A species is a group of animals that can interbreed freely amongst themselves. Some species contain subspecies – populations within a species that differ from each other by having different physical traits and their own breeding ranges. Northern giraffes have three subspecies that usually live in different locations to each other and red foxes have the most subspecies – 45 known varieties – spread all over the world. Humans have no subspecies.
van Holstein said: “We are standing on the shoulders of giants... My research investigating the relationship between species and the variety of subspecies proves that sub-species play a critical role in long-term evolutionary dynamics and in future evolution of species. And they always have, which is what Darwin suspected when he was defining what a species actually was.”
van Holstein’s research also proved that evolution happens differently in land mammals (terrestrial) and sea mammals and bats (non-terrestrial) because of differences in their habitats and differences in their ability to roam freely.
Here is the scientific paper: "Terrestrial habitats decouple the relationship between species and subspecies diversification in mammals" (The Royal Society)