The Beak of the Finch: A Story of Evolution in Our Time

by Jonathan Weiner

  In his mind’s eye, Lack saw this pattern as the aftermath of a great war across the whole archipelago, a war without generals or bloodshed, between the sharp beaks and the small beaks. Apparently, Lack said, these two species are so alike that whenever they breed side by side on the same island they are thrown into competition. If the island has only one niche for them, then either the line of the sharp beaks or the line of the small beaks is driven to extinction: one species outcompetes the other. The technical term for this outcome of a battle between two groups of living things is competitive exclusion. But if the island is tall enough to offer one species a new niche, a path out of the race, that species can evolve its way out of the competition. Then it changes in character. The beak bends, melts, morphs, changes shape, through evolution by natural selection, until that lineage of birds is freed from the dreadful war. The technical term for this outcome is character displacement.

  Lack pointed out that on Santa Cruz the small beaks are small and the medium beaks are medium. On Daphne, where there are very few small beaks, the medium beaks have become smaller. On Los Hermanos, where there are very few medium beaks, the small beaks have become larger. Lack saw this same pattern with many other pairs of finches in the islands: again and again, traces of character displacement.

  In the 1950s and 1960s, Lack and others explored competition theory and its ramifications. They pursued the theory so triumphantly and unanimously that a few ecologists and evolutionists with other interests began to feel competitively excluded. One of them was an American ornithologist, Robert Bowman, who went to the Galápagos in 1952, after Lack, and studied the Galápagos finches as his thesis project. This was before the invention of mist nets. To find out what the finches were eating, Bowman shot them by the hundreds and inspected the contents of their stomachs. He decided that what a Galápagos finch eats does depend on its beak—a conclusion that would later be confirmed and extended in such extraordinary detail by the Grants’ International Finch Investigation Unit.

  However, Bowman also noticed, more than Darwin or Lack had noticed before him, that Galápagos plants, like Galápagos finches, vary from island to island. Bowman wondered if this variation in itself might explain the branching of the finches’ lines. The birds might have evolved their distinctive beaks and habits by adapting to the local varieties of flowers and seeds as they settled on island after island. They might have speciated, in other words, according to Darwin’s first vision of the evolutionary process, before his eureka in the carriage. That was a simpler story, Bowman argued—and who could prove it wrong?

  Time and again Bowman watched the sort of mixed flocks that Darwin himself had noticed when he first got off the Beagle. Time and again he saw four or more species of ground finches feeding in the very same bush, small peaceable kingdoms of ground finches. “And since there is no direct evidence that competition is occurring at the present time,” Bowman declared in his thesis, “I see no logical reason to assume that it must have occurred in the past.”

  To argue that the absence of competition is a proof of its power seemed to Bowman infuriatingly circular reasoning. There were other ecologists who agreed with him. A pitched battle began between the competitive camp and the noncompetitive camp, and in the absence of hard evidence and actual observations it dragged on and on.

  The ecologist Joseph Connell declared in a celebrated manifesto that until someone came along with harder facts, he would no longer buy the old guard’s arguments for Darwin’s principle of divergence: “I will no longer be persuaded by such invoking of ‘the Ghost of Competition Past.’ ”

  The ecologist Daniel Simberloff was even more emphatic. He argued that Lack’s famous patterns might be nothing more than the faces we see in the moon, in clouds, in Rorschach inkblots. Even if we see species living together in different niches, that does not prove that they drove each other into those niches—that they repelled each other like the identical poles of two magnets. They might instead have diverged by random adaptations toward different seeds and different needs. Finches colonizing the islands at random would have produced patterns that could be interpreted in all kinds of ways. The finches’ distributions in the archipelago might be “most fruitfully treated as a random process.”

  Peter Grant suspected that Lack was right; but he too was annoyed by Lack’s dogmatic style, his willingness to issue pronouncements without having seen what is actually going on. Grant once accused Lack in print of a “distressing lack of objectivity.” .… “Future generations,” he wrote, “will surely wonder that we discuss competition so much yet know it so little and measure it even less.”

  THAT WAS ONE OF THE FASCINATIONS of the events the Grants witnessed in their first dry season, events they still see repeated each year. Again and again, when the rains end, their flocks of Darwin’s finches move apart and specialize, magnirostris going this way, fortis that way, scandens and fuliginosa going off on their own as well, each according to its beak. Not only does this divergence help to prove that the beaks of the finches are adaptive; it also helps to prove that their differences have survival value. It suggests that the forces Darwin envisioned in his carriage more than a century ago are at work right now among his finches.

  When birds that have been eating the same foods for months begin to fan out into specialties, according to the sizes and shapes of their beaks, we are seeing a kind of divergence in the course of a single season. And we can also see that the consequence is just what Darwin imagined: for as each dry season wears on and the birds are scrabbling for food, their choices as they hunt and peck diverge more and more, which reduces the competition between them. That they respond to the drought in this way, in effect making room for one another, may help to explain why they are able to coexist on the little island at all.

  Large ground finches. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  Of course in the dry season the birds are only changing their behavior, not their beaks. Their diets diverge as their stocks of food are depleted because so many individual birds stop trying to do everything and concentrate on what they do best, according to the beaks and bodies they were born with. Changing your behavior is much quicker than changing your anatomy. But the change is driven by the same sword of competition. Darwin’s finches are responding to the same forces that Darwin believed led to their creation.

  The traditional picture of the economy of nature, the picture most of us learned in school, is that nature is a stiff hierarchy, a food pyramid of more-or-less rigid trophic levels. Plants are at the bottom of the pyramid, making food from sunlight. Herbivores feed on the plants, and carnivores feed on the herbivores. Within each level there are assorted specialists, called guilds; any ecologist can rattle off long lists of them: “leaf eaters, stem borers, root chewers, nectar sippers, bud nippers”—much like the feudal guilds of shoemakers, tailors, butchers, bakers, and candlestick makers.

  As more and more ecologists and evolutionists watch life up close and long term, the way the Grants are observing Daphne Major, they see that these categories are not as fixed as they had imagined. More and more naturalists are shifting their efforts from the study of pattern and structure to the study of process and motion, watching change through time, and, like the Finch Unit in the Galápagos, what they see again and again is the lesson of Heraclitus: everything flows. Nature is fluid. The finches on Daphne start out their year in a single guild, the guild of the ground finches; then in hard times they break up into smaller guilds. Other ecologists and evolutionists are watching this same process of splitting and shifting guilds elsewhere. This fluidity of nature suggests that as divergent pressures are repeated year after year, animals and plants around the world are even now evolving differences in physique and taste that (if they continue) will make their guilds diverge farther and farther.

  Of all the watchers who have worked with the Grants in the Galápagos, the one who has grown most fascinated with Darwinian
divergence is Dolph Schluter. Schluter is a rising star now in the field of evolutionary biology, but he was about to drop out of school when he met the Grants. He had just completed an undergraduate degree in wildlife management at the University of Guelph, in Canada. He hadn’t applied to graduate school, even though he had the highest marks in his class. He was sick of school, burned out. He was planning to take a job trapping muskrat and mink in the Athabasca Tar Sands, in Alberta. Then he heard a student of the Grants give a seminar.

  “I’d never realized people did this work,” Dolph says now. “I knew about evolution, of course. But it had never struck me that someone could actually study it—in modern times. My idea somehow was poring through fossils. That someone could watch evolution! It was a revelation.

  “I wrote to Peter Grant. It was the only place I applied. Otherwise I’d still be trapping muskrats.”

  Dolph’s Ph.D. project under Peter Grant was a close-up study of the war between the small beaks and the sharp beaks on a single Galápagos island. Dolph read the latest manifestos of Simberloff et al., which called finch distributions “a random process,” and he went down to the islands in a state of radical doubt. “I was quite dizzy with the idea that everyone—including Grant—had overestimated the importance of competition.”

  The whole Grant family rode the boat with Dolph, escorting him to the island of Pinta, one of the most remote in the archipelago. “As Pinta came into sight—and it was spectacular! I’d never been south before, let alone the Galápagos; a school of dolphins was leaping in the boat’s bow wave—Peter Grant turned to me and said, ‘Well, Dolph, you know, there might only be a master’s in this.’ Only a master’s thesis, not a Ph.D.

  “We had no idea what we’d find, in other words,” Dolph explains, with laughter in his voice. “Grant was trying to say the idea might not be feasible. The plans might not work.”

  The Grants taught Dolph how to set up mist nets, how to catch a bird and measure it. They stayed a week. “Then it was, ‘Goodbye, we’re off to Genovesa. See you in five months.’ ” Schluter and his field assistant were now the only human beings on the island.

  Like all the taller islands in the Galápagos, Pinta has an amazing slope. At the bottom, it is a desert of bare black pahoehoe lava and thorny scrub: dry, hot sheets of bare rock. As you march up the slope you walk into a cloud, and a green forest canopy begins to close over your head. The highlands are moist and cool; the ground is thick and soft underfoot with deep forest litter. At the top is elfin forest: misty, green, with moss-covered trees, lichens, orchids.

  As soon as he had learned to tell them apart, Dolph could see that the small beaks’ and the sharp beaks’ territories overlapped. Fuliginosa went up to the summit of the island, and difficilis went not quite all the way to the bottom. “I saw that right at the beginning,” he says. He had expected to find the two rivals living in separate zones of the island, bottom and top, as Lack had described, with territorial skirmishes all along the border. “That possibility was completely shot,” Dolph says. All was quiet on the western, eastern, southern, and northern fronts. Fuliginosa and difficilis were ignoring each other. “This is typical of fieldwork,” he says now. “You plan and plan, and then you get there and realize you can’t do it.”

  In the next few months, he and his assistant set up study grids up and down the slope. They found to their astonishment that the finches, although they overlap in territory, overlap hardly at all in diet. On Pinta fuliginosa takes seeds out in the open, pecking on the hard hot lava, mostly down by the shore. But difficilis scratches with its long claws in the leaf litter, mostly up in the forest, kicking the leaves away, shoving pebbles aside, and snatching up spiders and snails, crickets and caterpillars with its beak. Fuliginosa eats hardly anything but seeds; difficilis takes hardly any seeds. So if Darwin and Lack were right about the cause of the divergence, there was no way to tell by watching these birds. “There’s very little trace of competition,” Dolph says cheerfully. “It really is … a ghost.”

  Because he could not settle the competition question on that one island, he went on to another island, and another. Over the next few years he mapped Lack’s patterns in ever-finer detail from one end of the archipelago to the other. The masses of measurements that he and others collected were consistent with Lack’s predictions, consistent to a remarkably detailed degree.

  Take fortis, the medium-sized ground finch. Its equipment represents a trade-off between the best beak for small soft seeds and the best beak for big hard seeds. The bird evolves toward the best deal it can make, and on each island the best beak is determined partly by the presence of fuliginosa, because small beaks in the dry season will lower the pile of small soft seeds. A fortis with a beak too small to crack a Tribulus will have to compete with fuliginosa, but fuliginosa are more efficient than fortis can be. So the smaller fortis will die younger than the bigger fortis. In this way competition with the small beaks drives up the average size of the medium beaks. On islands where it has been driven up in size by this competition with small beaks, it is much better with Tribulus, and it eats fewer small seeds. Fortis on Daphne has very little competition from fuliginosa, and there fortis throws down most of the Tribulus it picks up. On the other hand, fortis on Pinta, Marchena, and Santiago, where there are many fuliginosa, eat fewer small seeds but seldom meet a big seed they cannot crack.

  Meanwhile, where fuliginosa is competing with fortis it tends to specialize in the smaller seeds because fortis is so much more efficient with the big hard seeds of Tribulus and the prickly pear. But where it has an island all to itself, as on Los Hermanos, the beak of fuliginosa is big and deep, and it can handle fairly big seeds. On Los Hermanos its beak is almost identical to the beak of fortis on Daphne.

  On the islands where magnirostris is missing, the beak of fortis is bigger than average; on the islands where fuliginosa is missing, fortis is smaller than average. It fills both its own niche and that of its missing rival.

  There are far too many pairs of species like the sharp beaks and the small beaks, or the small beaks and the medium beaks, or the medium and the large, that have divvied up the islands in this neat way between them, converging on the same beak where they live apart but diverging where they are neighbors, making an infinity of finely calculated mutual adjustments, for the patterns to be due to chance. Lack was essentially right. “He was not right in every detail,” Dolph says, “but the mechanism was dead-on. I’ve since concluded that he was a pretty bright fellow. He’s my hero actually. He had a disarming writing style, and he could make very forceful arguments. In fact, I think they should be called Lack’s finches and not Darwin’s. Darwin didn’t see the significance of the birds. He thought there was just one species per island. He didn’t even try to pull it together—he didn’t do a bloody thing with them except collect them. That’s why they’re Lack’s finches and not Darwin’s.”

  AFTER A FEW YEARS in the islands, Dolph arrived at a new way of visualizing the Darwinian pressures on Darwin’s finches.

  Imagine a single population of birds on a desert island. As this population changes through time, generation after generation, it is perpetually moving toward the optimum design for that island. It is working toward its point of maximum fitness, toward what evolutionists sometimes call an adaptive peak.

  Picture that point of maximum fitness, the peak of optimal design, as the summit of a mountain. The slopes around the summit represent designs that are slightly inferior to the design right at the peak. The valleys far below represent designs that are fatally inferior to the design at the peak. A winner, a bird with the right beak, the right wings, the right tarsi, is on the peak; a loser is wandering down in the valley.

  If a bird is blown out to sea and lands on a desert island, it will not be perfectly adapted to its new home from the start. Its fitness will be somewhere down on the slope of the adaptive landscape. If it is very far down in the valley, it will die, and its line will die out. But if it can hang on and breed, its
descendants will move up and up, evolving and adapting, until they reach the peak.

  Evolutionists have been thinking about adaptive landscapes for a long time (the idea was introduced by Sewall Wright, one of this century’s great Neo-Darwinians, in 1932). The adaptive landscape is one of their favorite metaphors for evolutionary change: almost as popular as the tree of life itself. But Dolph was the first to see how revealing the idea could be when applied to Darwin’s finches. Peter Grant had asked him to find a way of pulling together all of the group’s work on beaks and seeds in one framework; to create an all-embracing mathematical model that would help the finch watchers understand what they were seeing. Dolph decided to draw their data in terms of adaptive landscapes, with the aid of computers. Dolph is one of the most sophisticated mathematicians in the Finch Unit, along with Trevor Price, his old field mate and office mate, with and against whom he has honed his skills.

  In picturing the forces that lead to character divergence, the adaptive landscape makes a better metaphor than the tree of life. It permits a finer level of description. A population of finches splitting in two (the forking of a branch, in the tree-of-life metaphor) can be imagined as setting off on a sort of lonely pilgrimage or mass migration across an adaptive landscape. Some birds leave one peak and migrate to another. First there is just one species in the landscape, clustered around one peak. Then there are two, clustered around two neighboring peaks. In between there lies a valley.

  It was in 1979 that Dolph realized how he could use this image of the adaptive landscape to test Darwin’s principle of divergence with Darwin’s finches. He realized that he and the other members of the Finch Unit had amassed so many measurements that he could now draw a fairly realistic adaptive landscape for the Galápagos ground finches.