Rapid evolution of reproductive traits has been attributed to sexual selection arising from interaction between the sexes. However, little is known about the nature of selection driving the evolution of interacting sex-specific phenotypes. Using populations of Drosophila melanogaster selected for divergent sperm length or female sperm-storage organ length, we experimentally show that male fertilization success is determined by an interaction between sperm and female morphology. In addition, sperm length evolution occurred as a correlated response to selection on the female reproductive tract. Giant sperm tails are the cellular equivalent of the peacock's tail, having evolved because females evolved reproductive tracts that selectively bias paternity in favor of males with longer sperm.
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