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

by Jonathan Weiner

  Once we accept that slight variations can help decide who lives and who dies, Darwin takes his thesis a step further. He argues that favorable variations will be more likely to be passed down. They will spread through the population, from one generation to the next, while variations that hurt individuals in the population will tend to dwindle and die out.

  Two medium ground finches on Daphne Major: same species, same age, same island, yet one beak is strikingly deeper than the other.

  Drawing by Thalia Grant

  During the eclipse of Darwinism this point seemed as much a matter of faith as the rest of Darwin’s theory. Believers accepted it, skeptics rejected it. In the 1930s, for instance, the British evolutionists Robson and Richards analyzed the handful of studies that purported to show evolution in action. Robson and Richards concluded that even where natural selection might just possibly have been detected in action, the case studies did not prove Darwin’s point because, in the opinion of Robson and Richards, the variations in question had not been passed down, and variations that are not passed down cannot lead to evolution.

  Peter Boag has a background in genetics. After the Grant team had been watching Daphne a few seasons, Boag decided, as part of his thesis project, to try to measure the relation of parent bill size to offspring bill size in Darwin’s finches. That is, he would measure how accurately the variations in their beaks are passed down—a factor that matters as much to their evolution as the presence of the variations themselves, or their influence on the lives of the individual birds. Improbable as it sounds, no one had ever actually tried to measure this factor, known in the jargon of genetics as heritability, in the wild. The more accurately the variations are reproduced—the more heritable they are—the faster the work of evolution could proceed among these finches. And without actually making the measurements, Peter Boag explains, “We didn’t have any basis to judge.”

  Boag looked over all of the finch group’s data, several years’ worth, and he compared the sizes of the offspring and the sizes of their parents. He found that the body size of a finch does indeed depend very strongly on the size of its parents. A finch’s size is highly heritable.

  Boag also compared the birds’ beaks to their parents’. The shape and size of the beak too is highly heritable. The beak of the finch is passed down faithfully from one generation to the next.

  There was one loose end that could invalidate Boag’s results. Suppose for the sake of argument that on Daphne Major in those years, fortis finches with bigger-than-average beaks were able to get more food. And suppose birds that eat better as babies grow up to be bigger-beaked adults. If so, a pair of big-beaked parents would have tended to provide more food for their babies, and their babies would have grown up big-beaked too. Big-beaked parents would have big-beaked offspring, small would have small, and yet the effect would have nothing to do with genetics. Despite the correlations that Boag had found, the size and shape of the beak of the finch might not be passed down from parent to chick.

  This was nothing more than the age-old question of nature versus nurture, and Boag knew how to test it. If he took some eggs from a pair of big finches and put them in the nest of a pair of small finches, would the young grow up looking like their true parents or their foster parents?

  Boag never did have a chance to perform this experiment during his watch on Daphne. And in retrospect, the finch watchers are glad he didn’t, because their study is now so sensitive that the large number of egg switchings that Boag planned would have caused unnatural disruptions from that day to this. In fact, locally, Boag might have changed the course of evolution.

  Instead, the egg switch was performed by Jamie Smith, after he left the Galápagos and set up shop among the sparrows on Mandarte Island. Smith switched many eggs from one sparrow nest to another, just as Boag had planned to do on Daphne. He found that the foster birds took after their true parents, not their adoptive parents. Being raised by a larger bird does not make you a larger bird. The young birds resemble their true parents, even though they are not raised by their true parents. This is very strong evidence that it is nature, not nurture, that plays the larger role in deciding the size of the sparrows and the shape of their beaks. As with the finches, the sparrows’ beak variations get passed down with remarkable fidelity from one generation to the next.

  Recent studies have shown that even the smallest details of bird life, everything from the exact size of the eggs to the number of the eggs and the date they are laid, are heritable too (at least to some degree). They are passed down from generation to generation in species after species of birds. This seems to be the rule rather than the exception in nature, just as Darwin imagined it to be, although not all variations in the living world are passed down as faithfully as the beak of the finch.

  ORIGINALLY THE GRANTS HAD PLANNED to Study Darwin’s finches for a few months and lug home as much data as they could. Then they would try to sort out some of the forces that have made the birds what they are. In other words, the Grants were planning to take a snapshot. And if Darwin had been right about the slow pace of evolution, no one could ever take more than a snapshot. Watching these birds would be like watching the stars for an astronomer, or the mountains for a geologist. Even one hundred years in the Galápagos would be a snapshot.

  But as the pieces fell into place, the Grants and their team began to understand that they had something worth watching. They would have to come back. The birds are exceptionally variable in their beaks. They are exceptionally sensitive to these variations. They pass on their variations with exceptional fidelity. Each of the requirements of Darwin’s process, each of the prerequisites for evolution by natural selection, is heightened in Darwin’s finches to an almost unnatural degree.

  “Slow but sure moves the might of the gods,” says the chorus in Euripides’ The Bacchae. Slow but sure moves the power of natural selection, says Darwin. But here on Daphne Major, among Darwin’s finches, the action might be swift and sure.

  No one had ever stood watch before among Darwin’s finches, so the Grants could only wait and see. As things turned out, they did not have long to wait.

  Chapter 5

  A Special Providence

  … there’s a special providence in the fall of a sparrow.

  —WILLIAM SHAKESPEARE,

  Hamlet

  And for myself I am fully convinced that there does exist, in Nature, means of Selection, always in action & of which the perfection cannot be exaggerated.

  —CHARLES DARWIN,

  Natural Selection

  Before he leaves Daphne’s north rim, Peter Grant stoops and scans the dirt by the path. With his floppy-brimmed sun hat and gray beard he looks both cheerful and grave, reading the dust. He is hunting for a Tribulus seed.

  “There was a time when you’d just say, ‘There’s one, there’s one, there’s one.’ Now you’ve got to search for them,” he says.

  He kneels beside a Tribulus plant, or what is left of it. After almost four years of drought, the plant has withdrawn to the roots. It looks like a black claw hiding from the sun. All around it the lava is coated with layers of old guano, and the glare on this white paint makes Peter squint, even though the morning sky is still overcast. He brushes aside a pebble or two.

  “Here’s a Trib,” he says at last, holding it up in his palm. The Tribulus plant dropped this mericarp during the last wet season, to wait for the next wet season. Now the plant is a withered stump, and the mericarp, still waiting, has bleached to the color of driftwood.

  Though it is guarded by two long sharp spines, this mericarp has been chipped open at one end. Within the broken place, Peter can see two dark pod holes, side by side, like tiny eye sockets, both empty. “Just two seeds taken out,” he says.

  At this moment, up and down the volcano, four hundred Darwin’s finches are doing what Peter has just done. They are turning over pebbles, inspecting the lava, raking the cindery dust with their claws, sometimes poking their heads down into dark crevices
, looking for the last bleached seeds. To open new ground, one of them will sometimes brace its head against a big rock and roll over another rock with its feet. A finch that weighed less than 30 grams was once seen rolling over a rock that weighed almost 400. That is like a man rolling a boulder that weighs one ton. It is the labor of Sisyphus, and unlike Sisyphus, Darwin’s finches cannot keep it up forever. They are wearing down the horny sheaths of their beaks. Some of them have scraped their feathered crowns almost bald. Their occasional reward is a treasure like the object in Peter’s palm, one more tough vitamin capsule, a husk with a few kernels no one else has eaten.

  In the Grants’ first four years on this island, they never saw the struggle for existence get this intense. Those were good years for Darwin’s finches. By the end of the Grants’ first season, for instance, there were about fifteen hundred fortis on Daphne Major. Nine out of ten of those fortis were still alive in December, just before the next rains came. There were also about three hundred cactus finches on the island that first April, and nineteen out of twenty of them survived the dry season and made it through to December.

  Their fourth year, in 1976, was especially wet and green. There were great bouts of rain in January and February, and light showers in April and May, a total of 137 millimeters of rain, which is a good year for Darwin’s finches.

  The fifth year of the study, 1977, began well too. Rain fell right on schedule in the first week of January. Within days, green leaves unfolded and flower buds opened all over Daphne Major, with here and there a few caterpillars crawling on the buds, fast food for Darwin’s finches. There were more than one thousand fortis and almost three hundred cactus finches on the island.

  By this time the Grants’ first pair of colleagues in the islands, Ian and Lynette Abbott, had gone back to Australia, and Jamie Smith had gone back to Canada. The Daphne watch had been taken over by Peter Boag and Laurene Ratcliffe. Boag was eager for the island’s fortis to start laying eggs, because he needed more parents and offspring for his study. He had also received special permission from the staff of the National Park Service of Ecuador to perform his egg-switching experiment. After the fortis eggs hatched, he was planning to band the foster chicks. The next season he would measure their beaks’ length, depth, width, and finish his Ph.D. with a bang.

  After the first rain, a few pairs of cactus finches mated. (Cactus finches often breed before much rain has fallen, perhaps because they make most of their living from cactus.) The birds laid their eggs in nests they built in the cactus trees, and the eggs hatched fine.

  Fortis do not breed until a little more rain has fallen. Peter and Laurene waited for the next good cloudburst, the one that would trigger the fortis to mate. But after the first week of January the sky above Daphne Major was like the sky that hangs over the Grants this morning, gray, low, and gloomily quiet. There was one more shower, a very light one, then nothing but clouds and heat.

  The small rain that fell in the first week of the year did not settle into the soil. There is no place on Daphne’s slopes for water to pool, and there is not much dirt to soak it up. The rain ran down the sides of the volcano as if pouring off a roof, and trickled away into the sea. Whatever was left was baked away by the sun or dried in the sea breezes on the rim and in the eddying winds that circle within the great bowl of the crater, stirred as if by a fire in the crater bowl each morning, as the rising sun begins to heat the lava.

  Peter and Laurene checked on the nests when the cactus finch chicks were seven days old. Cactus finches build domed nests deep inside the cactus bushes, where they are well protected from owls. The chicks were peeping away with as much energy as they do every year. Peter and Laurene reached into each cactus bush, lifted the chicks out, set them down in a hat, and measured them one by one. In a normal year, flies and moths would have been flitting around the cactus trees by now, and the chicks’ mothers and fathers would have been bringing bugs to the chicks. The air should have been so thick with bugs that three sweeps of a net through the air would come up with hundreds. But at the end of that January the island was still so dry that there were few flowers to bring out the insects. Three sweeps of the net came up with only a couple of bugs.

  When Boag and Ratcliffe sampled the contents of the chicks’ crops with an eyedropper they saw that most were almost empty. They found a little pollen, a few bits of flowers, or a seed kernel, sometimes a small spider.

  All over Daphne, leaves shriveled, flowers wilted. Boag and Ratcliffe had not planned to stay on the island this long. The way things usually went, they would go early in the year and band the fledglings in their nests on the seventh day. Then they would leave the island and come back later on to see how all the young birds were doing. But this year, Peter and Laurene were stuck. They could not leave the island until the fortis had their fledglings.

  From the upper rim of the crater, Boag watched the horizon. The wind usually comes from the south, and it is blocked by the much larger and taller island of Santa Cruz, which lies 8 kilometers to the windward of Daphne Major. Storms drop most of their rain on the south side of Santa Cruz, so that the north side of the big island, and all of Daphne Major, lies in a rain shadow. Boag could see regular downpours along the coast of Santa Cruz. The island of Santiago, 30 kilometers to the northwest, where Darwin passed a fortnight, got soaked too that spring.

  “We panted, drank water, and read books,” says Laurene. “I read The Agony and the Ecstasy. Peter read every single book on World War II.”

  “That’s the place to read The Rise and Fall of the Third Reich,” says Peter.

  A few scattered showers fell on a few scattered days, 24 millimeters of rain in all. This was not enough to move the fortis to pair off and mate, and it was not enough to fill the air with moths and flies. Two out of three of the cactus finches’ chicks died in the nest, and those fledglings that did make it out stayed close to their parents for twice as long as usual, some of them for more than a month. They hopped beside their mothers and fathers and begged with piercing cries and much shaking of their wings.

  During the previous June, when the island was wet and green, there had been more than 10 grams of seeds in an average square meter of lava. The finches had already eaten their way through many of those seeds during the dry season of 1976. Even if the rains had fallen as late as March or April of 1977 the seed supply would soon have rebounded, and the fortis would have begun to pair off and breed and lay eggs for Boag’s experiment. But the weeks went by, and the rains did not come, and the birds did not pair off. Day after day they went on pecking over the same square meters for the same diminishing supply of seeds. By June of that year there were only 6 grams of seeds per square meter. By December there would be only 3 grams.

  As they always do in dry times, the birds went on looking for the easiest seeds. But now they were sharing the last of the last of the pistachio nuts. They were down to the bottom of the bowl. In June of the previous year, four out of five seeds that a finch picked up were easy, scoring less than 1 on the Struggle Index. But as the small, soft, easy seeds of Heliotropium and other plants disappeared, the rating climbed and climbed, peaking above 6. The birds were forced to struggle with the big, tough seeds of the Palo Santo, and the cactus, and Tribulus, symbol of the struggle for existence, a seed sheathed in swords.

  Back in 1973, it had been quite rare to see a fortis try to crack one of the iron stars of the Tribulus, and when one of the birds did try, it needed an average of almost fifteen seconds to crack the mericarp. Boag and Ratcliffe got out the stopwatch again. A fortis could now get the side of its beak across a corner of a mericarp and twist, twist, until the capsule chipped in less than six seconds. The birds were getting a lot of practice with Tribulus—that is, those birds that could handle it at all.

  Some of the very smallest fortis on the island, the ones whose beaks were too small for caltrop, were poking around the Chamaesyce instead. The herb Chamaesyce has small soft seeds, but it also has a milky, sticky latex when its leav
es are wounded and its stems are broken. These little fortis, along with the very smallest birds on the island, the immigrant fuliginosa, began hunting for seeds in the Chamaesyce in spite of its latex. The feathers on the crowns of their heads got so matted, gummy, and sticky that they rubbed off afterward as the birds raked the cinders and gravel looking for more seeds. Their bare scalps were exposed to the sun all day. Boag and Ratcliffe began to find little bald finches lying dead on the lava.

  They kept up the routine of capturing and measuring finches, dangling them in the weighing cup and recording the numbers in their waterproof notebooks (that year they did not need them to be waterproof). By June, many of the birds’ weights were down as much as a quarter from the June before. A large number of these finches had failed to molt, although by now they badly needed a new set of feathers. Some of their contour feathers were so worn that the down underneath was exposed, as Boag would report later in his now-famous paper on the drought of 1977. He and Laurene found dead fortis lying on the lava with feathers so disheveled they looked as if they had been combed the wrong way. The fraying of feathers was hardest on the smallest species, the fuliginosa, which when not eating the small, soft seeds of Chamaesyce were poking about for bugs in the lichen on the torchwood trees, Boag says, or hawking for bugs from a naked branch. They needed all their contour feathers for flight.

  Even in good years, finch watchers have to be careful never to leave a bucket of water standing open in camp, or Darwin’s finches will jump in and drown. On one island another Galápagos biologist, an iguana watcher, did leave a jerry can open once, and in the morning it was full of finches. The bucket is like an oasis—it draws thirsty animals from far and wide. Once at the Charles Darwin Research Station a centipede a foot long crawled into an open bucket, and as grasshoppers hopped in, the centipede ate them one by one.