All life on Earth probably originated as a single species, and we certainly have abundant examples of where one species split, a process known as speciation. Biologists have long pondered whether the process ever reverses, with two species hybridizing until their differences disappear, and it seems the process is already underway among common ravens (Corvus corax).
"The bottom line is [speciation reversal] is a natural evolutionary process, and it's probably happened in hundreds or almost certainly thousands of lineages all over the planet," said co-author Professor Kevin Omland from the University of Maryland, Baltimore County. "One of our biggest goals is to just have people aware of this process, so when they see interesting patterns in their data, they won't say, 'That must be a mistake,' or, 'That's too complicated to be correct.'"
Speciation reversal has been observed before but has always involved human interference, such as transporting geographically separated lineages so that they encounter each other again.
Ideally, zoologists would like to watch a more extensive process of speciation reversal occur in real time, but that's probably unlikely. After all, it's only recently that speciation, which is much more common, was witnessed in nature with the appearance of a new Galapagos finch. Instead, Omland and his colleagues analyzed the genomes of hundreds of ravens from across North America. The process started in 1999 when Omland wanted to see if common ravens were actually two species.
In 2000, Omland reported the existence of California ravens in the southwest of the US, and others, which he called Holarctic ravens, across the rest of North America and Eurasia. Even this differed from his expectations, which had been for one species in the “New World” and another in the “Old”.
However, as Omland dug deeper he found further complexity. Students at his lab discovered that two species were interbreeding. That's not uncommon. Although high school biology teaches that when different species mate their offspring will be sterile, it's not always true. Animals can be considered different species if interbreeding is rare or non-existent.