Sexual Selection Favors Female-Biased Sex Ratios: The Balance between the Opposing Forces of Sex-Ratio Selection and Sexual Selection

In a verbal model, Trivers and Willard proposed that, whenever there is sexual selection among males, natural selection should favor mothers that produce sons when in good condition but daughters when in poor condition. The predictions of this model have been the subject of recent debate. We present an explicit population genetic model for the evolution of a maternal-effect gene that biases offspring sex ratio. We show that, like local mate competition, sexual selection favors female-biased sex ratios whenever maternal condition affects the reproductive competitive ability of sons. However, Fisherian sex-ratio selection, which favors a balanced sex ratio, is an opposing force. We show that the evolution of maternal sex-ratio biasing by these opposing selection forces requires a positive covariance across environments between the sex-ratio bias toward sons (b) and the mating success of sons (r). This covariance alone is not a sufficient condition for the evolution of maternal sex-ratio biasing; it must be sufficiently positive to outweigh the opposing sex-ratio selection. To identify the necessary and sufficient conditions, we partition total evolutionary change into three components: (1) maternal sex-ratio bias, (2) sexual selection on sons, and (3) sex-ratio selection. Because the magnitude of the first component asymmetrically affects the strength of the second, biasing broods toward females in a poor environment evolves faster than the same degree of bias toward males in a good environment. Consequently, female-biased sex ratios, rather than male-biased sex ratios, are more likely to evolve. We discuss our findings in the context of the primary sex-ratio biases observed in strongly sexually selected species and indicate how this perspective can assist the experimental study of sex ratio evolution.