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Evolution of sexual dimorphism from gametes to ornaments

Evolution of sexual dimorphism from gametes to ornaments


Department of Biological Sciences, Stanford University, Stanford, CA 94305, UNITED STATES OF AMERICA.


This thesis develops theories for the evolution of alternative reproductive morphs, in the process exploring the evolutionary consequences of sperm limitation and developing an alternative theory for the evolution of the sexes and sex roles.

Previous studies of the evolutionary consequences of sperm limitation have been restricted to morphological and phenological changes. I examine the potential of selection for increased gamete density to cause the evolution of alternative reproductive morphs--- large eggs and small sperm, specialized male and female individuals and independent haploid and diploid stages in a lifecycle.

The biological sexes are defined by the gamete sizes, and the existing theory attributes sex roles in mating and parental care among animals to the difference between the egg and sperm sizes. It is argued that gamete size dimorphism evolved due to the success of the small sperm in parasitizing the investment in eggs, dioecy evolves simultaneously with anisogamy and that pre-existing differences in parental investment between males and females selects for further differentiation in the same direction.

In contrast to this conflict narrative, I show in chapter 2 that gamete size dimorphism may be an adaptation that increases gamete encounter rates when large zygotes are selected. Using the Vance survival function to model the zygote fitness dependence on its size, I obtain ecological and life-history correlates of isogamy and anisogamy, which are successfully compared with data from Volvocales.

In chapter 3, I reconstruct the ancestral states corresponding to the separation of the sexes and the fertilization behavior in the animal kingdom to find that hermaphrodism and broadcast spawning of sperm are primitive and dioecy and localized fertilization behaviour derived. This suggests that dioecy did not evolve simultaneously with anisogamy, but may instead be a specialization that is adaptive when males are more effective than hermaphrodites in concentrating and delivering sperm to egg producers.

The evolutionary consequences of sperm limitation are further explored in chapter 4, where it is shown that the alternation of haploid and diploid generations in algae may be selected in environments with low fertilization probabilities as a strategy to amplify sperm concentration over subsequent generations. From the population genetics model, we expect the selected ploidy-cycle to be the one with the highest population growth rate, and hence can predict the conditions under which haplonty, diplonty and haplo-diplonty should be selected.

In chapter 5, I develop a novel and sex-neutral hypothesis for the meaning of the showy ornaments that motivated Darwin to propose the sexual selection theory. As an alternative to attracting mates, ornaments may serve as badges or bids to join cliques of individuals that dominate the resources available to a population. Using a population genetics model, we can calculate the optimal bid size and the expected fraction of individuals that should be ornamented. The distribution of the resources and the optimal sex roles for each species decide whether cliques are effective at controlling resources and if they should consist of males, females or both. Hence a new hypothesis for the evolution of intersexual monomorphism and dimorphism, and intrasexual polymorphism is obtained.

Together, chapters 2, 3 and 5 constitute an alternative account for the evolution of the sexes and sex roles based on cooperation: specializations to the sexes may be adaptations that increase the fitness of both sexes.