And while his seminal work laid the foundation for evolutionary biology
, one major point of his was never proven.
Nearly 140 years after his death, a University of Cambridge researcher has found strong evidence that one of Darwin's hypotheses is true. That hypothesis states that a species belonging to a larger genus should also include more subspecies.
The graduate student, Laura van Holstein, was able to prove this using a tool Darwin didn't have: Data modeling.
"I've just reported evidence in favor of a hunch of good old Darwin's," van Holstein, a biological anthropology PhD student at Cambridge and study lead author, told CNN. "I think this has big implications for evolutionary biology."
Darwin's subspecies theory
To understand the significance of this development, it's best to start with a refresher on the following taxonomy (or naming conventions): genus, species and subspecies.
- A genus is a group of animals with similar traits. This can include multiple species. For example, most bears belong to the genus Ursus.
- A species is a group of similar animals that can interbreed and exchange genes to reproduce. The brown bear is a species under the Ursus umbrella.
- A subspecies is a group within a species that looks phenotypically different from the rest of the species and has its own breeding range that doesn't overlap with the rest of the species. A grizzly bear is a subspecies of brown bear.
Darwin predicted that species in a larger genus should also include more subspecies. But he never elaborated on why.
The evolutionary scientists who followed Darwin suggested that a subspecies represents an early stage of species formation. But that was difficult to prove. After all, evolution takes time.
She used a tool Darwin never did
Van Holstein, however, had what those scientists didn't: Data modeling software.
She wanted to show that the number of subspecies in a species is correlated to the number of species in a genus. If she could prove that, she'd have more evidence to suggest that subspecies are the "raw material" for a new species, she said.
She ran a few of the models: First, she devised a model using taxonomical information about different species to show that a genus with more species also has more subspecies to prove a relationship.
Then, she took it a step further than Darwin: She created models to show the relationship between species richness (the number of species in a genus) and subspecies richness is stronger in mammals that don't live on land -- namely bats and whales.
One more model found that the number of subspecies in a genus is predicted by the size of a species' range -- and in land mammals, a bigger range was linked to a higher number of subspecies in a genus.
Her findings show subspecies are early versions of species
Not only did van Holstein prove one of Darwin's points, but she expanded on his findings: 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.
In species that live on land, environmental barriers impact how its species form. But in species that live in the air or the ocean, species formation depends more on population dynamics, she said.
"This is a different way of thinking about subspecies -- the (previously!) unglamorous units of evolutionary biology," she said. "Some people have made the case that they are merely nice but evolutionarily meaningless groupings, but we show here that they probably can be thought of as incipient species."
The findings were published this week in Proceedings of Royal Sciences
Next, van Holstein plans to use the same modeling to predict how quickly species form in both endangered species and species of least concern, findings that could be used to predict which biological factors predispose certain species to becoming endangered -- something even Darwin never did.