ABSTRACT
There is a strong temptation to treat the theory of natural selection solely as a claim about thc 'ccntral tcndcncics' in cvolution. In thc words of Stcrclny and Kitcher ([1988], p. 345): 'evolutionary theory, like statistical mechanics, has no usefor such a fine grain of description [as the biography of each organism]: the aim is to make clear the central tendencies in the history of evolving populations.' Not only does such an interpretation seem to do justice to the centrality of population genetics-a thoroughly statistical enterprise-to evolutionary biology, but it also bids fair to solve or dissolve the long-standing problem of explaining or explaining away 'fitness', and to account for the theory's character in terms familiar from physics. In this paper we show why the temptation must be resisted. We argue that these benefits are not obtained, and that, moreover, the approach obscures crucial facts about the theory of natural selection. In Section I we show that the probabilistic propensity account of fitness required by the 'central tendencies' approach is no substitute for the causal conception of comparative fitness as a pairwise relation between individual organisms. In Section 2 we refute recent attempts to purge the theory of a causal concept of fitness by interpreting the theory, along the lines of the second law of thermodynamics, as one exclusively about 'ensembles'.
