Evolutionary dynamics of the mating system in a population of Mimulus guttatus

A persistent controversy in evolutionary biology concerns the relative importance of directional versus balancing selection in mating system evolution. Observations of natural plant populations have revealed large numbers of species with mixed mating systems that involve both self- and cross-fertilization. Paradoxically, early models of mating system evolution suggested that intermediate selfing rates represent unstable evolutionary equilibria. These models predict a bimodal distribution of selfing rates in natural plant populations. More recent models have predicted a narrow range of parameter values over which stability of intermediate rates is possible. Empirical studies to date have examined population level estimates of model parameters. It is individual variation that is of interest for studies of evolutionary dynamics of the mating system. For this reason, this study focused at the individual level within a population of Mimulus guttatus, the common monkeyflower. Population estimates of the selfing rate for the study population were intermediate (0.41-0.16). The research combines estimates for 47 individuals in this population for selfing rate, inbreeding depression, and fecundity with population level estimates for genetic correlation between outcross mates, to develop a fitness function that provides relative fitness estimates for each individual. Linear and quadratic regression of relative fitness on the selfing rate show no evidence of directional, stabilizing or disruptive selection on the selfing rate. Flower production is the major factor influencing fitness. Reproductive morphology and potential for autogamous seed production were also correlated with individual selfing rate estimates. My data suggest that environmental effects on the selfing rate in this population and phenotypic plasticity in the study organism overwhelm the effects of selection on the mating system. Suggestions for further study include greater emphasis on description of ecological influences on the selfing rate including pollinator behavior.