PALEONTOLOGICAL INSIGHTS INTO THE ORIGIN AND MAINTENANCE OF THE PRESENT-DAY LATITUDINAL DIVERSITY GRADIENT

The fossil record, which permits quantification of large-scale evolutionary and biogeographic dynamics, can provide unique insights into biological processes and principles. The latitudinal diversity gradient (LDG) seen in most groups of organisms is one of the major global biodiversity patterns, but the processes underlying the origin and maintenance of this gradient remain poorly understood. The origin and maintenance of spatial patterns of biodiversity are commonly viewed as resulting from a balance between in-situ origination and extinction; for example, high taxonomic richness in the tropics might result from high origination rates (the tropics-as-cradle hypothesis), from low extinction rates (the tropics-as-museum hypothesis), or from both. A fundamental problem in testing how evolutionary rates vary with latitude stems from our lack of knowledge of where taxa first originate. Lacking direct evidence, most neontological studies must assume that taxa originated in the climate zones where they are currently found (e.g. tropical, extratropical or boreal). Yet the well-known positive relationship between a taxon’s geologic duration and its geographic range and the fact that most high-latitude taxa tend to be widespread suggest that shifts in geographic ranges of taxa over geological time may play a major role in shaping the present day LDG. Using the fossil record of marine bivalves as it relates to the group’s present-day biogeography, we show that the tropics are the primary diversity source and accumulator, where taxa first appear and then expand outwards without losing tropical occupancy, and the high latitudes are primarily a diversity sink. We thus present a dynamic model explaining the origin of LDG, in which the tropics are so rich today not only because they are the source of young taxa, but because the geographic ranges of old, mostly widespread taxa overlap there with young and spatially restricted taxa. Under this model, the greater age of high-latitude assemblages reflect more a lack of origination rather than a strong latitudinal difference in extinction rates.