Binomial Sex-Ratio Distribution in the Lesser Snow Goose: A Theoretical Enigma

Several social and/or ecological conditions can be recognized that would make a strict adherence to an independent (random) sex-determination mechanism, if not maladaptive, certainly nonadaptive. Some conditions should lead to parental control over sex determination being adaptive; in others it can be reasonably argued that no one sex-determining mechanism should be favored. The population structure, methods of mate choice, and breeding biology of the lesser snow goose are such that theory would predict a virtual absence of selective pressure maintaining an independent sex-determination mechanism. Such a situation should lead to more variance in the distribution of intraclutch sex ratios than predicted by the binomial model of independent sex determination. A large sample of goslings does show a sex-ratio distribution which closely fits the binomial, thus refuting the theoretical argument. It can be speculated that pleiotropic effects of sex-skew mutations place a selective disadvantage on such mutations or that such mutations are, for one reason or another, impossible.