"According to the classic view, horses would have evolved faster in when grasslands appeared, developing teeth that were more resistant to the stronger wear that comes with a grass-dominated diet,” said Juan Cantalapiedra, a paleontologist at the Museum für Naturkunde in Berlin, in a statement. “They [would have] also became bigger to more effectively digest this low-quality food, and as a strategy against predators in these new, open habitats."
Cantalapiedra and his colleagues used fossil records of horses to analyze changes in teeth and body size. Then, they computationally generated phylogenetic trees of 138 horse species (7 living and 131 extinct) over 18 million years. They discovered that rapid diversification appeared three times: the first burst occurred in North America between 15 and 18 million years ago, followed by two bursts 11 and 4 million years ago, when shifting sea levels would have allowed migration into Eurasia and Africa. While these periods of speciation were concurrent with major environmental changes, as evolutionary biologists suspected, their analysis revealed that these events did not coincide with major physical changes.
The results imply that environmental changes spur the advent of new species, but not necessarily new traits—a paradigm shift for evolutionary biology.
“We’d always thought you can only really become species-rich by adapting to new environments, but here it seems that the new species comes first, and then the anatomy changes later,” Alistair Evans, an evolutionary biologist from Monash University in Australia, who was not involved in the research told Cosmos Magazine. However, he also added that "there is much more to a species than just how big it is [and] how big its teeth are.”