Pleistocene glaciations caused the latitudinal gradient of within-species genetic diversity

Pleistocene glaciations caused the latitudinal gradient of within-species genetic diversity.

Abstract

Intraspecific genetic diversity is a key aspect of biodiversity. Quaternary climatic change and glaciation influenced intraspecific genetic diversity by promoting range shifts and population size change. However, the extent to which glaciation affected genetic diversity on a global scale is not well established. Here we quantify nucleotide diversity, a common metric of intraspecific genetic diversity, in more than thirty-eight thousand plant and animal species using georeferenced DNA sequences from millions of samples. Results demonstrate that tropical species contain significantly more intraspecific genetic diversity than nontropical species. To explore potential evolutionary processes that may have contributed to this pattern, we calculated summary statistics that measure population demographic change and detected significant correlations between these statistics and latitude. We find that nontropical species are more likely to deviate from neutral expectations, indicating that they have historically experienced dramatic fluctuations in population size likely associated with Pleistocene glacial cycles. By analyzing the most comprehensive data set to date, our results imply that Quaternary climate perturbations may be more important as a process driving the latitudinal gradient in species richness than previously appreciated.

Lay Summary

It is well appreciated among biologists that species richness is higher near the equator than at temperate latitudes. Using data from thirty-eight thousand species, we demonstrate here that a similar gradient is present for intraspecific genetic diversity. Results indicate that nontropical species were more likely to have undergone significant changes in their historical population demography than were tropical species. Decades of phylogeographic work have suggested that Quaternary glaciation forced temperate species into isolated glacial refugia. Under this scenario, genetic diversity would be lost due to reduced population size, population bottlenecks, and genetic drift. Our analyses represent the most comprehensive test of the Quaternary glaciation hypothesis to date and suggest that the latitudinal gradient in intraspecific genetic diversity is at least partially a result of Pleistocene glaciation.