Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

If we observe in organisms the presence of complicated, highly organized structures, whose random formation is highly improbable, we usually have a tendency to explain their formation through the action of natural selection and to look for their biological function. In many cases, we actually find such a function, but in some cases it need not exist at all. Even when we discover that the particular structure has a favourable effect for the survival of the organism, this does not automatically mean that it emerged through the action of the relevant selection pressure. In many cases, this can be a case of exaptation (see I.7.1) and, in other cases, their development need not even be connected with the action of natural selection.

An important category of these structures is currently designated by a term taken from architecture – spandrels (Gould & Lewontin 1979). Spandrels are architectural elements that develop not through the intention of the architect, but as a consequence of objective, e.g. geometric, laws (Fig. I.10). Pendentives are given as a special case of spandrels in evolutionary literature; these are spherical triangular areas permitting the placing of a circular dome over a square room. The best known case (at least for evolutionary biologists) consists in the pendentives in the Basilica of St. Mark in Venice. At the present time, these pendentives bear pictures of the four evangelists and thus seem to be an essential and intentionally created element of the artistic decoration of the cathedral. In actual fact, they were not created to bear these paintings, but because this was the most rational structural solution for joining a four-walled base with its cupola ceiling. It is typical that the pendentives in the Basilica of St. Mark acquired an adaptive significance, i.e. that mosaics of four evangelists were placed on them later, several centuries after the creation of the basilica.