Widely distributed plant taxa of western North America often display such variation due to divergent selection across heterogeneous landscapes that support environmental gradients as well as historical periods of isolation owing to biogeographical barriers and distributional shifts associated with Quaternary climate oscillations. Temporal and spatial variation in these processes often leads to a continuum of phenotypic and genetic divergence with variable reproductive isolation among populations and species. Polyploidization has additionally been a prominent driving factor in diversification with extended ecological consequences. My dissertation research is applying landscape genetic analyses of high throughput sequencing data to address factors underlying the formation and maintenance of diversification and reproductive barriers across two foundational shrub species of western North America, rubber rabbitbrush (Ericameria nauseosa) and big sagebrush (Artemisia tridentata). Overall, the two shrubs illustrate convergent evolution of subspecific genetic differentiation with limited admixture across the range that is strongly predicted by environmental variation (i.e., mosaic hybrid zone). Within subspecific designations of both species, there is strong population structure that coincides with both geography and environment elucidating that they are likely independently evolving lineages. Furthermore, A. tridentata differentiation and admixture is not only determined by the environment, but also by ploidal variation. Only individuals of the same ploidal level (e.g., diploid [2n] or tetraploid [4n]) can interbreed and those of different levels are reproductively isolated (i.e., triploid block). These results provide a range-wide perspective on how genetic variation and reproductive isolation is organized among and within subspecific lineages, ploidal levels, and environments, and have relevance for understanding the genomic and environmental factors shaping reproductive isolation across heterogeneous environments. Given its ecological significance of these two foundational shrub species, further understanding of these patterns and processes should inform ecological restoration and expectations for evolution across changing environments in western North America
