Scientists Map the Complicated and Epic Evolution of a 'Ring Species,' the Greenish Warbler

Scientists are learning a bit more about a certain bird's evolution. They have taken a closer look at the Greenish Warbler, a species long considered to be an idealized example of a single species that have diverged into two, and have found that this bird has a much more checkered family history than previously realized.

The Greenish Warbler is what is known as a ring species, which are a continuous loop of related populations with each adapting to its local environment. Two terminal populations in this loop meet, but cannot mate due to genetic differences. Yet while the Greenish Warbler is an example of a ring species, scientists have now found that this bird's genetic migration through central Asia involved periods of geographic separation and hybridization.

"We've shown that the evolution of ring species is much more complex than the smooth and continuous divergence envisioned by the class model," said Miguel Alcaide, one of the researchers, in a news release. "If you view the ring of Greenish Warbler subspecies as a river, over the years the flow of populations has experienced periods of isolation-as if forming ponds during a drought-which accelerated genetic differences. This could have been interspersed with periods of flooding, or rapid exchange between populations. Interbreeding after range expansion has been, however, much more restricted among neighboring populations exhibiting substantial differences in morphology and behavior."

The Greenish Warbler originally expanded out of southern Asia; then, the subspecies of greenish warbler diverged around the expanse of the Tibetan plateau over thousands of years. The two distantly related populations of warblers eventually met again in the north, in central Siberia. Scientists have no found that small regions of the western Siberian genome can be found in some of the eastern Siberian birds, which indicates some successful hybridization between the two.

"Overall, despite the complex patterns of gene flow, the Greenish Warbler still has the central characteristics of a ring species: two mostly distinct populations connected by a chain of populations in which traits and genes change progressively from one species to the other," said Darren Irwin, one of the researchers, in a news release.

The findings reveal a little bit more about the evolution of this species. In addition, it shows how the process of being a ring species is far more complicated than previously expected.

The findings are published in the journal Nature.