Divergent destinies of polymorphism.

Understanding why polymorphic traits—color, shape, or behavior differences across individuals—persist within a species is a long-standing challenge. Such variation is expected to disappear as selection favors a single optimal form. Yet despite this expectation, polymorphism occurs frequently across the tree of life. Geneticist Theodosius Dobzhansky argued that diversity could be actively maintained through "balancing selection," which prevents genetic variants from becoming dominant or being lost (1). Early empirical studies did not clearly support this model (2–4), but evidence for balancing selection has since accumulated (5). Nevertheless, the persistence of polymorphisms often depends on complex, dynamic interactions between genes, phenotypes, and environments that are challenging to study (6). On pages 64 and 69 of this issue, Uller et al. (7) and Corl et al. (8), respectively, report on the mechanisms that govern the origins, maintenance, and loss of color variation in lizards. [ABSTRACT FROM AUTHOR]