The aims of my dissertation are to understand diversification in the genus Penstemon at three taxonomic scales: across the entire genus, where I use phylogenetic comparative methods to assess diversification rate variation and trait (phenotype and niche) lability; across species in Penstemon subgenus Dasanthera (hereafter “Dasanthera”), where I examine the geographic extent of introgression in the context of species limits, and; within Dasanthera species, where I construct species distribution models, test models of phylogeographic history, and test hypotheses of the formation of taxa through hybridization.
In Chapter 2, I employ RAD-seq data to examine the phylogeographic history and geographic context of introgression in Dasanthera. Dasanthera is sister to the rest of Penstemon, and its species are typically found in montane regions of the Pacific Northwest (PNW) of North America, where they often hybridize when in sympatry. Contrary to most phylogeography studies on PNW taxa, I find that during the Last Glacial Maximum, most Dasanthera species likely had more suitable habitat than in the present. This suggests a relatively recent range contraction for most Dasanthera species and may factor into clade-wide introgression dynamics; the majority of introgression events appear restricted to the Klamath Mountains, where species’ distributions likely had considerable overlap during the Last Glacial Maximum. Estimates of species’ relationships reveal blurred taxonomic boundaries in the Klamath Mountains, further highlighting the importance of this region for understanding species limits in Dasanthera. These results question the classical paradigm of species’ responses to glaciation in the PNW and draw attention to the importance of interpreting phylogenetic inference in the context of species’ demographic histories.
In Chapter 3, I use the Penstemon phylogeny to study macroevolutionary patterns across the genus. Penstemon is the largest genus of angiosperms endemic to North America and exhibits exceptional variation in species’ ecological niches and floral morphology. Using phylogenetic comparative methods, I identify a decoupling of diversification in Penstemon, wherein a single, large shift in the rate of net diversification precedes many smaller shifts in phenotypic diversification rates. This pattern coincides with heightened glacial activity during the Pleistocene, suggesting that the evolutionary radiation of Penstemon may have been initialized by niche-neutral diversification processes related to the colonization of new geographic regions made available from glaciation. The subsequent slowdown in rates of lineage diversification may then be attributed to density-dependent speciation processes, such as the adaptation to specific pollinator regimes.
In Chapter 4, I once again employ RAD-seq data to examine diversification processes in Dasanthera. In this chapter, I test hypotheses about the origins of two taxa from hybridization between P. davidsonii and P. fruticosus. I found mixed support for these hypotheses, detecting evidence of introgression in only one of the taxa (P. fruticosus var. serratus). Phylogenetic inference places the other taxon, P. davidsoniii var. praeteritus, firmly in a clade containing P. fruticosus. These analyses suggest that rather than being the product of hybridization, P. davidsonii var. praeteritus may be more accurately described as a variety of P. fruticosus. Overall, my results provide evidence that introgression can act as a mechanism for diversification in Penstemon.