A lesson in evolutionary theory can be drawn from the work of Rick Charnov, who transformed Fisher’s sex ratio theory into sex allocation theory, but unfortunately, the lesson has not spread far enough. The lesson is that costs as well as frequencies need to be included. That is so whether we are talking about evolutionary ecology (e.g., density dependence), social evolution (e.g., sexual selection), origins, an extended evolutionary synthesis, multilevel selection, or whatever. The two dimensions can be expressed in a (...) variety of ways—investing as well as spending, quality as well as quantity, costs as well as frequencies, and somatic as well as reproductive functions, for example. Both are needed everywhere in evolutionary theory. (shrink)

This article utilizes three premises. There are commonly ecologically oriented, naturally selected specialized differences in frequency and/or quality as well as sexually selected differences between the sexes. Sex in the sense of coming together and going apart or going apart and coming together is trade in these naturally selected differences, i.e., there is a mating market in sexual species. While such trade is beneficial to the population as a whole, sexual competition and selection is conflict over the profits of that (...) trade and can be detrimental to the population as a whole. These premises yield a naturally selected sex allocation theory of the possible directions and forms of sexual selection, i.e., one that includes costs as well as frequencies. This can explain conventional sex roles, the sex-role reversed, inter- as well as intrasexual selection, and passive as well as active choice. Any of these alternatives may be over mates, over gametes, or both. A hypothetical example based on density dependence relative to resources is provided, one that suggests that centrioles may be a cytoplasmic resource in males in multicellular animals, and which are the target of active choice and the mechanism of manipulation in passive female choice. As a whole, the approach yields a truly general theory of sexual selection, provides an alternative to the mechanism for Fisher’s principle, and gives a theoretical explanation for Mayr’s biological species definition. (shrink)