ON THE GENERATION OF LATITUDINAL DIVERSITY GRADIENTS

Biodiversity is unevenly distributed across the surface of the Earth. The tropics harbor greater numbers of species than temperate zones, a pattern seen in both ancient and modern times. The causal mechanism(s) responsible for these latitudinal gradients (LDG) still remain debated, although centuries have passed since their initial discovery. It is still unknown whether rates of speciation and extinction directly follow a latitudinal gradient, or whether secondary dispersal contributes to higher species diversity in the tropics. We introduce a cellular automaton algorithm designed to simulate processes of extinction, allopatric speciation, and dispersal within the evolving climate system of the Pleistocene. We test whether differential rates of speciation and extinction in tropical versus temperate climates can alone produce LDGs. We additionally test whether the magnitude of the diversity gradient on a continental scale depends on the degree of environmental heterogeneity observed across continents through time. Simulations were seeded using empirical data on species’ macroecological attributes, in conjunction with a continuous 120,000-year record of high-resolution climate conditions. Preliminary results centered on Eurasia suggest that speciation rates are elevated in regions characterized by high climatic heterogeneity. These diversity hot spots developed without invoking higher extinction rates at higher latitudes. Importantly, differential speciation rates were obtained solely from the interaction of species’ macroecological traits, namely dispersal ability and climatic tolerance, with a dynamic geosphere over time. This work provides a null model for spatial patterns of biodiversity dependent only on the contributions of species’ physiological tolerances, dispersal, and the evolving climate system.