After up to two million years of separate evolution, two types of common raven have been ‘caught in the act’ of consolidation, say scientists

Speciation, where one species diverges into two, is a well-known concept in the theory of evolution. But a new study based on almost 20 years of research has revealed that “speciation reversal”, the merging of two previously distinct lineages, may also play an important role.

Scientists have discovered that two lineages of common raven that spent between one and two million years evolving separately appear to be in the process of such a consolidation. The findings raise intriguing questions about how science should define species – and whether the boundaries are as clearcut as once thought.

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“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 Kevin Omland, professor of biological sciences at University of Maryland, Baltimore County (UMBC) and co-author of the new study. “One of our biggest goals is to just have people aware of this process.”

Omland first began studying the raven 20 years ago, after he began to suspect that two separate species could have been lumped together. He reported the existence of two lineages: one concentrated in the southwestern United States, dubbed “California,” and another found everywhere else (including Maine, Alaska, Norway and Russia) called “Holarctic”.

The latest paper, published in Nature Communications, provides new intriguing details of the evolutionary history of the two groups. A genetic analysis of 400 birds spanning the geographical range of the two populations suggests that the California and Holarctic lineages diverged between one and two million years ago, but more recently have merged together again and have been hybridising for at least tens of thousands of years. The two populations now comprise pure Holarctic ravens and a group that are hybrids of the two original lineages (the pure California type no longer exists).



“The extensive genetic data reveals one of the best supported examples of speciation reversal of deeply diverged lineages to date,” said Arild Johnsen, professor of zoology and evolutionary biology at University of Oslo and another co-author. “The biggest thing is the degree to which we’ve caught them in the act.”

The genetic analysis also revealed that the mitochondrial DNA of the hybrids and the Holarctics differed by about 4%, which the scientists said was twice as much as would normally be seen for birds to be considered separate species.

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Despite being genetically distinct, though, the birds look the same, sound the same and behave the same – although it is possible that they were different before they started to merge into one group.

The paper also notes that a third group of ravens – known as Chihuahuan ravens – which branched off from the California lineage, have remained separate and do not interbreed with the other two groups, despite their geographical ranges overlapping. Scientists are not sure why this is the case.

“The Chihuahuan raven doesn’t want to play,” said Omland. “It stays by itself and doesn’t interbreed with the others.”

The team is now investigating what prompted the merger between the two populations, including whether humans played a role. They are analysing genetic data from ravens that lived in the early 1900s to investigate whether the hybridisation has accelerated since then.