THREE APPROACHES TO SELECTION

In this chapter we turn from the first to the second of the two great principles of Darwin's theory of “descent with modification”, from “the unity of types”, i.e. history, to “the conditions of existence”, i.e. natural selection. There are three broad approaches to understanding selection – the idiographic, the nomothetic, and a third evolutionary ecological approach which in some respects combines both.

Going back to history for a moment, an important thing to understand about taxonomy is that monophyletic “taxa” or groups of related organisms are historically specific entities, about which only idiographic (or particular) rather than nomothetic (or law-like) statements can be made (Ghiselin 1974b; Hull 1978a). At the species level for example, a human being is a human being not because of universally or even statistically shared properties with other human beings, but because of his or her physical connectedness to them. But according to cladistics, do not all members of a clade universally share one or more uniquely derived characteristics? Yes normally, but if these conflict, history trumps. For example, if habitually walking upright is one of the shared, derived characteristics which distinguishes human beings from the other living great apes, no taxonomist would claim that a person born with cerebral palsy and unable to walk is not therefore a human being.