standpoints, but the merging of these 2 areas of inquiry could result in interesting insights.
Integration of precopulatory and postcopulatory sexual selection. To completely understand the entire selective history of any sexually selected trait, we will need to resolve the entire set of paths depicted in Fig. 9.2, including those that affect fitness after mating.
Darwin presented an incredibly detailed and clear description of sexual selection in The Descent of Man. Even though Darwin’s account of sexual selection was by no means complete and he had a garbled understanding of inheritance, Darwin was correct about almost every topic related to sexual selection that he discussed. For instance, he laid out essentially the modern version of intrasexual selection, and he correctly realized that female choice was an important mechanism in sexual selection. He also recognized that sexual selection could sometimes act on both sexes or more strongly on females than on males, and he demonstrated a good intuitive understanding of the effects of the operational sex ratio and mating systems on the intensity of sexual selection. However, Darwin did not clearly identify the evolution of female choice as a key topic for study in its own right. Rather, he tended to invoke a human-like sense of aesthetics in animals to explain their preferences for ornaments. He also never produced a clear picture of why some lineages seem to be experiencing stronger sexual selection than others. Regardless, The Descent of Man is an impressive scientific work, and well worth a read for anyone interested in sexual selection. Not only does it provide a clear intuitive explanation of the process, but the vast array of empirical examples could serve as the launching point for countless new studies.
Since Darwin, progress in the study of sexual selection has been astounding. Two of the greatest triumphs included the proliferation of models explaining the evolution of female preferences and quantitative approaches to the measurement of selection differentials. These advances provide clear, plausible mechanisms for the evolution of female choice and allow us to begin to address why sexual selection varies among species. A third major triumph in the study of sexual selection, which we did not have space to discuss, was the introduction of molecular markers into behavioral ecology (Gowaty and Karlin, 1984; Burke and Bruford, 1987; Hughes, 1998; Jones and Ardren, 2003). Molecular studies of parentage provide unprecedented opportunities to study patterns of mating in natural populations, so they have become a cornerstone of sexual selection research.