Towards a genetic theory for the evolution of the sex ratio. III. Parental and sibling control of brood investment ratio under partial sib-mating.

A genetical model is formulated in which the sex ratio in broods and the relative size of broods are determined by the genotype at an autosomal locus. The results also apply to the case in which the sex-ratio locus is sex linked and expressed in the homogametic sex and to the case in which the locus is expressed in the diploid sex of a haplodiploid organism. Fisher (1930) argued that the sex ratio evolves under natural selection to a value such that parental expenditure is equalized between the sexes. Shaw and Mohler (1953) and MacArthur (1965) proposed that the sex ratio evolves to increase a certain expression for fitness. The sex ratio suggested by Fisher (1930) is in fact identical to the sex ratio specified by these maximization principles. Further, in our model, the Fisherian sex ratio corresponds exactly to the sex ratio at certain equilibria that are approached whenever they exist.