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

by Jonathan Weiner

  But it was time for another changing of the guard. Lisle Gibbs, a Canadian graduate student of Peter Grant’s, took the next watch. He went down in December 1981 with a field assistant, to be sure to be there when it started to rain. They sat there, two men on a desert island, and waited half a year for the gray clouds to shower on them. Some rain fell—but not enough. “It was like waiting for Godot,” Gibbs says now.

  They went home to Michigan. And late in 1982, as Christmas approached, Lisle got a postcard from the village of Puerto Ayora. “It’s raining.”

  Chapter 7

  Twenty-five Thousand Darwins

  … and the windows of heaven were opened. And the rain was upon the earth forty days and forty nights.

  —Genesis 7: 11–12

  Lisle Gibbs no longer remembers how he got to the island. He simply said to himself, “I’m going to be there.” Seventy-two hours later he leaped from a panga and landed on Daphne’s welcome mat.

  He had spent six months waiting for rain, and poor Price had languished three years. And in the end the rain had arrived so far ahead of schedule that not a single human being was there on Daphne to see it fall.

  Lisle and a field assistant scrambled up the cliff and looked around. The female finches’ beaks were dark already, and the males’ beaks were black. The cactus trees were loaded with freshly woven domed nests, and the nests were crowded with cheeping baby finches, the latest generation of Darwin’s finches: conceived and hatched before the coming of the new year. “We were banding birds as soon as we landed,” Gibbs says.

  No one in the Finch Unit had ever known a wet season to start so early. And for the next few weeks the rains, though so long awaited, were almost appalling. In retrospect it is as if the cup of the year to come was so overfull that it was spilling backward. December 1982 drenched Daphne Major as never before in human memory—which of course was less than a decade long. On the island of Santa Cruz, next door, it was the wettest year’s end since the founding of the Charles Darwin Research Station in 1960: almost four times more rain than ever before. Far to the north, on the island of Genovesa, when another team of finch watchers arrived on the very last day of the year, they found Rosemary’s rain gauge already filled to overflowing.

  Stupendous and stupefying thunderstorms rolled over the islands, terrifying the cattle on the island of Santa Cruz, for Galápagos clouds are mostly mute. There were landslides on the steep slopes of the islands’ volcanoes, flash floods and overnight waterfalls. Cactus and Croton trees as old as the century were swept down the sides of the volcanoes, tumbling like twigs. Just above Gibbs’s tent on the rim of Daphne Major the skies were low, black, loud, and flickering, and just beneath them the seas were high, and hurled green and white breakers up the cliffs. The island rode into the new year like a ship into a storm.

  OF ALL THE CHRONIC PATHOLOGIES of weather on this planet the worst single repeat offender is El Niño, the Child, so called for its tendency to visit the Pacific shoreline of South America around Christmastime. During an El Niño a patch of abnormally warm water appears in the eastern Pacific and spreads until much of the eastern half of the ocean is running a fever of several degrees centigrade. Such a vast acreage of abnormally warm water stirs strange winds and weather virtually around the world; the Galápagos Islands sit in the epicenter, as it were, of the fevered water.

  El Niños are born at irregular intervals, usually three to six years apart. The Grants had started their study just after the last Child left. And by January 1983 it was clear to everyone who knew the islands that the Child was back, and that this was to be no ordinary Child. The seas were hotter than normal even for an El Niño, and the clouds that rose from the Pacific seemed to the naturalist and sailor Godfrey Merlen, who lives in the islands, like “storms of the ocean itself.” “The effect,” he wrote in a memoir of that apocalyptic year, “was of huge mushroom-like trees growing from the sea with heights reaching many thousands of feet.”

  Lisle Gibbs not only got rain. He not only got an El Niño. He got the strongest El Niño in living memory—probably the strongest of the twentieth century.

  On Daphne Major, his erstwhile desert island, Lisle waded through sheets of rushing fresh water. He slogged through black muck, frantically banding the new finch fledglings, which each day appeared faster than he and his assistant could keep up with them. They worked bareskinned and bareheaded through the torrents. Ponchos would have been useless, slick with rain on the outside and slick with sweat on the inside.

  Unlike Boag and Ratcliffe in the drought, Gibbs in the flood was sure that an extraordinary selection event must be in progress, although he did not know what the finches would do. He banded fledglings in the rain and wondered.

  “It was ridiculous,” Trevor Price said long afterward, ruefully, looking at the pictures in one of Lisle Gibbs’s papers. “It was like going from desert to jungle.” Gibbs had vines growing up the tent poles, and he could see them grow from morning to noon and from noon to evening, a few centimeters per day. The Croton trees flowered not just once or twice but as many as seven times, so that each tree and bush set seven crops of seeds, and every one a bumper crop. One Croton seed fell to the ground in December, and by May the plant was level with the eyes of a tall man, whereupon it burst into flower too. By June the total mass of seeds on the island was almost a dozen times greater than it was the year before. It was as if nature had set out a dozen dishes for every finch, whereas in most years it sets just one. There were also more than five times as many caterpillars to eat, and every one of them about four times normal size.

  It was too wet for cactus, and thickets of creeping vines smothered the Tribulus. So the big-seed crop crashed while the small-seed crop boomed. This was rags to riches for Darwin’s ground finches.

  “The birds went crazy,” Gibbs says. “The year before there had been no breeding at all. Now they bred like hell.” On Daphne, females produced up to forty eggs and fledged twenty-five young. The most prolific pair on Genovesa laid twenty-nine eggs in seven clutches, and twenty fledglings hopped out of the nest, a record for the island. In the steamy rains more and more of the birds were turning bigamous or polygamous. On Genovesa one female finch went through four males, one after the other.

  The longer the copulatory frenzy lasted, the more finch fledglings were hopping on the wet lava for Gibbs to catch and band. By June there were more than two thousand finches on Daphne Major.

  Most finches do not breed until they are two years old, and by then the finch watchers have gotten personally acquainted with each one of them. But in the middle of that breeding season, Gibbs and his assistant started to see banded birds they couldn’t recognize. “Finally, we realized that they were kids—three months old,” Gibbs says. The young birds they had banded in their first weeks on the island were pairing off and mating in the cactus bushes. No one on earth had ever reported anything like this: passerine birds are not supposed to breed in the same season they are born. But as the rains kept falling, almost every finch on the island was caught up in a breeding rush, like a gold rush. Some of the very young males staked out territories without a single cactus bush and still found mates. Many of the new pairs also successfully fledged young, especially young females who paired with older males. The youngest to breed was a female fortis less than three months old. She laid four eggs in her first clutch, and two chicks survived to leave the nest.

  The cactus finches produced eight times as many clutches as they had in the previous breeding season. In fact, in that single year they produced more than half the young they would ever produce in their lifetimes. The numbers of cactus finches and fortis on Daphne Major rose by more than 400 percent.

  It was a phenomenal year for finches all over the archipelago, from Wolf to Santiago, from Isabela to Española. On Genovesa, however, the gold rush had a dark side. The record numbers of eggs led to record numbers of deaths. Finches would sit in their nests in chilly showers for just so long; then they abandoned their eggs a
nd sometimes even their begging young. Heavy rains and heavy winds snapped the branches, and down fell the babies, cradle and all. Meanwhile, in a development that proved tragic for the finches, many of the Genovesa mockingbirds took sick in the rain. Their legs and claws blistered and swelled with what was probably pox.

  Most finches on Genovesa did not catch the pox, but even so they were hurt by the plague among the mockingbirds. Normally mockingbirds are bound together in tight little social groups, and the young unattached males help the older ones tend the nests. (The discovery that mockingbirds cooperate like this was made by the Grants’ daughter Nicola on Genovesa when she was twelve years old.) But this year, as the plague felled the elders in group after group, the survivors drifted away. Eventually the adult mockingbirds settled down and built nests elsewhere. But the young kept on roving through the wind and the rain. Bob Curry, a mockingbird expert and a Ph.D. student of Peter Grant’s, saw young mockingbirds hanging around the nests of the finches, sometimes singly and sometimes in little bands, like juvenile delinquents, “a steady stream of wandering mockingbirds.” They terrorized finch nests across wide swaths of the lava. They drove off the finch parents and ate the finch nestlings. One hapless cactus finch laid eight clutches, a total of two dozen eggs, and not a single one of her chicks lived long enough to hop out of the nest.

  A Galápagos mockingbird Mimus parvulus. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  AFTER THE RAINS STOPPED, the islands began putting themselves back together. The sun beat down. Red mud caked and crazed. By autumn the freshwater ponds in the uplands, where pintail ducks had made themselves at home, were already drying up, and the innumerable rain pools in the islands’ lava hollows, where shrimp had appeared as if by magic, were bone-dry again. The giant tortoises helped return the upland landscapes to normal by crashing and smashing down the weeds; they filled in the gullies by knocking down their sides. As early as September a naturalist walking through the uplands of Santa Cruz wondered, “Has this year really been?”

  The numbers of finches on the islands were astonishing, and for the first year or two after the El Niño there were still small seeds all over the island to support their population boom, riches like the jewel heaps in the caves of the Arabian Nights. But the skies that gave now took away. Only 53 millimeters of rain fell the year after the flood, and only 4 millimeters fell the year after that. The plants did not set half enough seed to replace the bumper crop from the year of the Child.

  The finches had overshot the carrying capacity of their desert islands, and now Lisle Gibbs watched their populations crash. He went on observing the huge flocks of Darwin’s finches on Daphne Major in 1983 and 1984, banding the newcomers and marking the deaths in his field notebooks with little crosses. Finches were dying right and left, as they had died in Boag’s drought.

  Would evolution continue to shoot like an arrow in the same direction, or reverse? How were the birds evolving now? He could not tell until he had accumulated a long enough record and fed it into a computer.

  In September 1985 Lisle was back home at the University of Michigan, Ann Arbor, where the Grants were teaching in those days. It had taken Lisle a year just to enter all of the data from his waterproof notebooks into the computer. He had spent months and months checking and double-checking the data for errors and checking and rechecking the program with which he would analyze the data for evolutionary trends.

  Now he ran the program. “I cranked out the numbers, and I was praying,” he says. “I remember the actual moment when I hit the return key. After all that work …”

  What he saw on the screen was so dramatic that at first he refused to believe it. He checked and rechecked. It was true. Natural selection had swung around against the birds from the other side. Big birds with big beaks were dying. Small birds with small beaks were flourishing. Selection had flipped.

  Both big males and big females were dying, he noticed, but many more males than females—again, the reverse of the drought. Everything the drought had preferred in size large—weight, wingspan, tarsus length, bill length, bill depth, and bill width—the aftermath of the flood favored in size small.

  At first, Lisle Gibbs and the Grants were not sure why the flood year dragged the birds backward like that, although it did make intuitive sense that an epic flood would undo the work of an epic drought. But eventually they came to understand why the flood favored small finches over big ones. With ten times more small seeds lying around, the large finches had trouble finding large seeds. They could still eat small seeds, of course, but they had the tools for large seeds, and they had a lifetime of experience hunting and cracking large seeds; and of course being big birds they had to eat many more small seeds to stay alive.

  So as seed supplies ran lower and lower, the bigger birds had more and more trouble. They were in the same sort of predicament that big young finches experience in their first few months of life. They paid dearly for their large size, because it gave them a larger appetite, and they could not make it up to themselves with their large beaks. Some of the large-beaked birds made the shift, but slowly and not as well as those with the right equipment.

  The net result of natural selection during Gibbs’s watch was as stark as during Boag’s drought. The birds took a giant step backward, after their giant step forward.

  A terrible drought like the one in 1977 may come once or twice in a finch’s lifetime, and an El Niño like the one that came in 1983 is a once-in-a-lifetime event. So having witnessed both the year of the drought and the year of the flood the finch watchers were now staring at an extraordinary picture. Clearly, selection pressures on a creature in the wild are far more intense in some years than others. But more than that, even the most intense selection pressures can actually reverse themselves during the creature’s lifetime. Not only can evolution push a species fast in one direction. Evolution can reverse direction and push it back just as swiftly.

  A Galápagos iguana. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  This was not just a freak of Darwin’s finches. Naturalists are now documenting similar reversals of fortune elsewhere in nature as well, including populations of Darwin’s “imps of darkness,” the marine iguanas of the Galápagos. The iguanas forage for seaweed in the shallows and then bake in the sun to digest it, while finches hop over them and sometimes pinch a fly or two from the lizards’ foreheads and dragon crests. It is hard to imagine neighboring animals with lives that are less alike, and yet for the evolution of the lizards the pressures of the droughts and the flood appear to have been countervailing too.

  Most of us think of the pressures of life in the wild as being almost static. Robins sing in an oak tree year after year. We imagine that life puts more or less the same pressures year after year on the robin and the oak. But the lives of Darwin’s finches suggest that this conception of nature is false. Selection pressures may oscillate violently within the lifetimes of most of the animals and plants around us, so that the robin must cling to the oak, and the oak to the ground, in chafing and contrary winds. It is as if each living thing on earth is holding on at the very shore of an ocean, in rough and invisible seas, swaying in place as each wave shoves it toward the shore and then tottering as the broken surf drags it back again.

  The stutter-step quality of the action is yet another reason that natural selection has been missed in most studies of live populations in the wild. If you measure natural selection over the course of a whole generation you may miss the many slings and arrows that it has taken along the way, the conflicting pressures in the nest, in the first days out of the nest, and on the yearlings and the adults; or on the acorn, the green shoot, and the towering oak. Each stage of life may have experienced an intense episode of natural selection, and yet their effects may have obscured each other’s traces by the time the very last of the generation has shuffled off the earth. Spe
cies of animals and plants look constant to us, but in reality each generation is a sort of palimpsest, a canvas that is painted over and over by the hand of natural selection, each time a little differently.

  When the finch watcher Jamie Smith left the Galápagos and began watching the sparrows on the island of Mandarte, in British Columbia, he did not know if he would find selection events there. His study provides a good basis for comparison because the sparrows and finches are closely related, and in some ways the situation of the song sparrows on Mandarte is much like that of Darwin’s finches on Daphne. There is a small resident population of sparrows. They are there year round. They do not migrate.

  Much of the work that Smith and others have carried out since the early 1970s on Mandarte is similar to the work that El Grupo Grant has done on Darwin’s finches: catching and banding sparrows, measuring their beaks and wings, following their fates.

  A few years ago, Smith was working on a paper on natural selection in these song sparrows, looking at the same traits as in Darwin’s finches. His major result was there was no evolution in the birds. Smith was going to report no selection in song sparrows.

  Before Smith published this report, another veteran of the Galápagos finch watch, Dolph Schluter, joined him at the University of British Columbia, in Vancouver. Smith told Schluter that natural selection was not doing anything to his sparrows.

  “So, I didn’t believe it,” Schluter remembers now, with a laugh. Schluter was fresh from the Galápagos at the time, and he was full of the power of natural selection. “Jamie told me, ‘Fine, here are the data. See for yourself.’ ”

  Dolph took a look. He knew that Smith had checked for evolutionary trends by comparing a generation of sparrows at birth and the same generation at death. Dolph decided to look at the birds year by year instead. He also broke each year into three components, studying the young sparrows’ survival rates in their first year of life, as they weathered their first Canadian winter; their survival as adult birds in each succeeding winter; and their success in rearing offspring in each breeding season.