Timing and origins of Mexican and Central American oak diversity.

Abstract

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 (<i>Quercus</i>) 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 (<i>Q.</i> section <i>Quercus</i>) and red oaks (<i>Q.</i> sect. <i>Lobatae</i>) independently colonized Mexico <i>c.</i> 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.