Title:
The Metaphysics of Fitness
Abstract:
Natural selection and fitness are coordinate concepts. Selection occurs precisely when there is variation in fitness. An adequate definition of fitness must explain why selection and drift are different. This entails that fitness cannot be defined as an organism’s or a trait’s actual degree of reproductive success. In evolutionary theory, the fitness concept is “fragmented,” meaning that different biological settings call for different mathematical representations of fitness. It’s also true that a single mathematical representation of fitness is multiply realizable, in the sense that the genes, phenotypes, and environments that make one dog more fit than another differ from the genes, and environments that make one daffodil more fit than another. The probabilistic concept of screening-off is key to understanding the conceptual role that fitness plays in evolutionary theory. This essay defends a causal interpretation of natural selection; it opposes the statisticalist thesis that selection is epiphenomenal. Interventionist ideas about causation are used to defend the causal interpretation and a zeroing-out criterion is used to compare the strengths of selection and drift. Selection and drift are better viewed as distinct causal influences on evolution, rather than as two distinct physical processes.