EVIDENCE FOR NATURAL-SELECTION ON MATING SYSTEM IN MIMULUS (SCROPHULARIACEAE)

We used two approaches to examine the role of natural selection in the evolution of selfing in Mimulus. First, using cpDNA and isozyme varia tion we investigated the phylogenetic relationships among a number of populations of two selfing (M. micranthus and M. laciniatus) and two o utcrossing (M. guttatus and M. nasutus) species to determine the frequ ency of evolution of selfing within the genus. We hypothesized that th e independent evolution of selfing taxa from outcrossing taxa would be strong evidence that the mating system is under the influence of natu ral selection. Second, we used phenotypic selection analysis to measur e the presence and intensity of selection on floral characters associa ted with the mating system in extant populations of the outcrossing sp ecies M. guttatus. An unrooted Wagner tree constructed from cpDNA vari ation and a dendrogram constructed from Nei's genetic distances at iso zyme loci both indicated the independent origin of the two selfing tax a, M. micranthus and M. laciniatus, from different ancestral M. guttat us populations. Phenotypic selection analysis demonstrated that both d irect and indirect directional selection intensities were variable bet ween populations but could be high. Decreased stigma-anther separation was favored in the one population that was likely pollinator-limited. Our results indicated that natural selection was strong enough to cau se the repeated evolution of selfing in Mimulus and that the selective agent may be inadequate pollinator service.