An innovative view of the role of fitness concepts in evolutionary theory.   Natural selection is one of the factors responsible for changes in biological populations. Some traits or organisms are fitter than others, and natural selection occurs when there are changes in the distribution of traits in populations because of fitness differences.Many philosophers of biology insist that a trait’s fitness should be defined as an average of the fitnesses of individual members of the population that have the trait.   Marshall Abrams argues convincingly against this widespread approach. As he shows, it conflicts with the roles that fitness is supposed to play in evolutionary theory and with the ways that evolutionary biologists use fitness concepts in empirical research.The assumption that a causal kind of fitness is fundamentally a property of actual individuals has resulted in unnecessary philosophical puzzles and years of debate. Abrams came to see that the fitnesses of traits that are the