Comparative diversification analyses of Hydrangeaceae and Loasaceae reveal complex evolutionary history as species disperse out of Mesoamerica.

Premise: The movement of lineages into novel areas can promote ecological opportunity and adaptive radiation, leading to significant species diversity. Not all studies, however, have identified support for ecological opportunity associated with novel intercontinental colonizations. To gain key insights into the drivers of ecological opportunity, we tested whether intercontinental dispersals resulted in ecological opportunity using the Hydrangeaceae-Loasaceae clade, which has numerous centers of diversity across the globe.

Methods: A time-calibrated phylogeny was reconstructed from four molecular markers. We tested for bursts of speciation rates followed by a decrease as expected phylogenetic patterns under an ecological opportunity model. Ancestral ranges were estimated using historical biogeographic analyses to examine the relationships of ancestral distributions and habitats with speciation and extinction rates.

Results: Hydrangeaceae and Loasaceae originated in arid Mesoamerica, then dispersed into South America, Eurasia, and eastern North America. Six clades experienced increased diversification rates, but those increases were not associated with transitions into new continental areas. Mentzelia section Bartonia was the only clade that exhibited a burst of speciation followed by a decrease. Both families originated in arid environments and experienced multiple transitions into mesic and tropical environments, but Loasaceae experienced a higher speciation-to-extinction ratio than Hydrangeaceae in the western Nearctic.

Conclusions: Dispersal between continents did not trigger speciation rate shifts in Loasaceae and Hydrangeaceae. Instead, shifts occurred in regions inhabited by intrafamilial relatives and were likely driven by climate change in the Miocene, where species in drier microhabitats diversified into newly created habitats.