The sizes of organisms are determined by their interactions with their environment and related ecological and evolutionary processes. Recent studies of body size distributions across communities show evidence for multimodality. The multiple modes were originally explained as a consequence of textural discontinuities in habitat structure. Because communities consist of species that are drawn from lineages, body size patterns within lineages will affect those that are expressed in communities. We used a cellular automation model to argue that multimodality in body sizes within lineages can arise from a few fundamental evolutionary mechanisms alone. We tested the hypothesis using body size data for 138 fish genera and found strong support for the idea that evolution structures body size distributions. The results suggest, first, that we should expect the distribution of body sizes within lineages to be multimodal and second, that a coherent theory of community body size distributions will need to combine both evolutionary and ecological perspectives.