A Fourth ‘Eternal Metaphor’?

We have now considered both mutation (Chapter 8) and selection (Chapter 9) from an evolution-of-development perspective. Clearly, both are important. Equally clearly, they are complementary: mutation gives rise to an altered developmental pathway and hence to a morphologically different individual, while selection governs the fate of such individuals in populations. Yet despite their obvious complementarity, the relative importance of the two processes – particularly in relation to their influences on the direction of evolution – has given rise to considerable controversy, which has dominated discussion of evolutionary mechanisms more than once over the past century. Let us now examine why.

If we contrast the present-day biosphere with its equivalent two or three billion years ago, what is the most striking difference? Undoubtedly, it is the existence now, but not then, of a diverse array of complex multicellular organisms, each internally coadapted as well as being adapted to its external environment. It is of course true that many lineages have not undergone a significant increase in visible complexity; both prokaryotic and eukaryotic unicells are still abundant and successful. It is also true that neither coadaptation nor adaptation are ever perfect for a variety of reasons, including ‘constraints’ and environmental variation. But such points to not detract from the fact that the explanation of “adaptive complexity” represents a central task for evolutionary theory (Maynard Smith 1972).