Mechanisms defining ecological range limits in a Penstemon hybrid zone

Hybrid zones, areas where divergent species cross-fertilize, can be used to study the mechanisms that isolate species. Studies of selection in hybrid zones indicate that endogenous factors like genomic incompatibilities as well as exogenous factors like pollinators and climate can be important in structuring species boundaries. I studied the role of endogenous and exogenous factors in determining the range limits of two perennial wildflowers, Penstemon newberryi and P. davidsonii, and their hybrids, along an elevational gradient in the Sierra Nevada mountains.; First, I measured floral morphology and surveyed pollinator visitation along the elevational gradient, identifying over 62 species of floral visitors. Morphological measurements were used to construct a plant hybrid index and examine the correlation with elevation and with floral visitors. The visitor community that visited P. davidsonii was somewhat distinct from the community that visited P. newberryi and hybrids. Eleven common pollinator species visited both parent species and could be contributing to hybrid formation.; In the second chapter, I identified physiological differences between both species and hybrids. Penstemon davidsonii had less negative pre-dawn and mid-day water potential, lower water-use-efficiency, and took less time to produce mature fruits than its montane relative, P. newberryi. Hybrids were mostly intermediate, but gas exchange traits differed depending on genotype.; In the third chapter, I investigated the relative importance of endogenous and exogenous isolation. I conducted a hand-pollination experiment in which the pure species and hybrids were crossed in all possible combinations. Hybrid crosses did not result in few seeds or less viable seeds. I performed a reciprocal transplant experiment, planting both species and hybrids into gardens at low, middle, and high altitudes. In each garden, the hybrid with the native cytoplasm had a higher survival rate, suggesting local adaptation to different elevations.; This study indicates that exogenous isolating mechanisms, like physiological and pollinator differences, act to maintain P. newberryi and P. davidsonii as distinct at the elevational extremes of their ranges. My results illustrate the importance of studying hybrid performance in multiple environments and in generating reciprocal hybrids to effectively test for isolating mechanisms in natural hybrid zones.