This thesis is a collection of six manuscripts on sex allocation theory. Sex allocation is the pattern by which resources are apportioned to male and female offspring, or to the nuturing and dissemination of small and large gametes. The ratio in amount of resources allocated to male offspring relative to the amount allocated to female offspring is a trait potentially subject to natural selection, since (i) the tendency to invest more or less heavily in one sex may be inherited, and (ii) the ratio of investment may be correlated with the total reproductive potential of the brood. The simplicity of the trait, a ratio of numbers of males to females, or calories invested in each, combined with the fact that this ratio is associated with fitness through a remarkable number of behavioral, genetic and life history traits, has made sex allocation theory a model system for developing and testing ideas in many apparently disparate areas of evolutionary biology.; Three different methods are used to develop models, according to the particular problem at hand. The first is economic, relating resources allocated to offspring with the reproductive returns obtained for this investment. This method is used to analyze the robustness of Fisher's equal allocation theory, which is found to be robust for high fecundity organisms, but quite sensitive to the patterns of sexual selection and the mating system when fecundity is low, a situation to which Fisher's theory is often applied. Results of this general work are used to derive specific predictions for variation in sex allocation among colonies of ants. The second method extends Fisher's idea of reproductive value to include cases in which relatives interact. This method is applied to the problem of sex allocation when populations are genetically subdivided, and have complex demographic characteristics. Predictions are developed with these techniques for social spiders, which have both genetically subdivided populations and complex demographies. The third method analyzes the evolutionary dynamics of sex allocation by computer simulation when separate subsets of the genome favor different directions of evolutionary change. Predictions are developed for cytoplasmic male sterility in hermaphroditic plants, where cytoplasmic genes cause pollen sterility and autosomal genes restore pollen fertility in the presence of these cytoplasmic genes.
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