NOTES AND COMMENTS ON THE MEASUREMENT OF SEXUAL SELECTION The theory of sexual selection was formulated by Darwin (1871) to explain the origin of sexually dimorphic traits

The theory of sexual selection was formulated by Darwin (1871) to explain the origin of sexually dimorphic traits that are detrimental to survival. Darwin believed that sexual selection arises from either mate choice or intrasexual competition for mates, reasoning that such traits could evolve if they conferred a fitness advantage on their bearer. In part because of the obvious difficulties in quantifying the evolutionary effects of mate choice or intrasexual competition for mates, Wade and Arnold (Wade 1979; Wade and Arnold 1980; Arnold 1983; Arnold and Wade 1984a,b) operationally defined sexual selection as selection arising from variance in mating success; the clear intention of these authors was to allow for the empirical measurement of sexual selection while retaining the sense of Darwin's original definition. This definition has subsequently been used in several important studies (Fincke 1982; McCauley 1983; Price 1984), as well as in a recent influential reference (Partridge and Halliday 1984). Our purpose is to show that variance in mating success is not necessarily a valid measure of the opportunity for sexual selection as defined by Darwin. After presenting three criteria we believe are desirable in order to distinguish sexual selection, we use examples to detail some difficulties arising from Arnold and Wade's definition, and we conclude by pointing out the potential difficulty of separating "sexual" and "natural" selection at all. We feel that this discussion is needed because further understanding of sexual selection will be obscured unless terminology is concordant with Darwin's original conception and unless the ambiguities in distinguishing and measuring sexual selection are appreciated. For an excellent summary of' some of the alternative approaches that have been used to measure sexual selection, see Clutton-Brock (1983). We propose three criteria for identifying sexual selection. (1) One or both of Darwin's agents of sexual selection, intrasexual competition for mates or mate choice, must be occurring. (2) As a consequence of these agents, there must be a variance in fitness. Variance need not be in mating success per se, but only in some episode of fitness (sensu Arnold and Wade 1984~) . Note also that this criterion does not follow automatically from criterion 1; female choice, for example, may be strong but will not constitute sexual selection if each female chooses a mate according to independent criteria, thus resulting in no variance in