A latitudinal gradient in biodiversity has existed since before the time of the dinosaurs, yet
how and why this gradient arose remains unresolved. Here we review two major hypotheses
for the origin of the latitudinal diversity gradient. The time and area hypothesis holds that
tropical climates are older and historically larger, allowing more opportunity for
diversification. This hypothesis is supported by observations that temperate taxa are often
younger than, and nested within, tropical taxa, and that diversity is positively correlated with …