ASYMMETRIC GEOGRAPHIC RANGE EXPANSION EXPLAINS THE LATITUDINAL DIVERSITY GRADIENTS OF FOUR MAJOR TAXA OF MARINE PLANKTON

Using the highly-resolved fossil records of four major taxa of marine plankton, we show that latitudinal gradients of species diversity arise as a consequence of asymmetric geographic range expansion rather than latitudinal variation in diversification rate, as commonly believed. The mechanism explains both the normal gradients exhibited by planktonic foraminifera and calcareous nannoplankton, and the reversed gradients exhibited by diatoms and radiolarians. Despite a close association between diversity and environmental temperature, neither per-capita speciation nor extinction rates trend significantly with temperature (or latitude) for these taxa. Species of planktonic foraminifera and calcareous nannoplankton that originate in the temperate zone preferentially spread toward, and arrive earlier in the tropics to produce a normal gradient with tropical diversity peaks; by contrast, temperate-zone originating species of diatoms and radiolarians preferentially spread toward, and arrive earlier in polar regions to produce reversed gradients with high-latitude diversity peaks. Our results localize the mechanism underlying latitudinal diversity gradients to the effect of temperature on species’ range limits rather than on probabilities of speciation and extinction. We show that this mechanism also operates in various multicellular taxa, thus providing a general explanation for the origin of latitudinal diversity gradients, a major hitherto unsolved pattern of nature.