Abstract
The evolution from outcrossing to predominant self-fertilization represents one of the most common transitions in flowering plant evolution. This shift in mating system is almost universally associated with the "selfing syndrome," characterized by marked reduction in flower size and a breakdown of the morphological and genetic mechanisms that prevent self-fertilization. In general, the timescale in which these transitions occur, and the evolutionary dynamics associated with the evolution of the selfing syndrome are poorly known. We investigated the origin and evolution of selfing in the annual plant Capsella rubella from its self-incompatible, outcrossing progenitor Capsella grandiflora by characterizing multilocus patterns of DNA sequence variation at nuclear genes. We estimate that the transition to selfing and subsequent geographic expansion have taken place during the past 20,000 years. This transition was probably associated with a shift from stable equilibrium toward a near-complete population bottleneck causing a major reduction in effective population size. The timing and severe founder event support the hypothesis that selfing was favored during colonization as new habitats emerged after the last glaciation and the expansion of agriculture. These results suggest that natural selection for reproductive assurance can lead to major morphological evolution and speciation on relatively short evolutionary timescales.