More than one way to build a black bird: The quirks of remote island development

When it comes to the biological necessities 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 physical outcome: all-black feathers. This change was no accident. It was a result of nature specifically selecting for that trait.

The researchers’ new study is published in the journal PLOS genetics. “The chestnut-bellied flycatcher isn’t as well known as Darwin’s finches,” said lead author Leonardo Campagna, an evolutionary geneticist at the Cornell Lab of Ornithology. “But this bird complex has also undergone many evolutionary changes, many of which involve changes in the coloring and patterning of their plumage.”

The Scenario: A large population of chestnut-bellied birds lives on one of the larger 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 turned completely black. But the birds on each island evolved black plumage at different times from different genetic mutations that quickly moved through the small island populations.

One of these mutations has spread in the last 1,000 years – a mere blink of an eye in evolutionary time. “There’s definitely an advantage to an all-black plumage,” Campagna said. “We have traced this trait back through time by sequencing the entire genome of the chestnut-bellied flycatcher for the first time. The two mutations that result in black plumage occurred at different times, on different islands, and on different genes related to melanin pigment production. This level of convergence is wild.”

The various flycatcher populations are in the early stages of speciation—splitting off to form new species—but they haven’t diverged much genetically and are able to interbreed. But they rarely do, producing a few hybrids. Field experiments have shown that the chestnut-bellied and all-black birds each respond aggressively to a perceived intruder with their own plumage color, but do not respond in the same way to members of their species with a different color. And it turns out that mother nature isn’t done with the flycatcher’s genome yet.

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

Uy has been studying Solomon Islands flycatchers for about 15 years, assisted by a trusted group of indigenous islanders who he says have been “instrumental” in his work. “I think the pattern that’s emerging is that there’s something about small islands that favors these all-black birds — in the more distant archipelago, where melanism has evolved for the third time, we found that melanic and chestnut-bellied birds coexisting islands still exist, but as the islands become smaller, the abundance of melanic birds increases.”

There are several theories as to what drives the switch to back plumage, including female preferences, the longer durability of black feathers, and even a possible link to genes that control other beneficial behaviors. The study’s authors include computer scientists Ziyi Mo and Adam Siepel of the Cold Spring Harbor Laboratory, who wrote the machine learning program that helped the researchers dig deeper into the past and measure mutational patterns in the flycatcher “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, such as when a particular genetic trait began, how strong natural or sexual selection was, and how fast it moved through a population. We can then ask the trained algorithm to tell us the most likely scenario.” that generates the data we observe in populations today. It’s like going back in time.”