Spandrel

A spandrel is a phenotypic characteristic that evolved as a side effect of a true adaptation. The term is originally from architecture, and the meaning in evolutionary biology is analogous. The biological term spandrel was popularized by Stephen Jay Gould and Richard Lewontin in their influential paper "The Spandrels of San Marco and the Panglossian Paradigm: A Critique of the Adaptationist Programme". In the context of evolution, they introduced the term spandrel as a metaphor for characteristics that are or were originally side effects and not true adaptations to the environment. They are analogous to misbugs in hacker jargon.

Critics such as Dennett argue that the architectural spandrels (pendentives, to be precise) of St Mark's Basilica are not the undesigned gaps between design features that Gould and Lewontin describe, but that they are intentionally designed features themselves; deliberate solutions to an architectural problem where alternative solutions available to the designers included the use of corbels or squinches. The critics also argue that this misidentification of a design feature as an accident is an illustration, in parallel, of Lewontin's and Gould's underestimation of the adaptedness of evolved lifeforms.