Falacy of Seeking Too Much Purpose in Evolutionary Adaptation

Darwin’s concept of evolution is centered on natural selection, or survival of the fittest [1]. Although Darwin did realize that organisms possess structures and even entire organs that might not have an extant function, as is the case of rudiments [2], on the whole, selectionist thinking has heavily dominated the biological literature ever since. In its extreme but not uncommon form, the selectionist, or adaptationist, paradigm perceives every trait as an adaptation. Under this view of biology, the first and most important question a researcher asks about any structure (including any genomic sequence) is: what is it for? Often, this question is followed up with experiments aimed at elucidating the perceived function.

Is the pan-adaptationist paradigm valid, especially at the genomic level? In a classic 1979 article [3], unforgettably entitled “The spandrels of San Marco”, Stephen Jay Gould and Richard Lewontin mounted the first all out, frontal attack on pan-adaptationism, which they branded the Panglossian Paradigm after the inimitable Dr. Pangloss of Voltaire’s Candide ou L’Optimisme [4], with his “best of all possible worlds”. The argument of Gould and Lewontin is purely qualitative and centers on the metaphorical notion of spandrels, as they denoted biological structures that do not appear to be adaptations per se but rather are necessary structural elements of an organism [5]. The analogy comes from architectural elements that are necessitated by the presence of gaps between arches and rectangular walls, and that can be exploited decoratively to host images, as with the images of archangels and evangelists in the Venetian San Marco basilica (Fig. 1): the spandrels have an essential structural function and by no means have been designed for this decorative purpose. Analogously, biological spandrels can be exapted (recruited) for various functions, although their origin is non-adaptive (exaptation is a new term introduced by Gould and Vrba to denote gain or switch of function during evolution). Rather than hastily concocting adaptationist “just-so stories” (in reference to Rudyard Kipling’s book of lovely tales [6] on how the elephant got his trunk (Fig. 2) and the jaguar his spots—did Kipling actually sense the inadequacy of naïve adaptationism?), submitted Gould and Lewontin, a biologist should attempt to carefully and objectively reconstruct the evolutionary histories of various traits of which many will emerge as spandrels.