Horse study reins in evolutionary orthodoxy

Millions of years of horse development suggest that one of the key assumptions of evolutionary theory may be wrong.

The evolution of the horse has always been touted as a textbook case of how the appearance of new traits paves the way for evolutionary success. A new study published in Science, however, suggests that might not be the case after all.

The earliest horses appeared in North America 55 million years ago. They were dog-sized, three-toed forest dwellers. But as grasslands expanded 18 million years ago, many new species evolved.

They grew bigger, traded their three toed feet for one big fast hoof (derived from the middle toe) and developed strong grinding teeth – all good adaptations for life on the open prairie.

But did these adaptations indeed come first, as the textbooks suggest?

To find out, a team led by Juan Cantalapiedra from the Leibniz Institute for Evolution and Biodiversity Science in Germany carried out a study to see how horse traits changed over time and how they correlated with the emergence of geographically dispersed horse species and altered environmental conditions.

They made use of the rich fossil record to study the body size and tooth shape of 138 species of horses, all but six of them extinct, with the oldest dating from 18 million years ago.

They identified periods during which there were bursts of new geographically dispersed species, but these did not correlate with physical changes.

‘ THE RADIATION OF EQUIDS … HAS BEEN CITED AS A TEXTBOOK EXAMPLE OF ADAPTIVE RADIATION FOR MORE THAN A CENTURY.’

Instead new traits arose after changes in the environment and patterns of migration – the precise opposite of the prevailing theory.

This, they say, strongly suggests that evolution was driven by “extrinsic factors – such as geographical dispersals, increased productivity, or habitat heterogeneity – that release diversity

limits and promote speciation”.

In other words, new traits and new species evolved after environmental changes created a new niche that allowed greater genetic diversification.

Cantalapiedra and colleagues note the irony of their findings, saying, “the radiation of equids … has been cited as a textbook example of adaptive radiation for more than a century”.

Alistair Evans, an evolutionary biologist from Monash University, in Melbourne, concurs

“We’d always thought you can only really become species-rich by adapting to new environments,” he says, “but here it seems that the new species comes first, and then the anatomy changes later.”

That said, he points out “there is much more to a species than just how big it is and how big its teeth are” – suggesting the complex evolutionary history of horses is far from a closed case.