Two Species Become One: Ravens Found To Be Evolving In An Unexpected Way

These ravens represent a rare example of two species recombining into one. Bjørn Aksel Bjerke/University of Oslo

All life on Earth probably originated as a single species, and we certainly have abundant examples of where one species split, a process known as speciation. Biologists have long pondered whether the process ever reverses, with two species hybridizing until their differences disappear, and it seems the process is already underway among common ravens (Corvus corax).

"The bottom line is [speciation reversal] is a natural evolutionary process, and it's probably happened in hundreds or almost certainly thousands of lineages all over the planet," said co-author Professor Kevin Omland from the University of Maryland, Baltimore County. "One of our biggest goals is to just have people aware of this process, so when they see interesting patterns in their data, they won't say, 'That must be a mistake,' or, 'That's too complicated to be correct.'"

Speciation reversal has been observed before but has always involved human interference, such as transporting geographically separated lineages so that they encounter each other again.

Ideally, zoologists would like to watch a more extensive process of speciation reversal occur in real time, but that's probably unlikely. After all, it's only recently that speciation, which is much more common, was witnessed in nature with the appearance of a new Galapagos finch. Instead, Omland and his colleagues analyzed the genomes of hundreds of ravens from across North America. The process started in 1999 when Omland wanted to see if common ravens were actually two species.

In 2000, Omland reported the existence of California ravens in the southwest of the US, and others, which he called Holarctic ravens, across the rest of North America and Eurasia. Even this differed from his expectations, which had been for one species in the “New World” and another in the “Old”.

However, as Omland dug deeper he found further complexity. Students at his lab discovered that two species were interbreeding. That's not uncommon. Although high school biology teaches that when different species mate their offspring will be sterile, it's not always true. Animals can be considered different species if interbreeding is rare or non-existent.

A paper in Nature Communications, using genomic analysis not available when the work began, concludes the two species of ravens diverged 1-2 million years ago. After at least a million years of only limited interbreeding, they have come back together over 10,000 years or more. Meanwhile, Chihuahuan ravens (Corvus cryptoleucus), although overlapping in territory with the California Ravens, held themselves aloof, apparently uninterested in sharing any DNA. This is particularly surprising because California ravens appear to have branched off Chihuahuan ravens more recently than they separated from the Holarctic birds they're now combining with.

"The extensive genetic data reveals one of the best-supported examples of speciation reversal of deeply diverged lineages to date," said Professor Arild Johnsen of the University of Oslo. "The biggest thing is the degree to which we've caught them in the act."

The causes of the initial speciation, and the recent reversal, remain unclear. The timeline of the ravens' interbreeding at least possibly coincides with humans' arrival in North America 18,000 years ago, but is more likely to reflect climatic changes with the ending of the last Ice Age.

The geographic range of Holarctic, California, and Chihuahuan ravens, and the evolutionary path they are thought to have taken. Kearns et al/Nature Communications

To see if the process is accelerating, first author Dr Anna Kearns is working on extracting DNA from ravens collected by museums more than a century ago. She said the degradation of the specimens is making the process difficult, but if she succeeds she'll be able to determine whether industrialization has played a big part in bringing the ravens back together.

In the light of evidence that Neanderthals, Denisovans, and even an additional mysterious species contributed to the genome of most modern humans, the process of speciation reversal has taken on additional relevance. Ravens give us a chance to study a case where the process involves a more even genome mix.

Perhaps confusion among all these ravens is why Westeros seems to lose so many of the birds.


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