Abstract

Regulatory mutations that alter gene expression are likely to play key roles in phenotypic evolution. Because of the modular nature of cis-regulatory DNA, mutations can alter phenotypes without pleiotropic side effects, yet because regulatory mutations often result in systemic transformations of complex phenotypes, they can result in radical but functionally integrated changes to organismal form. Empirical evidence supports the importance of regulatory mutations with restricted pleiotropic effects in phenotypic evolution. Moreover, data suggest that more mutations affect gene expression than gene structure, so such mutations are likely to be a major source of the heritable phenotypic variation on which selection acts.