On Mar. 18, 2020, Laura van Holstein and and Robert A. Foley published “Terrestrial habitats decouple the relationship between species and subspecies diversification in mammals” in the Proceedings of the Royal Society.

Darwin’s hypothesis states that a species belonging to a larger genus should also include more subspecies.

The study show this hypothesis is weakly supported in mammals as a whole, but when taxa are separated by ecological substrate, non-terrestrial groups show much stronger correlations between taxonomic richness at the two levels than terrestrial mammals do.

As these processes also appear more tightly coupled in birds, we suggested fundamental factors unique to terrestrial habitats, such as increased exposure to ecological or physical barriers, increasingly become the causal drivers of divergence at two levels.

These results imply that the evolutionary relationship between subspecies and species might differ between mammalian taxa, and that this relationship is mediated significantly by ecological substrate.