Chapter I Models of sex ratio evolution Jon Seger & J. William Stubblefield 1.1 1 Summary 1.2 I Introduction Our understanding of sex ratio evolution de pends strongly on models that identify: ( 1 ) c o n straints on the production of male and female offspring, and (2) fitness consequences entailed by the production of different attainable brood sex ratios. Verbal and mathematical arguments by, among others, Darwin, Dtising, Fisher, and Shaw and Mohler established the fundamental principle that members of the minority sex tend to have higher fitness than members of the ma jority sex. They also outlined how various eco logical, demographic and genetic variables m ight affect the details of sex-allocation strategies by modifying both the constraints and the fitness functions. Modern sex-allocation research is de voted largely to the exploration of such effects, which connect sex ratios to many other aspects of the biologies of many species. The models used in this work are of two general kinds: ( 1 ) expected-future-fitness or tracer-gene models that ask how a given sex allocation will affect the future frequencies of neutral genes carried by the allocating parent, and ( 2) explicit population- genetic models that consider the dynamics of alleles that determine alternative parental sex al location phenotypes. Each kind of model has dif ferent strengths and weaknesses, and both are often essential to the full elucidation of a given problem. M ales and females are produced in approxi m ately equal num bers in m ost species w ith sepa rate sexes, regardless of the m echanism of sex de term ination, and in most hermaphroditic species individuals expend approxim ately equal effort on male and female reproductive functions. Why should this be so? Sex allocation is a frequency- dependent evolutionary game (Charnov 1982, Maynard Smith 1982, Bulmer 1994). The basic principle that explains why balanced sex ratios evolve so often was described in a limited and tentative way by Darwin (1871), farther devel oped by Karl Dtising (1883, 1884) and several early twentieth century authors, and then sum marized concisely by RA Fisher in The Genetical Theory of Natural Selection (1930) (Edwards 1998, 2000). Subsequent work has generalized the prin ciple and extended it to cover a great variety of special circumstances to which Fisher’s elegant but elementary account does not apply. Sex allocation is now remarkably well under stood, and this understanding is often hailed as a triumph of evolutionary theoiy. However, to say that the fundamentals may be well under stood is not to say that all of the interesting and important discoveries have been made. Despite its focus on a seemingly simple and singular phe nomenon, sex-allocation research has become a rich and diverse enterprise that makes contact CORE Metadata, citation and similar papers at core.ac.uk Provided by The University of Utah: J. Willard Marriott Digital Library