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What Makes a Crow Species?

Rather than clarifying how speciation occurs, peering into closely related animals' genomes can raise more questions about the process. At Quanta, Emily Singer discusses two crow species — the black carrion crows and the black and gray hooded crows — that can interbreed and yet mostly remain separate species.

"The crows present a puzzling question to biologists, which gets to the heart of what it means to be a species: Given that hooded and carrion crows can mate and swap genes, how do the two groups maintain their individual identities?" she says.

By examining the crows' genomes, researchers led by Jochen Wolf at Uppsala University have found that just a small section of DNA differs between the birds. Additionally, Singer notes that there seem to be gene expression differences between the birds in regions related to pigmentation, and that some of these genes are found in the area that differs between the crow species. This, she says, indicates that pigmentation genes may somehow be keeping the species separate.

While assortative mating is one possibility, she writes that the researchers are also investigating whether a confluence of mate choice, pigmentation, and vision may be at play. Perhaps black crows can better see other black crows, influencing their mate choice — a gene linked to vision is also located nearby.

Further, whether these crow populations long ago and remain similar through interbreeding or split recently remains unknown.

"So what does all this mean for the definition of species? Scientists still don't have a definitive answer. Simply defining species based on genetics doesn't solve the problem," Singer says.