Cryptic Sexual Selection - More Control Issues 1

Identifying the evolutionary processes that cause genetic and phenotypic divergence among species is one of the fundamental goals of evolutionary biologists. Darwin (1871) first pointed out that male traits such as plumage coloration, which he hypothesized to influence sexual selection, rapidly diverge among closely related species. In an earlier book (Sexual Selection and Animal Genitalia; Eberhard 1985) William Eberhard extended Darwin's observation by showing that male genitalia diverge very rapidly among closely related species which have internal fertilization but not among those with external fertilization. Eberhard hypothesized that the rapid divergence in male genitalia was driven by the process of "cryptic female choice." Traditionally the term female choice has been used to describe a process whereby females differentially mate with some males among a pool of potential mates. The term cryptic female choice was coined by Randy Thornhill (Thornhill 1983) to describe events occurring after the onset of copulation that determine the extent to which a female makes use of a male's sperm. Eberhard's new book (Female Control: Sexual Selection by Cryptic Female Choice) focuses on cryptic female choice and its importance to the process of sexual selection. Eberhard states in his first chapter that, despite the potential significance of cryptic female choice, this topic has been essentially ignored in the major texts on sexual selection that have appeared since the publication of his earlier book. The aim of the new book is to make it clear why cryptic female choice can no longer be ignored. For those who would like a thorough synopsis of the book, complete with references, you're in luck: a recent TREE article (Eberhard and Cordero 1995) reviews many of the major topics. Eberhard begins by defining cryptic female choice and argues for its potentially broad application. He defends his focus on the opportunity for female-controlled sexual selection as a response to, in his view, the long-standing bias in the opposite direction; e.g., he contrasts the phrase, and the direction of research on, "sperm competition" with the dearth of work on "sperm screening." He then discusses in detail "twenty different female mechanisms which could result in cryptic female choice." The traits proposed to underlie cryptic female choice can be summarized as: variation in copulation quality, mechanical and chemical interactions between mates, female transport and treatment of deposited sperm, differential opportunity for fertilization, and remating. Eberhard provides extensive evidence for courtship during copulation. These observations include: male