Horse study reins in evolutionary orthodoxy

The history of the horse family may hold the key to one of evolution’s classic conundrums: what comes first, new traits or a new environment?

In the journal Science, Juan Cantalapiedra from the Humboldt Museum in Germany and colleagues in Spain and Argentina present evidence for the latter.

They studied the body size and tooth morphology of 138 species of horses, all but six of them extinct, with the oldest dating from 18 million years ago, along with the rate at which the horse lineages diverged into separate species.

They found that migration patterns and changes in environment drove the development of new traits. This is the opposite of the prevailing theory of evolution, which holds that new traits – such as bigger teeth or a thicker pelt – develop first, allowing species to then move into new environmental niches.

Using fossils to gauge the body and teeth size of the specimens gave the researchers clues to the kinds of food the horses ate. Longer teeth and bigger body size, for instance, hinted that new grub was on the menu. 

Cantalapiedra and his team determined ancestral relations from the fossil record using phylogenetic analysis – a way of building a family tree that links related species, and where each new branch represents a split from a common ancestor. 

Computer software constructed these phylogenetic trees and the team analysed variation among different branches. They found that speciation bursts – comparatively rapid branch-splitting, resulting in multiple new species – did not correlate with the physical changes that were taking place in the animals at the time.

This suggests strongly that evolution was driven by “extrinsic factors – such as geographical dispersals, increased productivity, or habitat heterogeneity – that release diversity limits and promote speciation”, they write.

In other words, new traits and new species evolved because environmental changes allowed greater genetic diversification. This contradicts the idea of “adaptive radiation”, which holds that the evolution of new traits allowed species to move into previously unoccupied environments. 

Cantalapiedra and colleagues note the irony of their findings: “the radiation of equids […] has been cited as a textbook example of adaptive radiation for more than a century, as it is crucial in the development of evolutionary theory linking trait evolution and adaptive success.”

Within each branch of the evolutionary tree constructed, the researchers found an initial spike of speciation, followed by a slowdown. If this occurred due to physical changes in the horses, they’d expect to see rapid species divergence, marked by the equally rapid evolution of new traits. 

But they didn’t. None of the early stage speciation spikes were associated with a burst in body size or change of tooth structure. In fact, branches of the horse family that underwent fast speciation actually showed slower rates of tooth evolution.

So how was speciation occurring without notable physical changes in these horses? A possible explanation is what the horses ate changed before physiological adaptions caught up.

“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,” comments evolutionary biologist Alistair Evans from Monash University in Australia, who was not involved in the research.

But, Evans adds, “there is much more to a species than just how big it is [and] how big its teeth are”.

The complex evolutionary history of horses is far from being a closed case, with research in the area continuing to grow.