In the 30 November Nature, Kopp et al. report that altered regulation of the bric-a-brac (bab) gene drove the evolution of sexually dimorphic pigmentation in Drosophila (Nature 2000, 408:553-559). The fifth and sixth abdominal segments (A5 and A6) of male Drosophila melanogaster are fully pigmented, whereas those of the female or of males of many other Drosophila species are only partially pigmented. The D. melanogaster males discriminate strongly against females with extra pigmentation, so the pigmentation probably helps the males to pick out females. Kopp et al. find that the appearance of the male pattern correlates with the repression of bab expression in A5 and A6, which is under the dual control of homeotic (Abdominal B) and sexually dimorphic (doublesex) genes. The analysis of this rapidly evolving trait bridges the gap between previous comparative studies (analyzing slowly evolving traits such as limb development) and genetic analyses (analyzing traits in closely related species that can produce fertile hybrids). It offers insight into how selection creates new morphological characteristics through changes in DNA sequence.