Growing apart: an ontogenetic perspective on the evolution of sexual size dimorphism

Sexes play different roles in reproduction and the adaptive significance of the often remarkably distinct morphologies of adult males and females is documented frequently. Yet, in most vertebrates, the sexes are nearly identical in morphology during early development and undergo highly divergent growth to achieve different adult sizes. The mechanisms that enable the virtually genetically identical sexes to have such divergent growth are not well understood. Of special interest are the constraints that a shared gene pool imposes on sex-specific modifications of growth and the ways that males and females overcome these constraints in response to divergent selection pressures. Recent studies show that the rapid evolution of sex-specific developmental regulators and modifiers can produce sexual dimorphism in size whilst maintaining the integrity of the developmental program that is shared between the sexes.