Evolutionary Branching and the Evolution of Anisogamy

Populations of most sexual species are anisogamous, i.e. they consist of two types of individuals producing gametes of different size. The evolution of anisogamy is usually explained with models that either rely on mutations with large effects or are based on populations with pre-existing mating types. Here we present a model for the evolution of anisogamy that does not rely on either of those assumptions. We used the theory of adaptive dynamics to study the evolution of gamete size allowing arbitrarily small mutations in a population without mating types. As in previous models, we assumed that the survival of a zygote depends on its size, and therefore on the sum of the sizes of the gametes that formed it, and we altered the form of this relationship to investigate a broad range of ecological conditions. We found that the emergence of gamete size dimorphism is a frequent and general evolutionary outcome of our models. Using individual-based models we further show that gamete size dimorphism frequently leads to anisogamous populations consisting of two types of genotypes that differ in the size of the gametes they produce.