Genetic and ecophysiological consequences of natural hybridization between Ipomopsis aggregata and I. tenuituba

Natural hybridization has long been recognized as an important factor in the evolution of flowering plants, and hybrid zones provide natural laboratories in which to study the evolutionary processes that influence interactions between divergent populations. The studies detailed in this dissertation examine the genetic and ecophysiological consequences of hybridization in a natural Ipomopsis hybrid system in Poverty Gulch, Colorado, to understand the relative importance of different selective mechanisms to the evolutionary outcome of natural hybridization. Using maternally- and biparentally-inherited genetic markers, I first characterized the spatial genetic structure of the Ipomopsis hybrid zone. A strong incongruence between the markers suggests that gene flow in this hybrid zone primarily occurs via pollen transfer from I. aggregata populations into hybrid and historically I. tenuituba populations. However, at the center of the natural hybrid zone, hybrids and I. tenuituba have a survival advantage over I. aggregata that may counter the pollinator-mediated advance of I. aggregata genes. This survival difference appears to reflect physiological adaptations to abiotic factors that vary across the hybrid zone. Overall, the exposed, rocky slopes where I. tenuituba grows in Poverty Gulch are warmer and drier than the more vegetated, mesic I. aggregata sites, while the center of the hybrid zone is even more xeric than either parental habitat. In general, ecophysiological traits expressed in these plants were consistent with their natural habitat, even when grown under common greenhouse conditions. Ipomopsis tenuituba had higher water-use efficiency, maximal photosynthetic rate, and optimal temperature for photosynthesis than I. aggregata. Hybrids were largely tenuituba-like or positively transgressive for several of these physiological traits, which are consistent with adaptations reported for plants found in xeric sites. Consequently, while pollinators select for aggregata-like floral morphology, ecological selection appears to favor I. tenuituba-like ecophysiological traits, thereby putting hybrids with a mosaic of these characters at an advantage in the center of the hybrid zone. As a result, the overall genetic structure of this hybrid zone likely depends on a complex mixture of selection mediated by both pollinators and the environment.