Evolutionary biologists’ work often consists of trying to map the planet’s family tree. A branch extending through time and space eventually cleaves, splitting to create both chimpanzees and humans, dogs and wolves, whales and hippopotamuses.
But occasionally, according to recently published research, some of those forked branches fuse back together.
University of Maryland, Baltimore County biological sciences professor Kevin Omland is one of the authors behind a study recently published in Nature Communications. It explores the way two lineages of the Corvus corax — the common raven — merged back into one, following tens of thousands of years of interbreeding.
Researchers have spent more than a century studying speciation, the process in which a species splits into two. But Omland’s work has helped illuminate the more complicated idea of speciation reversal.
“We’ve been trying to build a ‘tree of life,’ but this adds some complexity to that,” Omland said. “Not only do you have a single trunk diverging into two, diverging into four, diverging into eight. But sometimes those lineages come back together.”
The research itself has spanned two decades, with teams of people collecting genetic samples from ravens across the world. Omland can rattle off a handful of sites: Maine, Minnesota, Alaska, Siberia, France, Italy — and perhaps most importantly, California.
The researchers found that while ravens in Southern California looked identical to the other common raven counterparts, they actually had segments of distinctive DNA.
“There was about a 4 percent difference in the DNA between the ones in California and the ravens in the rest of the world,” Omland said.
At first, researchers thought they had discovered a new species of raven. But, it turned out, they had actually caught the ravens “in the act” of going through speciation reversal.
Between one million and two million years ago, the two forms of ravens — dubbed California ravens and Holarctic ravens — somehow began evolving separately. It may have been caused by shifting glaciers or other changes in climate, Omland said. Perhaps at one point, these two raven species developed distinct appearances, vocalizations or mating displays.
But then something brought them back together, some tens of thousands of years ago. The birds began interbreeding, swapping their DNA and exchanging genetic traits.
By now, “whatever difference there may have been now have been genetically swamped out,” Omland said. There’s nowhere on Earth where a person can find a pure California raven.
The idea of one species “swamping out” another is not unique to these large, black birds. Humans are also a product of this phenomenon. While there are no longer any “pure Neanderthals” left on Earth, some of their DNA lingers within the broader mosaic of humanity’s genetic makeup.
“We are the products of evolution, so understanding the process of evolution in a complete way is very important,” Omland. “Understanding this most fundamental process of evolution is absolutely crucial.”
The research has forced those involved to question the way the scientific community thinks of the idea of a species and what that classification really means.
“It has definitely shaped the way I view ‘species,’ as being much more fluid and not permanent,” said Anna Kearns, a former postdoctoral fellow at UMBC. “These species distinctions can erode under certain conditions.”
Kearns is now working as a postdoctoral fellow at the Smithsonian Center for Conservation Genomics, and continuing this research by sampling more raven DNA from the early 1900s and late 1800s. She’s extracting and testing genetic material from birds kept in museum collections.
John Marzluff, a University of Washington professor who was involved with the study, echoed her assessment.
“To me, this work is about letting the public know that speciation is an ongoing, two-way street,” he said. “Animals can become more or less distinct through time, depending on our actions and the actions of our systems on this planet. … Species were once seen as immutable, fixed and created in a certain way.”
John McCormack, director of the Moore Laboratory of Zoology at Occidental College in Los Angeles, was not involved with the study. He said looking at this research showed him that species are more “porous” than originally thought.
“This is one of the first very well-worked studies of just how much gene exchange is happening between species, to the point where they can even collapse back into one,” he said. “As more and more species are looked at, I believe we’ll see this kind of pattern is more the rule than the exception.”
The researchers say it’s unlikely human action affected the speciation reversal of the ravens, though Kearns’ work will investigate that aspect further.
“In this case, it happened naturally,” Omland said. “But we’re probably causing it in hundreds of other cases around the planet now and we’re losing biodiversity in that sense.”
Marzluff said he hopes this work leads the way to studying the speciation reversal process in other species.
“I suspect the same process will be seen in a lot of organisms that came across the Bering Strait,” he said, referencing the area where archaeologists believe a land bridge once connected North America and Asia and allowed humans to migrate between the continents.
For Omland, his location in Baltimore County adds intrigue to his research on ravens. The birds have moved back into the metropolitan area within the last decade. He recently spotted a nest under Interstate 95.
“If you told me in 1998 that ravens would be common in Baltimore and D.C. ... I wouldn’t have understood how that could have happened,” he said. “But they’ve come in at this point in great numbers.”
The reason, he said, is a mystery.