Diversification, disparification, and hybridization in the desert shrubsEncelia

SummaryThere are multiple hypotheses for the spectacular plant diversity found in deserts. We explore how different factors, including the roles of ecological opportunity and selection, promote diversification and disparification inEncelia, a lineage of woody plants in the deserts of the Americas.Using a nearly complete species-level phylogeny along with a broad set of phenotypic traits, we estimate divergence times and diversification rates, identify instances of hybridization, quantify trait disparity, and assess phenotypic divergence across environmental gradients.We show thatEnceliaoriginated and diversified recently (mid-Pleistocene) and rapidly, with rates comparable to notable adaptive radiations in plants.Enceliaprobably originated in the hot deserts of North America, with subsequent diversification across steep environmental gradients. We uncover multiple instances of gene flow between species. The radiation ofEnceliais characterized by fast rates of phenotypic evolution, trait lability, and extreme disparity across environments and between species-pairs with overlapping geographic ranges.Enceliaexemplifies how interspecific gene flow in combination with high trait lability can enable exceptionally fast diversification and disparification across steep environmental gradients.