Data from: Environmental harshness is positively correlated with intraspecific divergence in mammals and birds

Life on Earth is conspicuously more diverse in the tropics. Although this
intriguing geographical pattern has been linked to many biotic and abiotic
factors, their relative importance and potential interactions are still
poorly understood. The way in which latitudinal changes in ecological
conditions influence evolutionary processes is particularly controversial,
as there is evidence for both a positive and a negative latitudinal
gradient in speciation rates. Here, we identify and address some
methodological issues (how patterns are analysed and how latitude is
quantified) that could lead to such conflicting results. To address these
issues, we assemble a comprehensive data set of the environmental
correlates of latitude (including climate, net primary productivity and
habitat heterogeneity) and combine it with biological, historical and
molecular data to explore global patterns in recent divergence events
(subspeciation). Surprisingly, we find that the harsher conditions that
typify temperate habitats (lower primary productivity, decreased rainfall
and more variable and unpredictable temperatures) are positively
correlated with greater subspecies richness in terrestrial mammals and
birds. Thus, our findings indicate that intraspecific divergence is
greater in regions with lower biodiversity, a pattern that is robust to
both sampling variation and latitudinal biases in taxonomic knowledge. We
discuss possible causal mechanisms for the link between environmental
harshness and subspecies richness (faster rates of evolution, greater
likelihood of range discontinuities and more opportunities for divergence)
and conclude that this pattern supports recent indications that
latitudinal gradients of diversity are maintained by simultaneously higher
potentials for both speciation and extinction in temperate than tropical
regions.