More Than One Way to Build a Blackbird: The Quirks of Remote Island Evolution

When it comes to the biological imperatives of survival and reproduction, nature often finds a way—sometimes more than one way. For one species of flycatcher in the remote Solomon Islands, scientists have so far found at least two genetic pathways that lead to the same natural outcome: all-black feathers. This change was not accidental. It was a result of nature specifically choosing this feature.

The researchers’ new study is published in the journal PLoS Genetics. “The chestnut-bellied flycatcher is not as well known as Darwin’s finches,” said lead author Leonardo Campagna, an evolutionary geneticist at the Cornell Lab of Ornithology. “But this group of birds has also gone through many evolutionary changes, many of which include changes in their plumage color and pattern.”

The scenario: A large population of chestnut-bellied birds lives on one of the largest islands in the Pacific chain. From there, some birds established new populations on some smaller islands. Over time, the birds on the two smaller islands lost their chestnut bellies and became all black. But the birds on each island developed black plumage at different times, from different genetic mutations that moved quickly through the islands’ small populations.

One of these mutations spread over the past 1,000 years—a mere blip in evolutionary time. “Clearly there is some advantage to having all-black plumage,” Campagna said. “We have traced this trait back in time by sequencing the entire Chestnut-bellied Flycatcher genome for the first time. The two mutations that lead to black plumage appeared at different times, on different islands, and in different genes associated with pigment production melanin This level of convergence is wild.’

The various populations of flycatchers are in the early stages of speciation—they split to form new species—but they have not yet diverged too much genetically and can interbreed. But they rarely do, producing some hybrids. Field experiments have shown that chestnut-bellied and all-black birds react aggressively to a perceiver with their own plumage color, but do not respond in the same way to members of their own species with a different color. And it turns out Mother Nature isn’t done with the flytrap genome.

“We find that there is a third melanic (all-black) population of flycatchers among islands located about 300 miles away from the original island,” said senior co-author Al Uy, a professor of biology at the University of Rochester. “The mutation underlying their plumage color is again different from those on the other two islands we studied.”

Uy has been studying the flies of the Solomon Islands for about 15 years, with the help of a trusted group of native islanders who he says have been “instrumental” in his work. “I think the emerging pattern is that there’s something about small islands that favor these all-black birds – in the most remote archipelago where melanism evolved for the third time, we found that melanism and chestnut-bellied birds still co-exist on every single island , but as the islands become smaller, the frequency of blackbirds increases.”

There are many theories about what is driving the switch to rear plumage, including female preference, the greater endurance of black feathers, and even a possible link to genes that govern other advantageous behaviors. The study’s authors include computer scientists Ziyi Mo and Adam Siepel of Cold Spring Harbor Laboratory, who wrote the machine learning program that helped the researchers dig deeper into the past and measure mutation patterns in the flycatcher’s “family tree.”

“The use of machine learning is an exciting new development in the field of population genetics,” said Campagna. “We train the computer to recognize specific evolutionary patterns of when a particular genetic trait started, how strong natural or sexual selection was, and how quickly it moved through a population. We can then ask the trained algorithm to tell us the most likely scenario created the data we see in today’s populations. It’s like going back in time.”