Sex Ratios under Natural Selection

Abstract

SEVERAL workers^1,2 have shown the numerical equality of two sexes (that is, a 1 : 1 sex ratio) to be selectively favoured in relation to the parental effort involved in the rearing of offsprings to maturity. More recently, Kalmus and Smith³ have argued in support of the adaptive superiority of this ratio on the basis of such considerations as those of hybridity, recombination potential and the level of inbreeding as influenced by sex ratio variations. However, their arguments are not valid in general and at best seem to apply only to the monogamous species. In fact, widely deviant sex ratios are reported to occur in the natural populations of both polygamodiœcious and gynodiœcious species^4,5, and Lewis⁵ has suggested that sex ratio adjustments frequently through a preponderance of females help ensure reproductive economy combined with the maintenance of certain hybridity optima in such species. In a few theoretical examples analysed by Shaw⁶, such adaptive changes were examined in the presence of certain specific sex-ratio factors. In this note a model of differential selection for an autosomal recessive in the two sexes is utilized to show that maximum levels of heterozygosity (say 0.50 where gene frequencies p, q = ½) could be attained with variable sex ratios, a result in disagreement with the view of Kalmus and Smith³.