Biodiversification and the Latitudinal Diversity Gradient over deep time: insights from mechanistic models

The rise in species richness from the poles to the tropics, known as the latitudinal diversity gradient (LDG), is one of the most widespread patterns in the distribution of modern ecosystems. Although first documented more than 200 years ago, its origins, evolutionary dynamics, and underlying mechanisms remain unresolved. While geological and climatic changes are recognized as key drivers of biodiversity patterns, the precise causal factors shaping the LDG and their relative contributions to species richness gradients are still debated. Here, we explore how spatiotemporal variations in the physical environment influence the LDG by simulating the global diversification of terrestrial mammals over the past 125 million years using a spatially explicit eco-evolutionary model (gen3sis). This approach allows us to investigate both the mechanisms driving the LDG and broader biodiversification processes in dynamic landscapes, integrating changes in geological, climatic, and surface processes. Our findings indicate that the modern LDG is largely shaped by paleoclimatic and paleogeographic factors, with limited influence from surface processes. This gradient has persisted since the Cretaceous, steepening and stabilizing in width from the early Tertiary. Over deep time, LDG drivers demonstrate a strong influence of tectonic activity on speciation rates. The modeled scenarios also support an "out of the tropics" model in which species primarily originate in the tropics and disperse toward the poles without losing their tropical presence. As a result, the tropics are defined not only as a cradle, fostering the origination of new species, but also as a museum, preserving biodiversity over deep time.