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Darwin proposed that lineages with higher diversification rates should evidence this capacity at both the species and subspecies level. This should be the case if subspecific boundaries are evolutionary faultlines along which speciation is generally more likely to occur. This pattern has been described for birds, but remains poorly understood in mammals. To investigate the relationship between species richness (SR) and subspecies richness (SSR), we calculated the strength of the correlation between the two across all mammals. Mammalian taxonomic richness correlates positively, but only very weakly, between the species and subspecies level, deviating from the pattern found in birds. However, when mammals are separated by environmental substrate, the relationship between generic SR and average SSR in non-terrestrial taxa is stronger than that reported for birds (Kendall's tau = 0.31, p < 0.001). By contrast, the correlation in terrestrial taxa alone weakens compared to that for all mammals (Kendall's tau = 0.11, p < 0.001). A significant interaction between environmental substrate and SR in phylogenetic regressions confirms a role for terrestrial habitats in disrupting otherwise linked dynamics of diversification across the taxonomic hierarchy. Further, models including species range size as a predictor show that range size affects SSR more in terrestrial taxa. Taken together, these results suggest that the dynamics of diversification of terrestrial mammals are more affected by physical barriers or ecological heterogeneity within ranges than those of non-terrestrial mammals, at two evolutionary levels. We discuss the implication of these results for the equivalence of avian and mammalian subspecies, their potential role in speciation and the broader question of the relationship between microevolution and macroevolution.

Ancient DNA is revealing new insights into the genetic relationship between Pleistocene hominins and modern humans. Nuclear DNA indicated Neanderthals as a sister group of Denisovans after diverging from modern humans. However, the closer affinity of the Neanderthal mitochondrial DNA (mtDNA) to modern humans than Denisovans has recently been suggested as the result of gene flow from an African source into Neanderthals before 100,000 years ago. Here we report the complete mtDNA of an archaic femur from the Hohlenstein-Stadel (HST) cave in southwestern Germany. HST carries the deepest divergent mtDNA lineage that splits from other Neanderthals ∼270,000 years ago, providing a lower boundary for the time of the putative mtDNA introgression event. We demonstrate that a complete Neanderthal mtDNA replacement is feasible over this time interval even with minimal hominin introgression. The highly divergent HST branch is indicative of greater mtDNA diversity during the Middle Pleistocene than in later periods.