Most people of Eurasian descent have some DNA from Neanderthals, our close relatives, strong in brow and nose. Some Homo sapiens bred with Neanderthals or other species, such as the Denisovans, who likewise shuffled their genes among ours. The concept of species might suggest easily sorted groups, but nature is messy. Species can mingle and weave and engulf one another.
We are not alone in this regard. Just ask the ravens. The common raven, Corvus corax, fused with a separate raven lineage, as spelled out in a study published Friday in Nature Communications. That other raven species, like the Neanderthals, no longer exists — except within strands of DNA.
“The ghost of interbreeding past is present in the genomes of humans and ravens,” said Rosemary Grant, a Princeton University biologist who has been observing the evolution of Galapagos Islands finches since 1973. Grant, who was not involved with the raven research, said this work adds to a “number of examples in the literature of species exchanging genes, with important and not trivial consequences.”
Corvus corax lives across the Northern Hemisphere in a band that passes through North America, Europe, Africa and Asia. Kevin Omland, an evolutionary biologist at the University of Maryland in Baltimore County, has been studying these birds since the late 1990s. There was an oddity in the birds in California, Omland and his colleagues discovered early into their investigation.
Common ravens in California look like other ravens. But their mitochondria contain a genetic jumble. Mitochondria, the tiny power-plant organs within animal cells, carry their own sets of DNA. Animals — humans and ravens included — inherit mitochondrial DNA from mothers. The mitochondrial DNA in ravens revealed a “deep genetic divergence between two different lineages,” Omland said.
Somewhere along the line, this discovery suggested, California ravens came from mothers that were unlike the rest of the world's common ravens.
The new study paints a clearer picture of raven evolutionary history, thanks to the addition of hundreds of bird samples and more sophisticated tools. In an analysis led by Anna Kearns, now a Smithsonian Conservation Biology Institute research fellow, the scientists combed through the DNA of 441 common ravens. The researchers pried DNA from the cell nuclei, genetic material inherited from both father and mother birds. By calculating the mutation rates in the mitochondrial and nuclear DNA, the scientists could spin back the clock.
About 1.5 million years ago, the birds in what is now California split from the rest of the ravens like a tributary that curves off from a great river. Their ongoing isolation allowed them to evolve into a different species. Much later, somewhere around 440,000 to 140,000 years ago, common ravens first contacted this separate raven species again. “This is definitely an ancient speciation reversal event,” Kearns said. The two species fused, the larger one swallowing the smaller. It was as though the land between the river and the tributary eroded, and once again it flowed as a single body.
The researchers compared these common raven genes with those of a raven species it does not mate with, the Chihuahuan raven, which lives in Mexico and some parts of the desert Southwest. Chihuahuan raven ancestors also split from Corvus corax about 1.5 million years ago. But Chihuahuan ravens still exist. The California ravens vanished after reuniting with the common ravens.
“There is no place on the planet where you can find pure California ravens,” Omland said. The common ravens have “swamped out” that species.
Previous research showed that two species could fuse within a narrow geographic region. Polar bears and grizzly bears can breed to make “grolar bear” hybrids, also called “pizzlies.” Separate types of fish belonging to the stickleback family are known to merge, Omland said, but their fusions might take place in a single stream or lake. The hundreds of bird samples in the study represent both the old and new worlds. “That’s one thing that differentiates our study from ones previous — the scale of it,” Omland said. Samples from along the American West Coast showed evidence of past interbreeding, the proportion of these ancient genes decreasing the farther ravens lived from California.
It was remarkable that the reversal took place after a million-year separation, said Alan Brelsford, an evolutionary geneticist at the University of California at Riverside who was not involved with this research. “What’s neat about the ravens is it seems like they were separated for quite a long time and still managed to fuse,” he said. Brelsford was impressed with the level of detail in the research: Mitochondria alone might tell an unrepresentative story, but the combination of nuclear DNA and mitochondrial DNA was convincing, he said.
It is not clear how the birds could reunite after being apart for so long. There are three main barriers to species fusion: mate choice, ecological differences and biochemical incompatibilities, Brelsford said.
An ice age, a few of which occurred within the 440,000-to-140,000-year timeline of first contact, might cause some of those barriers to crumble. Perhaps a glacier isolated a pocket of common ravens with the other species in California. Alone among distant relatives, the common ravens possibly mingled as their mating instincts kicked in. “You have no one else to breed with, so you’re going to breed with a raven that looks the same as you,” Kearns said.
“The authors argue convincingly that the ranges of the previously separated raven populations changed as a result of a natural change in climate,” Grant said. “This brought them together, and interbreeding followed.”
Last century, scientists had a laser-like focus on how species split, Omland said, looking for the hatchets that drove organisms apart. Advances in genomics changed that. In February, researchers studying the elephant genome revealed how elephants interbred with mastodons, Omland noted. Species fusion is “probably a way more common phenomenon than has been reported,” he said. In the new view, instead of a hatchet strike, speciation is a fraying tapestry whose threads might weave back together.
Human pressure did not cause the raven species to join, Kearns said, unlike recently observed species fusions. The merger probably began before humans came to North America. But it is possible we altered later raven generations. Kearns is hunting for this in the raven genome, extracting genetic material from birds in museum collections that date to the 1880s. “We’ll be able to see what the impact of urbanization, if any, has had on the fusion” of these birds, she said. Ancient raven genetic material continues to spread as the birds move into and out of the American West Coast.
Scientists like Omland are trying to grapple with what it means to define a species. Omland joked he could come up with a new definition, but “if I told you that I’d have to kill you.” The evolutionary biologist added: “It's so complex. It's just way, way more complex than the definition of species from the 1950s would have hinted at.”