Facultative sex ratios and population dynamics

THE theory of R. A. Fisher 1 for the effect of natural selection on the sex ratio predicts the population to be in evolutionary equilibrium when half of the parental reproductive resources are devoted to sons, and half to daughters. Provided that sons and daughters are of equal cost, this implies a sex ratio of ½ males at or near conception, and the result is independent of differential male and female survival after the period of parental care 2–5 . Most models demonstrating these results assume discrete generations, but some show that they also follow with overlapping generations 6–8 . These latter models typically assume the population to be in stable age distribution. Here, we relax that assumption, and show that selection can favour genes which result in the temporary overproduction of one or the other sex, under certain general conditions. The conditions are; overlap in generations; different temporal changes in life history expectations for the two sexes; parental ability to vary the sex ratio in response to the life history changes. The results are of interest because stable age distributions are probably uncommon in nature, with fluctuations in survival more the norm.