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.

Their findings show subspecies are early versions of species, Moreover, the more species in a genus typically means more subspecies are in that genus, and the relationship between species and subspecies depends on whether the species live on land.