Penstemon davidsonii - Penstemon newberryi eastern Sierra Nevada hybrid zones
A central goal in evolutionary biology is to understand how species boundaries are maintained in the face of gene flow. While gene flow between species often accompanies the formation of hybrids, the genome-wide effects of hybridization will depend on the presence and nature of reproductive isolating barriers. Two North American wildflower species with divergent floral syndromes, Penstemon davidsonii (bee syndrome) and P. newberryi (bird syndrome), have formed hybrid zones in the eastern Sierra Nevada for at least 1940. Despite decades of hybridization, each species appears to have maintained phenotypic integrity while forming phenotypically intermediate hybrids in narrow elevational bands. The genetic data used for this study was used for genome-wide association studies (GWAS) and hierarchical Bayesian genomic cline analysis to ask whether divergent floral syndromes enforce reproductive isolation in this species pair as predicted by classic models of pollinator-driven ecological speciation.