Sexual selection has been studied intensively, and is often strong in natural populations. Theoretical models, comparative studies and laboratory selection experiments all suggest that evolution driven by sexual selection should be rapid and common.However, there are relatively few documented cases of the contemporary evolution of secondary sexual traits in natural populations. Moreover, the causes are not always due to sexual selection, but often due to altered natural selection regimes, or an altered balance between natural and sexual selection.Here we discuss recent empirical studies that have demonstrated the contemporary evolution of secondary sexual traits in natural populations. Our examples include both continuous traits and discrete polymorphisms. Taxa in which the contemporary (rapid) evolution of secondary sexual traits have been demonstrated include fish, insects, mammals, reptiles and birds. The evolutionary rates of these changes range from 0.005 to 0.570 haldanes (arithmetic mean = 0.14; geometric mean = 0.095; median = 0.12).The relative rarity of examples could be explained by different genetic architectures of sexually selected traits compared with naturally selected traits, or by sexual selection regimes not being sustained in the long term. These factors could potentially slow down evolutionary rates of secondary sexual traits and make the detection of the contemporary evolution empirically more difficult.Promising and underutilized approaches to sexual selection research include reciprocal transplant experiments and estimation of sexual isolation between conspecific populations.
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