Timing and origins of Mexican and Central American oak diversity

The origins and assembly of temperate biodiversity hotspots remain poorly understood. This knowledge gap is particularly evident in low-latitude montane regions of the Americas, where northern lineages have repeatedly colonized and diversified. Here we investigate the evolutionary history of oaks (Quercus) in the Americas, with a focus on their parallel radiation into Mexican and Central American montane forests. Using a time-calibrated phylogeny, we show that white oaks (Q. section Quercus) and red oaks (Q. sect. Lobatae) independently colonized Mexico c. 25 Mya. This coincides with a variety of ecosystem changes at the Oligocene-Miocene boundary, including aridification. Diversification analyses reveal that montane habitats acted as cradles of oak diversity, with higher speciation rates associated with movement into topographically complex, higher-elevation landscapes. Despite 20 Ma of independent evolution in Mexico and Central America, red and white oaks show remarkable convergent evolution in climate niche. Our results highlight how extrinsic factors-migration into novel environments-coupled with intrinsic niche lability can facilitate rapid diversification. Our study thus provides insights into the origins of temperate biodiversity and the evolutionary processes shaping species-rich montane ecosystems.