Evolutionary dynamics of a quantitative trait in a finite asexual population.

In finite populations, mutation limitation and genetic drift can hinder evolutionary diversification. We consider the evolution of a quantitative trait in an asexual population whose size can vary and depends explicitly on the trait. Previous work showed that evolutionary branching is certain (\"deterministic branching\") above a threshold population size, but uncertain (\"stochastic branching\") below it. Using the stationary distribution of the populations trait variance, we identify three qualitatively different sub-domains of \"stochastic branching\" and illustrate our results using a model of social evolution. We find that in very small populations, branching will almost never be observed; in intermediate populations, branching is intermittent, arising and disappearing over time; in larger populations, finally, branching is expected to occur and persist for substantial periods of time. Our study provides a clearer picture of the ecological conditions that facilitate the appearance and persistence of novel evolutionary lineages in the face of genetic drift.