THE EVOLUTION OF SELF-COMPATIBILITY IN  NON-EQUILIBRIUM POPULATIONS

Nathan Charles Layman

One major goal of evolutionary biology is to explain patterns of variation. Often traits with the highest levels of between species diversity are intimately tied to breeding systems. Angiosperms in particular are noted for their tremendous diversity in the form and function of their flowers and much of this variation is thought to reflect a history of natural selection favoring outcrossing. Surprisingly, while outcrossing predominates in most angiosperm lineages, transitions in breeding system toward higher amounts of selfing represent the most common evolutionary trend in flowering plants. Despite the generality of this pattern, it is often difficult to identify why these transitions originally occurred due to the obscuring effects of subsequent, rapid secondary evolution. The purpose of this dissertation was to identify the conditions leading to the invasion of selfing mutations using a combination of field experiments and modeling techniques. These methods were used to address the following questions: 1) Why are outcrossing adaptations such as self-incompatibility maintained, despite persistent input of selfing mutations?; 2) Does polyploidy facilitate the evolution of selfing?; and 3) How does demographic uncertainty influence the distribution of breeding systems on islands? Field experiments show that the invasion a self-compatibility mutation in Leavenworthia alabamica is generally inhibited by strong inbreeding depression and seed discounting. It is only when these costs are temporarily reduced or when mate limitation becomes periodically intense that selfing is favored. However, capturing these kinds of rare, transient events can be exceedingly difficult in nature. Using computational models, we examined two of the major transitions thought to facilitate the evolution of selfing. First, polyploidy has been thought to have profound consequences on inbreeding depression. Using an individual based model, we show that the bottleneck associated with the formation of newly polyploid lineages provides one of the most permissive conditions for the spread of selfing. Second, a similar model shows that the reduction in inbreeding depression and enhanced mate limitation imposed by long distance colonization shapes patterns of breeding systems on islands. Overall, these results suggest that transient, non-equilibrium conditions likely play a vital role in the evolution of breeding systems in flowering plants.

THE EVOLUTION OF SELF-COMPATIBILITY IN NON-EQUILIBRIUM POPULATIONS

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