The jewel in the crown

We began this chapter with three questions. In brief, how have we answered them? First, separate sexes may evolve when an individual functions better as a single gender than as two. Second, biased sex allocation may evolve when one of two symmetries are broken. Finally, sex-determining systems probably evolve under the same processes and circumstances as sex allocation. We also began this chapter with the statement that sex allocation is the jewel in the crown of evolutionary ecology. We have seen that in one respect it is not, for our understanding of the evolution of cosexuality and dioecy, while certainly promising, is not as advanced empirically as our understanding of sex allocation bias. Both sex allocation and sex determination probably evolve by a common mechanism, and under common ecological circumstances. Sex allocation takes place in the context of sex determination, and sex determination can evolve in response to sex allocation decisions. Our

Wolbachia

S ZZ ?ZZ + Wolbachia

Wolbachia

Femininzing factor, f

ZZ ZW

Fig. 5.9 Evolution of sex determination in A. vulgare (after Rigaud 1997).

understanding of the evolution of sex determination is not far advanced, but an integrated study of sex allocation and sex determination may yet prove to justify a central place in evolutionary ecology, not simply because it is a shining example of what is possible, but because of numerous potential impacts on ecology and evolution, such as population extinction and in reproductive isolation. Sex allocation, from Hamilton's extraordinary example, illustrates the power of genetic conflict to change living systems.

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