How playing songs to Darwin's finches helped biologists confirm link between environment and the emergence of new species

Research recently published in Science by biologists from UMass has identified a connection between ecology and speciation in Darwin's finches, famous residents of the Galápagos Islands, Ecuador. Prior work on these birds had established that birds' beaks adapt to changing ecological environments, and that beak changes impact how the birds sing. The new study shows that beak-driven changes to song themselves can impact species recognition, and thus drive the separation of species.

"I started working with these birds 25 years ago," says Jeffrey Podos, professor of biology at UMass Amherst and the paper's senior author. "In my very first publication on the finches, back in 2001, I showed that changes in the beaks of Darwin's finches leads to changes in the songs they sing, and I speculated that, because Darwin's finches use songs to attract mates, then song changes related to beak evolution could perhaps catalyze ecological speciation."

But, at the time, Podos had no smoking-gun experimental evidence with which to prove his hypothesis that environmentally driven changes to beak shapes were driving the emergence of new finch species. Part of the difficulty is that speciation is an historical process, which makes it tricky to document. Being able to watch it unfold as it happens would be like catching lightning in a bottle. As a workaround, Podos devised an experimental study that built on simulations, and he had some helpful leads to work with.

He knew, for instance, that beaks can either evolve powerfully to crush hard seeds or they can remain more delicate thereby enabling the faster movements necessary for more elaborate singing. "It takes serious motor performance to sing an intricate song, like that of the swamp sparrow," says Podos, "and a big, powerful beak is just too clunky to manage the movements required."

Thanks to decades' worth of quantitative research that the wider biology community has conducted on the exact changes that the beaks of Darwin's finches undergo due to different environmental changes, Podos realized that he could model how beaks would change into the future. In this case, he chose drought as the ecological driver, which tends to select for thicker-beaked finches. And he also knew that he could both predict, and then simulate, the songs of the finches as they would change through successive future episodes of drought.

"Essentially, we engineered the calls of future finches," says Podos.

In general, the thicker the beak, the slower the songs and the narrower the bandwidth of the frequencies. Each subsequent drought event is predicted to render beaks that are increasingly thicker, which further should further slow the rate and decrease the bandwidths of the songs.

Finally, Podos and his team returned to the specific population of Darwin's medium ground finches and played them the calls of the future finches.

"We found that there were no changes in the finches' responses to our modified calls even when the simulated songs had changed by the equivalent of three drought events," says Katie M. Schroeder, the paper's co-author who participated in this research during her doctoral training under Podos at UMass Amherst. "But by six drought events, the had changed so much that the finches barely responded at all."

These findings suggest that, because of the links between beaks and song, an entirely new species of Darwin's medium ground finches could evolve in response to six major Galápagos droughts.

"Our research is not a conceptual revolution," says Podos, "but it is an empirical, experimental confirmation of ecological speciation and its plausibility."

This research was supported by the Charles Darwin Research Station and Galápagos National Park Service and the U.S. National Science Foundation.