This is a defence of the statistical interpretation of fitness. I distinguish three interpretations of the evolutionary explanations in which fitness figures: the Two Factor Model, The Single Factor Model and the Statistical Interpretation. They differ in their degrees of causal commitment. The first two are committed to fitness distribution being a cause of population change. The last maintains that fitness distribution correlates with population change, but does not cause it. The defence of the statistical interpretation relies upon a distinctive feature of fitness--that fitness is a function of the mean and variance of reproductive output and population size--and a distinctive feature of causation--that it conforms to the Sure Thing Principle.
Denis Walsh is Canada Research Chair in the Philosophy of Biology. He is a member of the Department of Philosophy, Institute for the History and Philosophy of Science and Technology, and the Department of Ecology and Evolutionary Biology at the University of Toronto. He did his PhD in biology at McGill University on the systematics of amphibians and his PhD in Philosophy at Kings College London on modal logic and modal metaphysics. He is currently doing research on the modes of explanation in evolutionary biology. Dr. Walsh is currently a visiting fellow at the KLI.