Comparing prezygotic isolating mechanisms in unimodal and bimodal plant hybrid zones

Natural hybrid zones provide insights into the evolution of reproductive isolation and the possible outcomes following its breakdown. The strength of reproductive isolation will affect the frequency of hybrids in a contact site, resulting in a unimodal site where isolation is weaker, and a bimodal site where isolation is stronger. Variation in the frequency of hybrids among contact sites of the same species pair allows assessment of the importance of individual reproductive isolating mechanisms in determining rates of hybridization, and suggests that local ecological circumstances might affect the evolutionary outcome of contact between species.; I surveyed Ipomopsis aggregata - I. tenuituba contact sites in the western Rockies, and quantified variation in frequency of hybrids and spatial distribution of parent species using floral morphological markers. I identified two sites that provided a contrast between a unimodal, clinal site (Poverty Gulch: PG) and a bimodal, mosaic site (Grizzly Ridge: GR). I further characterized these two sites using molecular genetic markers, and found evidence of more extensive introgression of neutral markers at GR than was suggested by floral morphology. However, there was still more genetic structure at GR than at PG, consistent with a prediction of less gene flow at that bimodal site.; Surveys of hybrid zones of many taxa have suggested that hybridization is primarily limited by prezygotic isolating mechanisms. I tested that prediction by comparing conspecific pollen advantage and pollinator-mediated reproductive isolation at PG and GR. Pollen seed-siring success was asymmetrical, and fitness of mixed pollinations was reduced compared to single-species pollinations at GR, but not at PG. Pollinator preference and constancy were stronger at GR in both natural populations and artificial arrays. Temperature differences between the sites might affect hawkmoth foraging behavior directly and hummingbird behavior indirectly through hawkmoth effects on nectar resources. Differences in spatial structure might also influence pollinator foraging choices, creating stronger preference at GR. These factors likely combine to produce stronger pollinator-mediated reproductive isolation at GR than at PG.; This study supports the conclusion that prezygotic isolation is important in limiting hybridization even in the absence of postzygotic isolation. It also provides evidence that reproductive isolation can depend in part on local conditions.