Ravens turn evolution on its head

Two lineages of the common raven (Corvus corax) in western North America have been fusing for thousands of years, researchers found, in what they say is one of the first and broadest instances of “speciation reversal” ever uncovered.

“Species reversal has been hypothesized in species restricted to smaller geographic areas — fishes in a lake or river,” said Anna Kearns, first author on the paper published in Nature Communications. “We have the best-worked example of speciation reversal for ancient lineages on this broad geographic scale.”

From 2013 to 2015, Kearns, who was a postdoctoral fellow shared between the University of Oslo’s Natural History Museum and the University of Maryland, Baltimore County during the study, and her colleagues analyzed the evolutionary history of two separate raven lineages through genomics. They discovered that the California lineage, which occurs in the southwestern United States, and the Holarctic lineage, which spans Maine, Montana, Wyoming, Alaska, British Columbia, Norway and Russia, have converged after their initial divergence 1.5 million years ago. A sister species, the Chihuahuan raven (C. cryptoleucus), has remained distinct even though its territory overlaps with the California lineage. The scientists sequenced nuclear genes and variable sites, called single nucleotide polymorphisms, randomly collected across the nuclear genome from the blood or feathers of museum specimens, roadkill and live birds at rehabilitation centers.

“We find huge recombination and mixture in the western U.S., where the two lineages are remerging,” Kearns said. “That was unexpected and neat. It’s ancient speciation reversal. It looks like the Californian lineage has been swamped out by the Holarctic lineage. In another system where speciation reversal may have only just begun, this could have big conservation implications where you can see one species get swamped out genetically and go extinct.”

Now a postdoctoral fellow at the Smithsonian Conservation Biology Institute’s Center for Conservation Genomics, Kearns is using the same cross-genome sequencing technique on DNA from the toes of preserved ravens dating back over a century. She’s probing into how genetic diversity and hybridization between the Californian and Holarctic lineages have changed since, and she’s examining if urbanization may have helped shape this speciation reversal.

Few cases of speciation reversal have ever been detected, researchers wrote, but the phenomenon is expected to increase as climate change and habitat loss lead to more overlap between once-separated populations.