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

Abstract

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.