Stone, B. W. | bioRxiv Science

bioRxiv2021

Asynchronous rates of lineage, phenotype, and niche diversification in a continental-scale adaptive radiation

Rapidly diversifying clades are central to the study of diversification dynamics. This central importance is perhaps most apparent when rapid evolution occurs across several axes of diversification (e.g., lineage, phenotype, and niche); such clades facilitate investigations into the interplay between adaptive and non-adaptive diversification mechanisms. Yet, empirical evidence from rapidly evolving clades remains unclear about the relationships, if any, across diversification axes. This is especially apparent regarding the timing of diversification rate shifts. We address this knowledge gap through comparisons of the rate and timing of lineage, phenotypic, and niche diversification in Penstemon, a rapidly-evolving angiosperm genus. We find that diversification rate shifts in Penstemon are asynchronous; while we identify a burst and subsequent slowdown in lineage diversification rate [~]2.0-2.5 MYA, shifts in phenotypic and niche diversification rates either lagged behind temporally or did not occur at all. We posit that this asynchronicity in diversification rate shifts is the result of initial niche-neutral diversification followed by adaptive, density-dependent processes. Our findings contribute to a growing body of evidence that asynchronous shifts in diversification rates may be common and question the applicability of expectations for diversification dynamics across disparate empirical systems.

evolutionary biology↗

bioRxiv2020

Phylogeographic analysis of shrubby beardtongues reveals range expansions during the Last Glacial Maximum and implicates the Klamath Mountains as a hotspot for hybridization

Quaternary glacial cycles often altered species geographic distributions, which in turn altered the geographic structure of a species genetic diversity. In many cases, glacial expansion forced species in temperate climates to contract their ranges and reside in small pockets of suitable habitat (refugia), where they were likely to interact closely with other species, setting the stage for potential gene exchange. These introgression events, in turn, would have degraded species boundaries, making the inference of phylogenetic relationships challenging. Using high-throughput sequence data, we employ a combination of species distribution models, models of demographic history, and hybridization tests to assess the effect of glaciation on the geographic distributions, phylogenetic relationships, and patterns of gene flow of five species of Penstemon subgenus Dasanthera, long-lived shrubby angiosperms distributed throughout the Pacific Northwest of North America. Surprisingly, we find that rather than reducing their ranges to small refugia, most Penstemon subgenus Dasanthera species experienced increases in suitable habitat during the Last Glacial Maximum. We also find substantial evidence for gene exchange between species, with the bulk of introgression events occurring in or near the Klamath Mountains of southwestern Oregon and northwestern California. Subsequently, our phylogenetic inference reveals blurred taxonomic boundaries in the Klamath Mountains, where introgression is most prevalent. Our results question the classical paradigm of temperate species responses to glaciation, and highlight the importance of contextualizing phylogenetic inference with the demographic histories of the species of interest.

evolutionary biology↗