Raven research defies common wisdom on 'species'

Raven research defies common wisdom on 'species'
Dr. Kevin Omland is an evolutionary biologist and professor of biological sciences at UMBC. (Amy Davis, Baltimore Sun)

As the Baltimore Ravens' march to victory in Super Bowl XLVII defied the common wisdom of the sports world, so, too, has an examination of the genetics of their winged namesakes in the western United States led one local biologist to evidence he says defies the common wisdom of his field.

Kevin Omland, an evolutionary biologist at the University of Maryland, Baltimore County, said he's the department's "mascot biologist." It happens that he teaches at a university in Baltimore County and has studied both the raven and the Baltimore oriole, although not from homegrown rooting interest. Those projects started before he moved here.

The Vermont native has been on the trail of the common raven, Corvus corax, for more than 15 years, and will embark on the next phase of his research with a trip to Alaska this summer. While his argument is challenged by at least two biologists, his research could shed light on the meaning of "species" and the complex puzzle of evolution.

In nature, the most common evolutionary pattern is for creatures to, over time, develop new characteristics, separate breeding patterns and eventually distinct genetic profiles as they come to be understood as distinct species. In very rare cases, this process of "speciation" reverses: two species interbreed and become one.

Omland and two colleagues say they have found evidence of such a case in ravens in Washington state. Members of the Holarctic, or northern, group — which has a remarkably wide international range — and the more narrowly ranged California, or western, group show the distinct genetic profiles researchers associate with different species.

Yet Omland and his colleagues found interbreeding on the Olympic Peninsula. That's not supposed to happen. If creatures of different species do mate, the common wisdom goes, their offspring are supposed to be infertile.

Yet the offspring of mixed pairs from the two raven groups were producing just as many offspring as same-group pairs, Omland and his colleagues found in research published in the journal Molecular Ecology in 2011.

Omland and his colleagues argued the two had once been separate long ago. The DNA distinction appeared to be a "a reflection of likely past isolation" that had more recently broken down due to geographical proximity and similarity of appearance. Ravens can also range so widely that they are more likely to merge than creatures demanding a more specialized environment.

The work engages the definition of "species," a "fundamental unit of biodivesity," as Omland put it. The definition had been relatively stable for about 50 years, until detailed analysis of DNA became more common laboratory practice in the early 1990s. That complicated matters considerably.

Now it's possible to find what Omland called "cryptic species," or genetic fingerprints of one species hiding in the DNA profile of another, suggesting an ancient evolutionary link.

"There's every shade of gray," said Omland. "That's why defining species is so difficult."

There's no absolute numerical benchmark in genetic difference, Omland said, but scientists tend to use a range of 4 percent to 6 percent difference between one species and another in mitochondrial DNA, which has been the focus of Omland's research so far. Indeed, the difference between the Holarctic ravens group and the California group is up to 5 percent in mitochondrial DNA.

The genetic difference between humans and our nearest relatives, the chimps, for instance, is put at about 10 percent for mitochondrial DNA, which shows inheritance from the mother.

Omland's work so far provides evidence, but not conclusive proof, of speciation in reverse. At least a couple of his fellow scientists at other universities praise aspects of the research but remain skeptical about the argument.

Trevor Price, a professor of ecology and evolution at the University of Chicago, praises the work for showing that a split between animal groups can go as far back as 2 million years — the period roughly accounting for the 4 percent genetic difference — yet they can still interbreed. Whether that means they were distinct species is another matter, he said.

"I can see someone saying to you they weren't separate species to start with," said Price. "It's more of a semantic issue, whether you want to call them species."

Darren E. Irwin, an associate professor at the Biodiversity Research Centre at the University of British Columbia, called the research "very clear and interesting," but he's not sure it argues for a case of speciation in reverse.

As the two groups are not isolated from each other now, at least in that part of the world, Irwin said in an email, "it is difficult to argue that there ever was any reproductive isolation between them."

He said the study is limited by the fact that it only considered mitochondrial DNA, from the mother, and not nuclear DNA, which shows information about both parents.

Indeed, that's where the work is headed next, as Omland and two colleagues travel to Alaska for a week or two this summer armed with a $1.3 million research grant from the Norwegian government, hiking gear and a contraption used to trap the wily ravens. While Omland has studied birds his whole career, this research on the complexities of speciation could shed light on other beings, as suggested in recent studies of contemporary humans carrying Neanderthal DNA.

"We ourselves may be the product of speciation in reverse," said Omland.