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Ambika Kamath | | Updated on: Jan 20, 2018




Changing the shape of beaks helped Darwin’s finches on the Galapagos survive droughts

Outside the public eye, there are creatures — a bird here, a fish there, particular flies, worms, and flowers — that have been the cornerstones of evolutionary biology for decades. In this final-ish instalment of Creature Feature (the column is going on hiatus while I finish my PhD), I’m going to tell you about the most famous of evolutionary biology’s organisms: Darwin’s finches on the Galapagos Islands.

I haven’t seen Darwin’s finches, or been anywhere near the Galapagos (they are a chain of islands in the Pacific Ocean, at the equator, I’m told). But these birds are so iconic to students of evolutionary biology that I feel like I’ve known them for years. And what we all know best about them is the shape of their beaks.

What a finch can eat — small soft seeds or large hard seeds — depends on the shape of its beak. But you can flip this equation around: the shapes of seeds that are not eaten, that remain in the environment, depend on the finches’ beak shapes. Both sides are important to understanding how finches evolve.

Charles Darwin, one of the first people to fully describe the process of evolution by natural selection, noticed these rather nondescript birds on his travels. But Darwin soon returned to England and turned his attention to pigeons, barnacles, and earthworms. Finches in Galapagos, however, have been the sole focus of evolutionary biology’s pre-eminent power couple, Peter and Rosemary Grant, for the last 40 years or so.

This level of undivided scientific attention to a single population of animals in the wild is almost unheard of. But such long-term studies can show us how we’ve come to live in a world so filled with creatures of all shapes and sizes and kinds. This is because the Grants have been able to measure changes in the shapes and sizes and kinds of finches — changes that we often predict and infer but rarely see — by paying attention to them for long enough.

When you watch an ecosystem for long enough, you start to see not just the commonplace but also the rare. You move from the messiness of an unknown system to the order of system well-comprehended; only then can you spot those unpredictable bolts of lightning (literal or metaphorical) with the power to shake up everything. For the Grants, the first of these bolts was a drought in 1985.

They had already seen the aftermath of a drought in 1977. Plants stopped producing new seeds, but the existing seeds kept getting eaten by finches. As the dry spell continued, the birds depleted the island’s supply of small soft seeds. Small birds with small beaks that were incapable of feeding on large, hard seeds soon starved to death. Eighty-five per cent of the adults in the population died; only one of 388 chicks survived. The survivors were big with robust beaks, who lived because they could crack open the harder seeds that had remained uneaten.

In this climatic carnage, the Grants saw evidence in real time for evolution by natural selection — the finches evolved bigger beaks. At this point, they could have been satisfied that they’d understood the effects of drought on finch beaks, and called it a day. But they persisted.

And it’s good they did, because the effect of drought in 1985, on the same finch species living on the same island, was entirely different. This time, smaller birds with the pointiest beaks were more likely to survive than large birds with large beaks. What changed? In between the two droughts, the world experienced an El Niño event, much like what we’re undergoing this year, minus some global warming. The 1982 El Niño brought to the Galapagos over 50 times as much rain as had fallen during the 1977 drought. Vines and shrubs that produce small soft seeds thrived, smothering the plants that bear bigger, tougher seeds. So when the next drought rolled around, many more of the seeds available to the finches were small and soft, and the small, pointy-beaked birds did just fine.

This complete reversal in how the beaks changed shape in response to drought proves that the natural world is messy. The Grants’ ability to understand the reversal proves the value of long-term studies of nature. As the Grants put it in their book 40 Years of Evolution , “if we had stopped before [1982, the El Niño year], after the 10th field season, we could have concluded, incorrectly, that droughts invariably select for large body and beak size.” But, because “long-term processes require long-term study”, they didn’t stop.

And while most of us don’t have the luxury of returning each year to a Pacific island to study birds named after the forefather of evolutionary biology, we do all have plants and animals in our gardens and on our streets that we can pay attention to. So if the last two years of Creature Feature haven’t convinced you that any organism can be interesting, let’s switch things up. Pick just one organism, and in the months ahead, look at it closely. Look for changes — across days, across seasons. Maybe you’ll start to make sense of this organism’s life, like I’m trying to do in my PhD. Not quite 40 years, but it’ll be a start. We can exchange notes when I’m back.

(Ambika Kamath studies organismic and evolutionary biology at Harvard University; ambikamath@gmail.com)

Published on April 22, 2016
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