Revisiting the Axelrod’s Hypothesis: What is the Origin of the Evergreen Sclerophyllous Oaks in the Northern Hemisphere?

Abstract

Axelrod proposed a hypothesis to explain the origin and evolution of the evergreen sclerophyllous flora in the Northern Hemisphere, suggesting an insitu differentiation of clades at low-mid latitudes during the Cenozoic. This hypothesis highlights a continuous belt of sclerophyllous vegetation that was fragmented over time due to tectonic movements and climatic changes, including cooling and drying. The genus Quercus exhibits numerous sclerophyllous species, which reach their highest diversity in the latitudes he identified. However, the evolutionary histories of the two subgenera within Quercus-the New World and Old-World clades-show distinct patterns. This review examines whether Axelrod’s hypothesis holds true for Quercus by synthesizing recent molecular phylogenies, fossil records, and biogeographic analyses. Findings reveal that while Old World oaks largely align with Axelrod’s hypothesis, the recent diversification of the tropical NewWorld oaks challenge the framework he proposed.