Berg, JasonThe mating system of a colonizing plant population will influence establishment success in a new region, as well as the propensity to invade locations beyond the initial point of introduction. In mixed-mating plant species, defined as those that are capable of both self-fertilization and outcrossing, the mating system of nascent populations introduced to regions outside of the native distribution is often free to evolve. While theories exist that attempt to model the relationship between colonization, spread, and mating system, few studies have examined this dynamic in nature among native, naturalized, and invasive populations of a species. My dissertation addresses several questions pertaining to the evolution of mating system in the context of an invasive plant species, namely Mimulus guttatus. I use molecular approaches and crossing experiments to determine the importance of selfing and outcrossing in several native, naturalized, and invasive populations of M. guttatus. I first use data from highly variable molecular markers designed for M. guttatus to assess outcrossing rates, inbreeding coefficients, and inbreeding depression in nature in the native and non-native populations. This demonstrates the role of selfing and outcrossing, as well as the fitness consequences of each, in non-native populations compared to native populations in their natural setting. Next, I use the same molecular markers to examine population structure within and among the M. guttatus populations to determine genetic diversity and relationships between the native and non-native populations. The results from this chapter demonstrate how mating system dictates the amount of genetic diversity in the populations and allows for inferences as to the native sources for naturalized and invasive populations. Finally, I conduct a greenhouse crossing experiment to experimentally determine the fitness consequences of selfing and outcrossing. I conclude that a history of outcrossing in some invasive populations translates to greater inbreeding depression compared to native populations, and that highly clonal naturalized populations experience less inbreeding depression than the invasives. Throughout, I explore the role of mating system in invasion success and underscore that different establishment pathways are possible in an invasive plant species. Therefore, these studies contribute to the scholarship on evolution in invasive plants.enDissertationEcologyEvolution & developmentGeneticsheterosisinbreeding depressioninvasive plantsmating system evolutionMimulus guttatusphenotypic plasticity