In most vertebrates, males and females are nearly identical in morphology during early development, but as a result of highly divergent growth, achieve often remarkably different adult morphologies. Although numerous studies have documented selection pressures that favor distinct morphologies of adult males and females, the mechanisms that enable the initially genetically identical sexes to end up so different in morphology are not well understood. Of special interest are the constraints imposed on sex-specific adaptations by a common gene pool and the ways by which males and females overcome these constraints. Recent studies show that the rapid evolution of sex-specific developmental regulators and modifiers of the otherwise conserved development of the sexes can produce sexual size dimorphism while maintaining the integrity of the developmental program that is shared between males and females.