More Than One Way to Build a Black Bird: The Evolutionary Oddities of Outlying Islands

When it comes to the biological imperatives of survival and reproduction, nature often finds a way, sometimes more than one way. For an isolated Solomon Islands flycatcher species, scientists have so far discovered at least two genetic pathways leading to the same physical outcome: all-black feathers. This change was no accident. It was the result of nature specifically selecting for this trait.

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 bird complex has also undergone many evolutionary changes, many of which involve changes in the coloration and patterning of their plumage.”

The storyline: 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 a few small islands. Over time, the birds on the two small islands lost their brown bellies and turned completely black. But the birds on each island developed black plumage at different times, from different genetic mutations that spread rapidly through the small island populations.

One of these mutations has spread over the last 1,000 years – a mere blink of an eye in evolutionary time. “There’s clearly something beneficial about having all-black plumage,” Campagna said. “We have traced this trait back in time by sequencing the entire genome of the Rufous-bellied Flycatcher for the first time. The two mutations that lead to black plumage appeared at different times, on different islands and on different genes linked to the production of melanin pigment. . This level of convergence is insane.”

The different populations of flycatchers are in the early stages of speciation – splitting apart to form new species – but they haven’t yet diverged much genetically and they can interbreed. But they rarely do, producing a few hybrids. Field experiments have shown that brown-bellied birds and all-black birds each react aggressively to a perceived intruder with their own plumage color, but do not respond the same to members of their species with a color. different. And it turns out Mother Nature isn’t done tinkering with the flycatcher’s genome.

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

Uy has been studying the flycatchers of the Solomon Islands for around 15 years, aided by a group of trusted natives who he says have been “instrumental” in his work. “I think the emerging trend is that there’s something about small islands that favors these all-black birds – in the most remote archipelago where melanism evolved for the third time, we found that melanistic birds and chestnut-bellied birds still co-exist on each island, but as the islands get smaller, the frequency of melanistic birds increases.”

There are several theories as to what drives the switch to back feathering, including female preference, the greater durability of black feathers, and even a possible connection to genes that govern other advantageous behaviors. Study authors include computer scientists Ziyi Mo and Adam Siepel of Cold Spring Harbor Laboratory, who wrote the machine learning program that helped researchers dig deeper into the past and measure mutation patterns in ” the flycatcher family tree.

“The use of machine learning is an exciting new development in the field of population genetics,” Campagna said. “We train the computer to recognize specific evolutionary patterns to know 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 algorithm trained to tell us the most likely scenario that generated the data we observe in current populations. It’s like stepping back in time.”