Theoretical Considerations of Unequal Sex Ratios

Abstract

A generalized, single-locus model is developed to investigate the evolutionary behavior of an autosomal locus capable of modifying the primary sex ratio. The model analytically incorporates effects of any differential fecundity and viability associated with different primary sex ratios. Computer simulation based upon the model confirms, for the most part, MacArthur's prediction that evolution will produce a sex ratio that maximizes the product of the number of daughters in the population times the number of sons in the population. Some exceptions to MacArthur's principle-arising from certain kinds of dominance relations at the modifier locus-are noted. Analysis of the effects of differential viability shows that the stage in the life cycle to which MacArthur's principle applies is that stage at which male mortality and female mortality both cease to differ between families possessing different sex ratios. In particular, as previously shown by Shaw and Mohler, differences in viability that depend solely upon an individual's sex have no effect upon the primary sex ratio and cannot, by themselves, be invoked as an explanation for non-one-to-one primary sex ratios. Some possible sources of differential fecundity and viability capable of producing unequal primary sex ratios are discussed.