Abstract
FISHER'S theory of sex-ratio evolution1 explains why 1:1 sex ratios are so prevalent2–8. Recent work has therefore emphasized situations in which biased sex ratios tend to evolve (for example, local mate competition7, non-mendelian inheritance of sex-ratio factors7,9,10, haplodiploid social insects11,12 and others6,13). In species where individuals change sex during their adult lifetimes, and in species where sex is determined by environmental conditions experienced during pre-adult development, biased sex ratios are expected and frequently observed6. Sex-ratio models for particular kinds of sex change (SC) and environmental sex determination (ESD) have predicted excesses of the first sex6,14,15 under SC, and of the sex developing in the 'worse' environment under ESD6,14–16. Here we generalize and unite these results in a single model that applies to a wide range of SC and ESD biologies. The principle that emerges from this model appears to explain strong female and male excesses seen in sex-changing fish and invertebrates and in nematodes with ESD. It is not known whether the female excess commonly seen in reptiles with ESD are also explained by this principle.
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