The sex ratio favoured by natural selection is studied in a single-locus model of a random-mating population. The population inhabits a "patchy" (spatially heterogeneous) environment, and offspring reared in different patches have corresponding differences in fitness, depending on their sex. Thus, some patches produce the best males; these or other patches produce the best females. The mating pool is comprised of the individuals from all patches. Sex ratio is allowed to evolve so that a zygote's probability of becoming male or female is determined in response to the patch in which it is reared. Results include the following:
(1) For many conditions, the sex ratio at equilibrium is one in which only males develop in patches relatively beneficial to maleness and/or only females develop in patches relatively beneficial to femaleness; the favoured sex ratio in some patches may include both sexes.
(2) The average population sex ratio in zygotes is not generally ½ at equilibrium.
(3) If the frequency of patch types fluctuates between generations, there is a greater tendency for selection to favour a sex ratio of ½ in each patch type than if the environment is temporally constant. This may help explain why so few animals control sex ratio in response to environmental cues.
(4) The sex ratio may coevolve with the offspring's choice of patch type and the environmental cue it uses in determining sex; when possible, embryos are selected to avoid patches detrimental to both sexes, and are selected to determine sex in response to the cue which indicates the greatest differential benefit to maleness versus femaleness. Available sex ratio data from haplo-diploids and invertebrates with environmental sex determination lend qualitative support to some of the predictions, but the widespread occurrence of environmental sex determination in reptiles offers a possible contradiction.
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