The latitudinal species richness gradient in New World woody angiosperms is consistent with the tropical conservatism hypothesis

Abstract

Significance The diversity of living things generally peaks in the tropics and declines toward the poles. This “latitudinal gradient” is Earth’s most prevalent biogeographic pattern, but biologists do not agree about its cause. Here, we use geographic and evolutionary data for over 12,500 species of woody flowering plants to test the “tropical conservatism hypothesis,” which attributes the phenomenal diversity of tropical environments to their large extent over the past 55 million years (My) and the evolutionary conservatism of environmental tolerances. As predicted, we find that transitions between tropical and temperate environments are quite rare over the evolutionary history and that most temperate lineages originated as Earth cooled over the past 34 My. Thus, the correlation between diversity and climate reflects plants’ evolutionary history.