S08.3: Theoretical models of sex allocation

Department of Genetics, University of Groningen, PO Box 14, 9750   AA Haren, The Netherlands, e-mail f.j.weissing@biol.rug.nl

Ever since Darwin, adaptive theories of evolution have been regarded as applicable to sex allocation. Fisher's (Fisher, R.A. 1930. Oxford University Press) assertion that equal investment should be made in sons and daughters within a population is based on the insights that: (1) Fitness is not always adequately captured by lifetime reproductive success. (2) Selection may be frequency dependent. Although Fisher's ideas still form the basis of sex allocation theory, a plethora of factors have been described predicting systematic deviations from a 1:1 allocation ratio at the population level. In particular, Trivers and Willard (1973. Science 179:90-92) predicted sex ratio variation when male and female offspring are differentially affected by parental condition. Although sex allocation theory is well developed and a huge body of data has accumulated, relatively few studies of higher vertebrates have found the theoretically predicted deviations from a 1:1 population sex ratio. Explanations include insufficient sample sizes, the constraints of chromosomal sex determination (although recent evidence suggests that this is not important), and a high cost to sex ratio manipulation mediated by selective mortality. These factors create a need in relation to the growing number of studies demonstrating intraspecific variation in avian sex ratios for specifically tailored life history models making quantitative a. priori predictions with which to confront the empirical data.

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