The Splendid White-eye (Zosterops splendidus) is found only on the tiny island of Ranongga and is one of seven species endemic to islands of the New Georgia Group, Solomon Islands.
Birds within the family Zosteropidae — named white-eyes for the feathers that frame their eyes — evolve at a faster rate than any other known bird, researchers said today.
“White-eyes have long been dubbed ‘great speciators’ for their apparent ability to rapidly form new species across geographies where other birds show little or no diversification,” according to a news release about the study published in the Proceedings of the National Academy of Sciences.
Ernst Mayr and Jared Diamond coined the term after encountering white-eyes in the Solomon Islands decades ago, the statement continued. “Each island they visited had distinct white-eye species, whereas most other birds varied little from island to island.
“Mayr and Diamond could only guess at an answer, but both thought that some intrinsic trait was driving the extreme patterns observed among the white-eyes.”
Their idea was spot on, said Christopher Filardi, biodiversity scientist for the Pacific Programs at the Center for Biodiversity and Conservation at the American Museum of Natural History. “There’s something special about these birds. White-eyes quickly diverge into new species across water gaps as narrow as a couple of kilometers — gaps that other birds easily bridge to maintain gene flow.”
Filardi collaborated with Rob Moyle (University of Kansas), Catherine Smith (Missoula, Montana), and hypothesis originator Jared Diamond (currently at the University of California at Los Angeles) to investigate the white-eye’s evolutionary history. The research was funded in part by the National Geographic Society’s Committee for Research and Exploration.
Looking at DNA, the team found that most of the 100-plus species in the Zosteropidae evolved very recently — in the last two million years — even though the group is spread from Asia to Africa and into Oceania.
Only a few vertebrates in the world, such as the cichlid fishes found in lakes of the African Rift Valley, exceed this rate of diversification, the researchers noted. “But while cichlid diversification can be explained by climate shifts and geological changes within their narrow geographic range, rapid speciation among white-eyes cannot be linked to environmental factors because of their recent hemispheric spread.”
Instead, the scientists suggest, white-eyes form new species rapidly because of their sociability, ability to survive in a variety of habitats, and a short time between generations relative to other birds.
Some white-eye species may also have minimized further dispersal and gene flow by becoming sedentary over the course of evolution, similar to historically dispersive human populations that “settled down,” the researchers said.
“Our results indicate that high rates of diversification may have as much to do with a species’ ‘personality’ as they have to do with more classical geographic or geological drivers of speciation,” Filardi said.
“I am delighted to see this molecular evidence supporting ideas that I had only been able to guess at over the last several decades,” Jared Diamond said. “I know that Ernst Mayr, if he had still been alive, would have been delighted at this confirmation 78 years after he visited the Solomons.”