Refuge of spandrels

There is a strong and widely held belief that all organisms are perfect and that everything within them is there for a function. Believers ascribe to the Darwinian natural selection process a fastidious prescience that it cannot possibly have and some go so far as to think that patently useless features of existing organisms are there as investments for the future. I have especially encountered this belief in the context of the much larger quantity of DNA in the genomes of humans and other mammals than in the genomes of other species. Even today, long after the discovery of repetitive sequences and introns, pointing out that 25% of our genome consists of millions of copies of one boring sequence, fails to move audiences. They are all convinced by the argument that if this DNA were totally useless, natural selection would already have removed it. Consequently, it must have a function that still remains to be discovered. Some think that it could even be there for evolution in the future — that is, to allow the creation of new genes. As this was done in the past, they argue, why not in the future? Some years ago I noticed that there are two kinds of rubbish in the world and that most languages have different words to distinguish them. There is the rubbish we keep, which is junk, and the rubbish we throw away, which is garbage. The excess DNA in our genomes is junk, and it is there because it is harmless, as well as being useless, and because the molecular processes generating extra DNA outpace those getting rid of it. Were the extra DNA to become disadvantageous, it would become subject to selection, just as junk that takes up too much space, or is beginning to smell, is instantly converted to garbage by one’s wife, that excellent Darwinian instrument. But even this fails to convince. It was therefore with great interest that I belatedly read Stephen Jay Gould’s paper on The exaptive excellence of spandrels as a term and prototype, which was published in the Proceedings of the National Academy of Sciences, USA last September (94:10750-10755). The paper has an important message and I strongly urge my readers at least to look at it, even if all the words in it can’t be understood. I offer this brief summary as a guide. The term spandrel originates in architecture and is used to describe spaces left over as a consequence of some other design decision, such as the triangles that remain behind when a rectangular wall is pierced by an arched opening. No selfrespecting architect would simply leave such spaces, especially in a grand cathedral with a rich patron. Instead they would be decorated, as is the case of the four pendentives under the dome of San Mario in Venice, which are decorated with the four evangelists. This example is a good one, because the historical sequence of events is known. The spandrels are the consequence of a structural design decision, a byproduct of placing a dome on rounded arches; three centuries later, mosaicists decorated these spaces. Thus spandrels are not primary adaptations but, because they can have later uses, they become, in Gould’s terminology, exaptations. In biological systems, we are confronted with the final result and we are required to distinguish between primary adaptations and historical products, some of which may have become co-opted for use. We need to separate the survival of the survivors from the survival of the fittest; as Manfred Eigen pointed out, the former is an historical inevitability whereas the latter involves choice and has a value function governing that selection. Gould points out that we can make the separation in several ways by analysing the historical evidence or by comparing many examples of the same structure. While in the case of San Marco’s pendentives it is clear which came first, this may not be always the case. For example, had the architect, in the same conception, provided both the main design and the use of the triangular space, it would be hard to separate the spandrel and its use from the primary construct. What can be shown to be unlikely is that the entire design was generated for the purpose of the decoration and that it is the dome that is the spandrel. This is because there are many domes on arches without any decorations. Being aware of degenerate organisms, however, we’d need to show that these unadorned specimens are not spandrels that were originally decorated but had their decorations removed at a later date. I suspect that the term spandrel will not survive. It is both too fancy and not catchy enough. But the main difficulty with Gould’s article is its anthropomorphism. We are constantly urged to explore the intent of the architectural designer, to distinguish between what he wanted and what he had to live with as a secondary consequence. There is too easy a transition from the analogy to the Great Designer and his intentions. We should be looking at the problem directly and be studying the grammar of systems that can evolve complexity by natural selection rather than seeking the comforts or discipline of analogies. These morsels could become wastrels. R669