For my 50th birthday in 2015, my partner, Jen, and I flew to Venice for a week's holiday in one of our favourite countries. We have visited Italy many times before, but this was our first trip to Venice. Believe the hype: it's a beautiful city. Feel free to check out my photos.
No visit to Venice would be complete without a trip to St Mark's Square and its famous cathedral. “We have to go and see the spandrels,” I explained to Jen before we left. She rightly guessed that this must, in some convoluted way, have something to do with Darwin.
The spandrels of San Marco in Venice were the inspiration for an important evolutionary metaphor coined by Stephen Jay Gould and Richard Lewontin in their famous 1979 paper The Spandrels of San Marco and the Panglossian Paradigm: a Critique of the Adaptationist Programme. A spandrel is a three-sided architectural feature that fills in the spaces above the curve of an arch. In the case of arches that support domes, such as at San Marco, the three-dimensional spandrels that fill the gaps between the arches and the dome are more correctly known as pendentives—although Gould and Lewontin stuck with the more general (and, in my opinion, more aesthetically pleasing) term spandrel.
Gould and Lewontin's point was that the impressively decorated spandrels of San Marco cathedral ‘are necessary architectural by-products of mounting a dome on rounded arches’, rather than ‘the cause in some sense of the surrounding architecture’. In other words, the impressive decorations are not the reason why the spandrels are there; the fact that the spandrels are there of necessity means that they happen to provide a surface which can be used for decorative purposes.
By analogy, Gould and Lewontin went on to point out that many features found in living organisms, rather than having a strictly adaptive purpose, might be the biological equivalents of architectural spandrels: features which arose as necessary by-products of other features. In other words, not every feature in an organism needs to have a primarily adaptive explanation.
Many people, especially those of a strictly adaptationist persuasion, have raised numerous, sometimes valid, objections of Gould and Lewontin's paper. But I for one find the concept of a biological spandrel a useful, short-hand metaphor for suggesting that a particular organic feature need not necessarily require an adaptive explanation.