Sexual dimorphism makes life interesting for many species. In the case of the fruit fly Drosophila melanogaster, the rear end of a male is so much darker than that of a female that a seasoned fly pusher can distinguish he from she even without the aid of a microscope. A telling investigation by Artyom Kopp and Sean Carroll at the Howard Hughes Medical Institute and Laboratory of Molecular Biology at the University of Wisconsin-Madison and Ian Duncan at the Department of Biology at Washington University in St. Louis suggests how sexual selection could have maintained the distinctive abdominal differences in two steps.1

Sexual selection is a key part of Charles Darwin's theory of the origin of species. Inherited traits that foster mating obviously boost certain gene variant frequencies in the next generation, and therefore drive microevolutionary change. Darwin knew nothing of genes, but he described the ins...

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