progress in some areas of sexual selection research, we still have much to accomplish.
Charles Darwin proposed the concept of sexual selection 150 years ago in On the Origin of Species by Means of Natural Selection (Darwin, 1859), but his definitive work on sexual selection was undoubtedly The Descent of Man, and Selection in Relation to Sex, which was published in 1871. Now—200 years after Darwin’s birth—is an excellent time to reflect on the modern relevance of his work and on progress that has been made in the study of sexual selection since his time. In typical Darwinian fashion, The Descent of Man, and Selection in Relation to Sex is a colossal tome (with a tongue-tripping title that we will henceforth abbreviate as The Descent of Man) that strolls through myriad topics relevant to evolutionary biology and ecology. Consequently, it is futile to attempt to characterize the full spectrum of topics addressed in this book, which touched upon such sundry issues as species concepts, taxonomy, correlated evolution, sex-limited inheritance, and group selection, to name a few. Rather, we focus on the contributions of Darwin in light of modern research in sexual selection, and in so doing we identify a few important topics for which a thread of reasoning can be traced from Darwin to the present.
One of the great strengths of Darwin was that he often constructed his literary works with a clear argument in mind and marshaled vast amounts of evidence to support his case. The Descent of Man is most famous for Darwin’s contribution to the hypothesis of sexual selection, but the main goal of the book was to provide evidence that evolutionary principles applied to humans and that humans descended from some ape-like common ancestor. Darwin believed that sexual selection played a major role in the evolution of humans and the divergence among distinct human populations, so he felt a lengthy description of sexual selection was necessary. Indeed, the bulk of the book concerns sexual selection, but many of Darwin’s insights regarding sexual selection appear in his chapters on humans.
Darwin’s most lasting achievement with respect to sexual selection must be his definition of the term, as it is essentially the same as the one still in use today. It is difficult to find a quote from Darwin that captures the full essence of his concept of sexual selection, but he provides the following definition:
We are, however, here concerned only with that kind of selection, which I have called sexual selection. This depends on the advantage which certain individuals have over other individuals of the same sex and species, in exclusive relation to reproduction.
Darwin (1871, Part I, pp. 254–255)