Hundreds of thousands of years ago, a glacier advanced upon the Pacific Northwest, its ice forming a natural barrier that fractured crows into two populations. These populations began to diverge into two separate species, which ornithologists decided were distinguishable by small differences in body measurements and the sounds they made. At least, that’s what they thought.
It appears today that, after undergoing the speciation process over hundreds of thousands of years, these two crow species have begun merging back into one, according to new genetic evidence. The crows have been hybridising over a 900-kilometre-long swath of the Pacific Northwest. The study paints a more complex picture of what constitutes a “new” species.
“It means that speciation isn’t a one-way process,” Dave Slager, the study’s first author and a PhD candidate in biology at the University of Washington, told Gizmodo. “It can even go in reverse, sometimes.”
Scientists have considered the two crow species separate since 1858. The Northwestern crow is a bird of Pacific Northwest beaches and mudflats, slightly smaller than the ubiquitous American crow with a huskier voice, according to ornithologists and birders. The extreme difficulty in telling these birds apart has made it tricky for scientists to know whether they hybridise and, if so, how often and over how broad a range. Newly analysed genetic information has revealed a surprising answer to that question.
The team of scientists from the University of Washington, the Burke Museum of Natural History and Culture, and the U.S. Geological Survey gathered samples from crows identified as either species, including 218 frozen tissue samples, 35 blood samples, and six feather samples, plus tissue samples from the European carrion crow for comparison. They analysed DNA from the birds’ cellular nuclei and mitochondria and compared data from the genetic libraries they generated in order to determine whether the crow genomes suggested separate evolutionary histories of the two populations, as well as how often the birds hybridised.
Northwestern and American crows do indeed represent two separate populations with separate evolutionary histories, according to the paper published in Molecular Ecology. The mitochondrial DNA, a kind that animals inherit from their mothers, suggested that the birds began to separate into different species around 443,000 years ago, when glaciers were advancing and retreating across North America in 100,000-year cycles. One of these ice sheets would have fractured the populations and formed a barrier across which speciation could have begun.
But the birds haven’t let their differences stop them from intermingling and boning: They hybridise in a 900-kilometre-long zone across western Washington State and British Columbia, an area that is seven times larger than the average hybrid zones between species pairs, according to the study. Perhaps with the glaciers melting, the opportunistic birds expanded their range, coming into contact and hybridising. Though separate species typically hybridise over only a limited area, and though hybrids are typically less-favoured via natural selection, such was not the case here. The genetic data showed that the crow hybrids were part of hybrid lineages dating back multiple generations.
One researcher not involved in the study, Martin Stervander, a postdoctoral fellow at the University of Oregon, told Gizmodo that this was an exciting piece of research. “Over the years, many have claimed observations of consistent differences between American and Northwestern crows, but scrutinisation has revealed that this has partly been based on people perhaps rather wanting to find consistent patterns,” he wrote in a Twitter message. But he also pointed out that it would be valuable to study the appearances of these birds in conjunction with their genetics, in order to establish which traits each lineage was contributing to the hybrids.
Slager wouldn’t explicitly say whether he thought the two populations should be merged into one species or remain separate species, since that’s up to the American Ornithological Society. But he pointed again to the data, which shows that over this vast swath between Washington State and the British Columbia Coast, you can’t tell the two species apart because there are likely no pure American or Northwestern crows. I’ll note that other species pairs, such as the myrtle warbler and Audubon’s warble (now just called yellow-rumped warblers), and the Thayer’s gull and Iceland gull (now just called Iceland gulls), have been merged by the American Ornithological Society for less.
Animals continue to show that our invented concept of species is flawed and doesn’t always line up with what happens in nature. Perhaps, the paper’s authors conclude, there are other cryptic hybrid zones hiding between pairs of other morphologically similar species.