Biological Adaptation: A Reply

I I claim that if adaptation and function were identical, then sentences of the form "The function of T in systems of kind S was G, and T was not adaptive for the species" ([2], p. 206) would be contradictory. But they are not, for it is possible for T to have some function G and yet not be adaptive. Only empirical evidence can determine whether or not such a sentence is true. Ruse's criticism of this argument has two aspects. First, he objects that my argument "rests on [the] decision to construe 'adaptive' solely in the sense of 'adaptive for the species"' ([3], p. 525), and he goes on to explain that a trait can be adaptive for individuals as well as species. (Ruse quite rightly points out that I also make this distinction ([2], p. 210).) Second, he claims that if adaptation for individuals is allowed, then "The function of Tin systems of kind Swas G and Twas not adaptive for anything (individuals or species)" is contradictory ([3], p. 526). It is true, first of all, that I do not mention traits as adaptive for individuals at the stage at which I present the argument in question. However, I can see nothing serious resulting from this. At the very worst my example becomes inappropriate, for as Ruse observes, the large antlers of the Irish Elk might be adaptive for individuals that possess them. My original sentence form need be emended only by appending "or individuals of the species" to it. The claim, then, is that a sentence to the effect that a trait has a function and yet is not adaptive (for a species or for individuals) is not contradictory-which it would have to be if adaptation and function were the same. But it is just sentences of this form that Ruse (second aspect) claims to be contradictory. Unfortunately, he contents himself with merely asserting this and makes no attempt to demonstrate it. It is a consequence of his general thesis ([3], p. 526) that adaptation and function are identical concepts, but he does not argue directly for this position either. It is, however, an easy matter to show that sentences of the relevant form are not contradictory. One of the functions of the fur of large mammals like polar bears is to reduce the loss of body heat. In a cold climate the possession of fur is thus