they were often employed. Properties that they singled out were the necessary number of four and their specific three-dimensional shape. In the biological sense, a 'spandrel' or 'exaptation' (as Gould and Lewontin referred to them) might be the result of an architectural requirement inherent in the Bauplan of an organism, or to some other constraint on adaptive evolution.

Their suggestive proposal generated a large literature of critique, which Gould characterised (Gould 1997) as being grounded in two ways. First, a terminological claim was offered that the "spandrels" of Basilica di San Marcomarker were not spandrels at all, but rather were pendentives. Gould (1997) responded, "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, such as Daniel Dennett, further claimed that these pendentives are not merely architectural by-products as Gould and Lewontin supposed. Dennett argues that alternatives to pendentives, such as corbels or squinches would have served equally well from an architectural standpoint, but pendentives were deliberately selected due to their aesthetic value. Critics argue that Lewontin and Gould's oversight in this regard illustrates their 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'.

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