Two groups of common ravens are merging into a single species in a case of speciation reversal

Common raven (Corvus corax) (Photo National Park Service)
Common raven (Corvus corax) (Photo National Park Service)

An article published in the journal “Nature Communications” brings evidence of an speciation reversal in two lineages of the species common raven (Corvus corax). A team of researchers examined genomic data from hundreds of ravens collected in North America in a study begun in 1999 concluding that this species diversified into three genetically distinct groups: the species Chihuahua raven (Corvus cryptoleucus) emerged from one of them but two of them are collapsing into one by a hybridization process.

Evolution leads mainly to continuous ramifications with the emergence of new species. However, sometimes two populations that remained distinct for long enough to become even two different species can come into contact and, by hybridization, reunite and give rise to a further new species. Advances in genetic analysis are revealing this type of results caused by hybridization and speciation reversal. For example modern humans also show genetic traces of ancient hybridizations with other hominins.

The study of the common raven began in 1999 and over the years took advantage of the increasingly sophisticated tools of genomics, a branch of molecular biology based on computer applications to this field that studies the genome of one or more organisms. In this case, genomics tools were used to reconstruct North American ravens’ genetic tree.

The conclusions of the analysis of the genetic data of hundreds of North American ravens collected in different data sets indicates that about 3 million years ago there was a single species of raven in Western North America, a direct ancestor of today’s common raven. This species gradually branched into two distinct groups called clades in jargon, one called California and one called Holarctic, each including a number of subspecies.

The situation has become even more complex because over the last two million years or so there was a further branching with a group that diversified into what is classified as a different species, the Chihuahua raven (Corvus cryptoleucus). However, the mitochondrial DNA of the California common raven is more similar to that of the Chihuahua raven than that of the Holarctic common raven.

Finally, genetic analyzes indicate that the two common raven clades are interbreeding in a hybridization that’s been going on for at least a few tens of thousands of years. In essence, two different clades are not increasingly branching into two more and more separate lineages to evolve into two different species but are going to merge into one species.

One question the researchers asked is the possible role of human beings in the common ravens’ speciation reversal. To try to get an answer, they intend to start a new phase of the study by analyzing the DNA of ravens that lived at the beginning of the 20th century.

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