Abstract
The latitudinal diversity gradient is Earth’s foremost biodiversity pattern, persistent across clades and geologic time. Several recent studies have shown that diversification rates are similar among latitudes, and therefore cannot explain the latitudinal diversity gradient. An alternative explanation is that the tropics were colonized earlier than the temperate zone, allowing more time for speciation to build richness. Here we test the diversification-rate and colonization time hypotheses in freshwater ray-finned fishes, a group comprising nearly a quarter of all living vertebrate species and with a longer evolutionary history than other vertebrates. To build a global timeline for colonization and diversification, we performed ancestral area reconstructions on a time-calibrated phylogeny of all ray-finned fishes using occurrence records from over 3,000 freshwater habitats. We found that diversification rates are not systematically related to latitude, consistent with analyses in other groups. Instead, the timing of colonization to continental regions had 2–5 times more explanatory power for species richness than diversification rates. Earlier colonization explains high richness in the tropics, with the Neotropics in particular supporting the most diverse fauna for the past 100 million years. Most extratropical fish lineages colonized shortly after the end-Cretaceous mass extinction, leaving limited time to build diversity even in places where diversification rates are high. Our results demonstrate that evolutionary time, reflecting colonization and long-term persistence of lineages, is a powerful driver of biodiversity gradients.
