. "12 Postcopulatory Sexual Selection: Darwin's Omission and Its Consequences--William G. Eberhard." In the Light of Evolution III: Two Centuries of Darwin. Washington, DC: The National Academies Press, 2009.
The following HTML text is provided to enhance online
readability. Many aspects of typography translate only awkwardly to HTML.
Please use the page image
as the authoritative form to ensure accuracy.
In the Light of Evolution Volume III: Two Centuries of Darwin
than before genital coupling, including male-female dialogues during copulation.
Picture a pile of freshly cut weeds at the sunny edge of a tropical forest. Metallic green flies dart and circle over it, chasing one another in short dashes. Your eye is caught when a chase ends as one fly grasps another in midair and the pair immediately lands on the pile of weeds. Their genitalia are already coupled, and the male immediately turns to face away from the female. After a few seconds, paradoxically (because he is already securely attached), he begins to court, rhythmically waving his colorful hind legs and tapping the female’s abdomen. The courtship continues for a few minutes as the pair remains coupled, and then the flies separate. The female walks down into the pile where she lays eggs (her larvae will feed on the rotting vegetation), while the male rejoins the frenetic chases above the pile.
Why would a male fly wait to court a female until after he has already achieved his evolutionary objective of copulating with her? The answer (recently worked out by a Brazilian graduate student, F. Barbosa, personal communication) had to wait for >100 years after Darwin’s great book on sexual selection (Darwin, 1871/1959) that explained so many other aspects of male-female sexual interactions.
THE PUZZLE OF DARWIN’S OMISSIONS
Darwin was uncannily on target about most of the topics he discussed, and he seldom missed general phenomena that had important consequences for his ideas. Strangely, however, there is a major missing piece in Darwin’s thinking on sexual selection. He discussed at length how competition between males for sexual access to females leads to sexual selection (Darwin, 1871/1959), but failed to realize that sexual selection [sperm competition and cryptic female choice (CFC) in Table 12.1] can also occur even after males have initiated copulation. Simply stated, Darwin missed the fact that not all copulations result in insemination, and that not all inseminations result in fertilization of the female’s eggs. Any male ability to improve the chances that his copulations will lead to fertilizations of eggs will give him an advantage in competition with other males who mate with the same female.
It was not until 99 years after Darwin’s 1871 book that Geoff Parker (1970) awakened evolutionary biologists to the evolutionary importance of processes that occur after the male has already achieved genital coupling (conventionally called, somewhat imprecisely, postcopulatory pro-