However, Darwin makes it clear that not all selection related to reproduction constitutes sexual selection, as primary sexual traits—like ovaries and testes—can evolve as a consequence of natural selection. Even though he never spells it out in so many words, Darwin’s working definition of sexual selection is essentially identical to the one used by Andersson (1994) and most other scientists studying sexual selection. In particular, “sexual selection arises from differences in reproductive success caused by competition for access to mates” (Andersson, 1994, p. 3). This definition admittedly focuses primarily on precopulatory sexual selection, so a more complete definition should also include postcopulatory processes, which can be accomplished by tagging the phrase “or fertilization opportunities” onto the end of Andersson’s definition.
Aside from the definition of sexual selection, what did Darwin accomplish in The Descent of Man? In a book as rich as Darwin’s, every reader could potentially identify different sets of key conclusions, depending on the reader’s background and research emphasis. Here we try to focus on those parts of the book that are still relevant to modern research, and from our perspective Darwin identified 2 major themes that set the stage for work on sexual selection. The first theme concerned the question of why sexual selection occurs in the first place. In the context of this question, Darwin identified the 2 major categories of sexual selection, namely intrasexual and intersexual selection, although he didn’t use those terms (Darwin, 1871). The second theme is related to the question of why sexual selection is strong in some lineages but not others. This question continues to be a major theme of modern research, but Darwin expressed an amazingly modern, intuitive understanding of some of the explanations for the patterns of sexual selection among diverse evolutionary lineages (Darwin, 1871).
In this chapter, we summarize Darwin’s contributions to these 2 major topics and consider how far we have progressed in our understanding of them. Rather than review all of the relevant literature, which for many of these topics has been done recently, we try to paint the current state of sexual selection with broad strokes. Like Darwin (1871), we focus on precopulatory sexual selection, leaving the treatment of postcopulatory processes for a different chapter in this volume (Eberhard, Chapter 12). Our sojourn through sexual selection literature leads to the identification of at least 2 major triumphs of precopulatory sexual selection research since Darwin. It also identifies numerous areas ripe for additional work. We hope that this paper will inspire our fellow scientists by showing (i) that Darwin, despite having tremendous insight given the state of biology in the 19th century, did not get everything right, (ii) that we have made tremendous progress since Darwin’s time, especially in the last several decades, and (iii) that many important questions regarding sexual selection remain to be answered.