Under Strict Control

Abstract

EVOLUTION In many species of animals, males and females are strikingly (and visibly) different. Three evolutionary genetic pathways can produce such sexual dimorphism. From the outset, the expression of the alleles for dimorphic traits could be limited to one sex. Or the alleles could be expressed initially in both sexes but subsequently promoted only in one sex or repressed only in the other. Darwin believed that the bright colors and ornaments of males were male-limited from their first appearance, whereas Fisher held that they had been lost from females through the accumulation of suppressors. Coyne et al. weigh these alternatives by examining female hybrids between bird species [such as Costa's (left) and black-chinned (right) hummingbirds] whose males show distinct ornamental traits (for example, plumage patterns and the presence or absence of elongated feathers). Unless the suppressors are completely dominant, these females should show some expression of the male traits. In none of the 13 hybrid cases (from four orders and eight families) that met their search criteria did the females express a male-limited trait. This suggests that male ornaments were sex-limited at their first appearance (perhaps by hormonal or physiological differences between the sexes) or are now controlled by completely dominant modifier alleles or regulatory elements. — SJS Evolution 62 , 214 (2008).