The paper explains why polymorphic traits, such as differences in colour, shape or behaviour in individuals within a species, persist despite the expectation that selection will favour one optimal form. Geneticist Theodosius Dobzhansky proposed that diversity is maintained by balancing selection that prevents the dominance or loss of genetic variants. Early empirical studies did not clearly support this model, but later evidence of balancing selection accumulated. However, the persistence of polymorphisms depends on complex interactions between genes, phenotypes and the environment. On pages 64 and 69 of this issue of Science, Uller et al. study the mechanisms of the origin, maintenance, and loss of color variation in lizards. Uller et al. describe how the adaptive spread of a sexually selected syndrome eliminates an ancient color polymorphism in wall lizards. Corl et al. investigate other mechanisms controlling color variation in lizards[1].