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.
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.