This picture – that our minds were formed by processes of evolutionary adaptation, and that the environment they are adapted to isn’t the one that we now inhabit – has had, of late, an extraordinarily favourable press. Darwinism has always been good copy because it has seemed closer to our core than most other branches of science: botany, say, or astronomy or hydrodynamics. But if this new line of thought is anywhere near right, it is closer than we had realised. What used to rile Darwin’s critics most was his account of the phylogeny of our species. They didn’t like our being just one branch among many in the evolutionary tree; and they liked still less having baboons among their family relations. The story of the consequent fracas is legendary, but that argument is over now. Except, perhaps, in remote backwaters of the American Midwest, the Darwinian account of our species’ history is common ground in all civilised discussions, and so it should be. The evidence really is overwhelming.
But Darwin’s theory of evolution has two parts. One is its familiar historical account of our phylogeny; the other is the theory of natural selection, which purports to characterise the mechanism not just of the formation of species, but of all evolutionary changes in the innate properties of organisms. According to selection theory, a creature’s ‘phenotype’ – the inventory of its heritable traits, including, notably, its heritable mental traits – is an adaptation to the demands of its ecological situation. Adaptation is a name for the process by which environmental variables select among the creatures in a population the ones whose heritable properties are most fit for survival and reproduction. So environmental selection for fitness is (perhaps plus or minus a bit) the process par excellence that prunes the evolutionary tree.
More often than not, both halves of the Darwinian synthesis are uttered in the same breath; but it’s important to see that the phylogeny could be true even if the adaptationism isn’t.