Three hybrid zones between the predominantly outbreeding Mimulus guttatus and inbreeding M. platycalyx were examined within an ephemeral watershed in California. Cline widths in most polygenic traits corresponded with summer soil moisture. Concordant clines were found for most traits, suggesting recent contact or selection against hybrids. At one site, we are using cline theory to measure the strength of selection in situ on these traits, and to test the feasibility of several competing hypotheses to explain the observed patterns. At this site, characters associated with mating system appear to have introgressed to a significantly greater degree than other distinguishing features. Previous studies and crossing experiments show a large degree of inbreeding depression and heterosis throughout this zone, and greatly enhanced autofertility as a result of hybridization (and hence moderation of floral morphology), relative to M. guttatus. We hypothesize that balancing selection acting on floral morphology is responsible for these patterns, as it balances the need for reproductive assurance vs. the cost of inbreeding depression. We also suggest that by invading a niche in which early flowering is favored (because of soil drying), both M. platycalyx and M. nasutus are forced to flower in weather that pollinators find unattractive (cold and wet). Hence selection for reproductive assurance may be the driving force behind the evolution of selfing in these taxa. Our analyses show that it is more profitable to consider evolutionary dynamics on a trait-by-trait basis than to study these from the perspective of pure species and hybrids.

Key words: autofertility, clines, hybrid zones, hybridization, mating systems, Mimulus