A new study shows that the common raven is anything but commonplace in its evolution.
Using DNA samples taken from ravens for nearly twenty years, the study provides evidence that common ravens on the western coast of North America have split into three genetically distinct groups. What’s more, two of these lineages appear to be in the process of melding back into one, scientists report Thursday in the journal Nature Communications.
For a long time, we’ve tended to think of the evolution of species as a branching tree, with new species splitting off as their own branches, says Anna Kearns, an evolutionary biologist at the Smithsonian Conservation Biology Institute.
“You see a split and then you see another split, and then you see another split, but you rarely see those two branches that are split come back together again,” says Kearns, who led the new study.
But occasionally, the branches of a family tree do merge back together, and two lineages—or groups that were on their way to becoming separate species—become one. Scientists call this “reticulate evolution,” says Kearns, and it’s been seen in only a handful of other species, including finches and two kinds of fish.
The findings suggest that many other species may have evolved “in reverse” from the merging of species, and highlights how complex the very notion of a species actually is. Though many of us learned in school that two species can’t interbreed, scientists say, biology isn’t always so clear-cut.
A Tale of Three Ravens
Today, the common raven is a large, black bird with a wingspan approaching four feet. As its name suggests, the common raven can be found nearly everywhere across the northern hemisphere, probably owing to its high intelligence and willingness to eat just about everything, from rodents and insects to berries and garbage. (Read: We Knew Ravens Are Smart. But Not This Smart.)
But the raven we see across North America today hasn’t always been the same bird.
Three million years ago, there was one species of raven in western North America, a direct ancestor of today’s ravens. This species would gradually cleave into two, the Holarctic raven and an ancestral western North American raven.
But over the course of the last two million years, the western North American ravens appear to have split into a species known as the Chihuahan raven, as well as a California lineage—not quite a new species, but a unique genetic signature within the western North American raven branch, so to speak.
What's more, DNA analysis suggests that these California breakoffs have been swapping DNA with the Holarctic ravens by interbreeding with them, causing these two branches on the raven family tree to join back into one species, the common raven—a merger that is apparently still ongoing today.
However, even if the birds are technically merging into one species, it’s important to note that changes are occurring along the way, says Bridgett vonHoldt, an evolutionary biologist at Princeton University who was not involved in the new study.
In other words, the common raven created by the merging of the California and Holarctic lineages is not precisely the same common raven as the one that came before.
“This is a fascinating study, demonstrating, once again, that what we think of as the ‘tree’ of life is only approximately a tree,” says Adam Siepel, geneticist at Cold Spring Harbor Laboratory on Long Island.
Siepel studies gene flow between humans and neanderthals, which Kearns and her co-authors note is similar to the gene flow seen in ravens.
New Technology, New Questions
Kaeli Swift, a graduate student studying crow behavior at the University of Washington, spends her free time helping people tell the difference between crows and ravens on Twitter. But even experts like her wouldn’t be able to spot the difference between the California and Holarctic raven lineages without genetic analysis. And that underscores some of the larger issues represented in by the new study.
It gets at just how hard it is to answer seemingly simple questions like “What is a species?” she says.
“With the new genomic analysis tools at our disposal, we can look at these questions in a deeper way than ever before,” says Swift.
“What were once two distinct organisms can collide and become one, weaving their previously distinct genomes into a single tapestry. It's a complex but beautiful work of nature, really."