The spandrels of San Marco (or rather, the pendentives)
When a round-headed or pointed arch is set in a rectangular frame, two roughly triangular spaces are left on either side, above the arch. These are spandrels. Often they are filled with painted or sculpted ornament. When a dome is supported on four columns in a square, four curved triangular pieces can be used to connect the dome to the columns. These triangles are sometimes also called spandrels.
The photo above shows spandrels of this kind beneath one of the domes of the Basilica of Saint Mark in Venice. They are richly decorated with mosaics of Biblical subjects on golden backgrounds. In 1979 these became the unlikely focus of an intense and at times bitter controversy in the theory of natural evolution. The argument was prompted by a paper in the Proceedings of the Royal Society, ‘The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist programme’, by two distinguished evolutionary biologists, Stephen Jay Gould and Richard Lewontin. Gould had just recently visited Saint Mark’s.
The ‘adaptationist programme’ that the authors were concerned to challenge was a widespread belief among evolutionary theorists that all features of organisms are adaptations to their environments, brought about through natural selection. Every trait, that is to say, confers an advantage of some kind in the competition to survive and reproduce. The ‘Panglossian paradigm’ of the paper’s title refers to Professor Pangloss in Voltaire’s novel Candide. Pangloss is an incorrigible optimist and believes that “all is for the best” in this, “the best of all possible worlds”. (The character is a misconceived satire on the philosopher Gottfried Leibniz: but that is another story.) All adaptations of organic form are in the creature’s best interests. “Our noses”, Pangloss says, “were made to carry spectacles.”
Gould and Lewontin’s argument is that, on the contrary, evolution is a historical process, pushed this way and that by accidents and contingencies in the environments of organisms. Selection cannot begin from scratch with animal or plant species, but must build on and modify the forms at which they have presently arrived. Organic forms are not ‘optimal’. There may be bodily traits that are not produced through selection, but are survivals or by-products. They may be there just to connect other parts, to ‘fill in the gaps’ so to speak. Hence the supposed analogy with the spandrels of San Marco, which – so the authors argued – serve simply to fill the spaces between dome and columns.
The paper provoked a powerful reaction. It became one of the most celebrated and widely discussed publications in evolutionary biology, welcomed in some quarters as a healthy corrective to extreme adaptationism. Biologists needed to be more careful in telling ‘just-so stories’ about the imagined adaptive functions of traits, without carefully testing those stories against evidence and experiment. In 1993 an entire book was devoted to reviews and a chapter-length response by Gould. Some readers however were less impressed. Richard Dawkins said the paper was ‘overrated’. The philosopher Daniel Dennett criticised the basic architectural analogy and claimed that the dome of San Marco might alternatively have been supported on brackets or on ‘squinches’ – small arches spanning across the corners of the square.
For architects the basic idea as it applies to spandrels in the first sense, the triangles above arches, is I think quite uncontroversial. These are indeed leftover pieces. They have no important structural purpose, they do not directly serve the larger functions of the building, and they are places where ornament can flourish. The ‘spandrels’ in Saint Mark’s are a different matter.
Sticklers for proper technical language pointed out that Gould and Lewontin’s terminology was strictly incorrect, and that referring to San Marco they should have used the word ‘pendentives’. The historian of structural engineering Robert Mark wrote a response explaining how architectural pendentives are very far from being by-products or filler pieces. They have a crucial structural role in transmitting the load of the dome to the supporting columns. Each triangle carries a ‘surcharge’ behind it of heavy stabilising masonry, to prevent the dome pushing the columns outwards. (And pendentives are themselves the outcome of a long process of ‘architectural evolution’.) Also, Dennett was wrong about brackets and squinches, which would have served only for much smaller domes.
All this argument about the finer points of medieval engineering was of course quite tangential to the central biological point of Gould and Lewontin’s paper.
Stephen Jay Gould and Richard Lewontin, ‘The spandrels of San Marco and the Panglossian paradigm: a critique of the adaptationist paradigm’, Proceedings of the Royal Society B: Biological Sciences, 205 (1161) 1979 pp.581-598
Robert Mark, ‘Architecture and evolution’, American Scientist Vol.84 No.4 1996 pp.383-389