Is sexual selection defined by dimorphism alone? A reply to Padian and Horner.

In their letter in response to our article in TREE [1], Padian and Horner [2] make three main points, the two most relevant concerning firstly the definition of sexual selection itself and secondly, the issue of how we might test for its presence in fossils organisms. Their third point pertains to species recognition and is addressed by Mendelson and Shaw [3]. Padian and Horner [2] employ what we think is a restrictive definition of sexual selection which requires sexual dimorphism to be present. They claim that ‘Because Darwin invented sexual selection, and based it on observations that have never been falsified, his definition cannot be wrong.’ It may be true that Darwin’s definition cannot be considered ‘wrong’ without falsification, but this does not mean that scientists cannot or should not modify or use alternate definitions based on evidence that has accumulated since Darwin’s time. Scientific definitions are subject to change because we alter theories and definitions to account for evidence, or use multiple definitions depending on context. Thus researchers should not be dogmatic about definitions of sexual selection, regardless of who created that definition or their intent at the time. Furthermore, we think the basis of Padian and Horner’s argument is questionable: although the observation of sexual dimorphism spawned Darwin’s idea, in neither The Origin of Species nor The Descent of Man can we find an unambiguous definition of sexual selection requiring sexual dimorphism. In fact, we find quotations in Darwin’s work that instead demonstrate an openness to the possibility of sexual selection operating in the absence of sexual dimorphism: ‘There are, however, many animals in which the sexes resemble each other, both being furnished with the same ornaments, which analogy would lead us to attribute to the agency of sexual selection. In such cases it may be suggested with more plausibility, that there has been a double or mutual process of sexual selection; the more vigorous and precocious females selecting the more attractive and vigorous males, the latter rejecting all except the more attractive females’ ([4] p. 225). Although Darwin goes on at this point to reject sexual selection as an explanation in this instance (for what we would now term ‘genetic correlation’), it is not because of a lack of dimorphism.