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

by Jonathan Weiner

  Evolution discloses a meaning in death, although the meaning is like some of the berries that Darwin tasted in the Galápagos, “acid & Austere.” There is a special providence in the fall of a sparrow. Even drought bears fruit. Even death is a seed.

  Chapter 6

  Darwin’s Forces

  And the more we learn of the nature of things, the more evident is it that what we call rest is only unperceived activity; that seeming peace is silent but strenuous battle. In every part, at every moment, the state of the cosmos is the expression of a transitory adjustment of contending forces; a scene of strife, in which all the combatants fall in turn. What is true of each part, is true of the whole.

  —THOMAS HENRY HUXLEY,

  Evolution and Ethics

  The next watch fell to Trevor Price. He landed on the island early in 1979, and made camp in the craterlet on the east rim, the same small flat sandy spot where the Grants, and the Abbotts, and Peter Boag and Laurene Ratcliffe had camped before him.

  Because he was standing watch after the others, Trevor could stand on their shoulders. He inherited all their painfully accumulated evidence demonstrating that variations in the finches’ beaks are important, and that natural selection moves swiftly and surely among them. As Peter Grant’s latest graduate student he also inherited the cooperation of the Charles Darwin Research Station, the routine of the pangas and chimbuzos, the names of local fishing captains, and (on buggy nights) the martial arts of the tent. Camping in the islands, he and his field assistant Spike Millington swatted mosquitoes beneath their tattered canvas with “Comparative Ecology of Galápagos Ground Finches,” by Abbott, Abbott, and Grant, published in the Ecological Monographs of 1977, and already a classic.

  After Trevor’s first year on the island, seven finches in every ten were banded, and after the next year, nine out of ten. Trevor was the first who could recognize every single finch on the island on sight. Ever since Trevor, the finch watchers have been able to recognize at a glance not only the banded birds but even the handful of rogues that are still at large. When a Finch Unit graph shows 1,250 finches on the island in a given month of a given year, it is not an estimate. The watchers have made a head count, like shepherds in a fold.

  Trevor was able to follow Darwin’s finches more closely than anyone else before him, not only because he had the benefit of so much data, and because so many of the finches on Daphne were banded, but also because after the drought of 1977 there were only a few hundred birds left. He could spot every one of their nests before its dome was woven. He had time to peer into it, mark its crook in the cactus tree with red flagging tape, and check it often on his rounds—around and around the desert island. Very few young fledglings hit the lava without one of Trevor’s bright-colored bands around their ankles, the bands that would identify them as surely as rank, file, and serial numbers—or given, middle, and family names.

  During Trevor’s watch the flocks on Daphne Major became known so comprehensively and microscopically that the whole island seemed as small—for a brief interlude, at least—as a Petri dish. He moved toward the kind of near omniscience we expect in a laboratory. Now he could begin to see what happens to Darwin’s finches when they are squeezed by not one but several conflicting selection pressures at once. For there is more than one force at work at a time in evolution, and their collisions are unruly.

  Trevor measured all the finch chicks on Daphne when they were eight days old. He measured them again as eight-week-old juveniles, and as eight-month-old adults. He could see that the beak of the finch is full grown, or very nearly so, at eight weeks. If a young fortis has a beak depth of 9.45 millimeters at eight weeks, the bird will still have a beak of 9.45 millimeters at eight months, and at eight years, if it lives that long.

  However, looking over his data, and the data of finch watchers before him, Trevor noticed something peculiar that the others had missed. When he looked at each generation of fortis, he saw that the average beak depth did not stand still as the birds grew up. In 1976, for instance, the average beak depth of the juvenile fortis on Daphne had been 9 millimeters. Six months later, the average beak depth of that same cohort of birds had dropped to 8.73 millimeters.

  The individual birds had not changed. But the cohort as a whole had changed, because the smallest of the young birds, the ones with the shallowest beaks, were surviving, while the biggest, the ones with the deepest beaks, were dying. Trevor saw the same thing happen to generation after generation of young Darwin’s finches. Not every small young finch survived, and not every big young finch died, but the small ones were the most likely to succeed.

  After some thought, Trevor figured out why. At that tender age, the young finches’ skulls and beaks are still soft, like the skulls and jaws of human babies; the bony puzzle pieces are not yet firmly sutured together. So even the biggest birds cannot crack the big hard seeds that some of them will learn to manage later on. During their first dry season, as the small seeds on the island grow scarce, and as the biggest of the adults on the island begin to eat the biggest seeds, the juveniles still have to hunt and peck only small soft seeds. Even the very biggest juveniles will not pick up a Tribulus seed.

  Big birds need more food than young birds, and big juveniles need the most food of all, because they are still growing. But because they are young, their big soft beaks do not help them get it. For a finch, being young and big is all liability and no opportunity. After a while, as each dry season drags on, some of these big juveniles get so thin and sluggish that the finch watchers on Daphne Major can reach down and pick them up with their bare hands.

  So bigger is not always better. While these birds are young, natural selection forces them in the direction of small size. When they get older, selection can force them in the direction of large size. Trevor measured these small conflicting waves of natural selection as they passed through each generation, like ripples bouncing back and forth across the face of a pond, pushing each cohort first one way and then the other.

  LIFE IS NOT SIMPLE, even for a flock of birds on a desert island. Just staying alive from one life-stage to the next is a full-time job. And of course survival is only the first step. After the birds get a little older they also have to meet, mate, and raise families—while continuing to survive. Sex adds a whole new set of struggles to the struggle for existence, and the pressures of sexual selection sometimes conflict with the pressures of natural selection.

  Darwin mentions sexual selection in the Origin, but he writes about it at greater length in The Descent of Man, published in 1871. In fact the subject fills half the book, whose complete title is The Descent of Man, and Selection in Relation to Sex.

  In one way the process that Darwin called sexual selection is less harsh than natural selection. The worst penalty in the game of natural selection is death. The worst penalty in sexual selection is life without a mate. But then, a failure to mate is equivalent to genetic death.

  In the dry season, natural selection metaphorically scrutinizes these birds, “daily and hourly,” as they strive to keep body and beak together. Some birds make it, and some don’t. In the wet season, which is also the breeding season, the survivors are scrutinized daily and hourly by one another, not metaphorically but literally, as males begin jousting for territory, building nests, and singing from the highest cactus in their territories, while females troop by and inspect the males’ nests and plots of lava and listen to their songs.

  In other words, as soon as nature stops selecting among these birds, the birds start selecting among one another. Again, some make it and some don’t.

  Darwin was convinced of the power of sexual selection, just as he was convinced of the power of natural selection. But he never saw evolution happen through either process, and his sexual selection theory went into a long eclipse after his death. It began to reemerge only after The Descent of Man was reprinted in a centennial edition in 1971.

  The process has now been demonstrated in action many times and in many places,
and one of the most dramatic demonstrations, of course, was the aftermath of Boag’s drought on Daphne Major. There, because the females on the island chose only the largest males with the largest beaks, the process of sexual selection worked in the same direction as the process of natural selection and magnified it.

  The skewed sex ratio on the island, a legacy of the drought, lasted a long time. During Trevor’s watch on the island, the male fortis outnumbered the female fortis by two to one, or even three to one. This provided Trevor with an ongoing demonstration of the power of sexual selection. In fact it turned the island into as dramatic a theater for the comedy of sexual selection as the drought of 1977 had been for the tragedy of natural selection.

  No females went without a mate in those years, and some of the females also had a second mate on the side. But males all over the island languished through the wet seasons without a mate, building nests in the cactus trees on their territory and singing from the highest point and winning nothing. Not one of the males on the island ever had two mates at a time, as far as Trevor could tell—and he was watching as closely as a Washington reporter.

  Once again, Darwin’s finches were not only meeting but surpassing Darwin’s expectations. Darwin had assumed that the pressures of sexual selection would be higher in polygamous species than in monogamous species. Among the sea lions of the Galápagos, for instance, one male has a harem, and all the rest are bachelors and out in the cold—so the selection pressure among the males is enormous. Among more or less monogamous birds like these finches, on the other hand, Darwin assumed (reasonably enough) that the pressure of sexual selection would be milder, because there would usually be about equal numbers of males and females to pair off in each generation. Yet because of the remarkably skewed sex ratio after the drought the pressure of sexual selection on Daphne Major approached the intensity it attains among sea lions, where males play winner-take-all, losers-take-nothing. Year after year, many of the males remained bachelors, while other males mated and fathered many chicks.

  Trevor could not tell, by eye, why one finch was winning round after round of sexual selection and others were losing. But after he left the island he entered into his computer the weight, wingspan, beak length, beak depth, and beak width of all the finches on the island in 1979, 1980, and 1981.

  In two of those years, 1979 and 1981, the finches were breeding after having survived a terrible time, first the drought of 1977, and then the more moderate drought of 1980. During those breeding seasons, Trevor saw that it was the biggest males with the biggest beaks that were winning mates. The females were choosing the very features that had allowed the birds to pull through those hard times.

  In one of the years of his study, Trevor also measured the sizes of the males’ territories and their wealth—how many fruit- and seed-bearing trees there were in each male’s territory. Here too he found a pattern. Males with bigger territories were more likely to win females than males with smaller territories.

  Plumage mattered too. For a jet-black fortis male on Daphne Major, the chances of finding a female in 1979 and 1980 were better than fifty-fifty. But for a male that was not yet in full black feather—a male that still looked a little immature—the chances were less than one in three.

  Black plumage indicates age and experience, and age and experience can make a difference in the number of offspring a couple can turn out. On the island of Genovesa, for instance, the nests of first-time fathers get dive-bombed by owls significantly more often than the nests of experienced breeders. Females with experienced, jet-black males are more likely to breed early in the wet season than females with first-time males. Experienced couples can sometimes lay not just one but two clutches of eggs before the breeding season is over.

  If black males win more females, why don’t all males turn black as fast as they can? Why do some of them turn black in their first year while others linger for years in plumage of such unattractive brown, in spite of the powerful sexual selection pressure that Trevor measured? The huge amount of variation in the finches’ plumage suggests that there are hidden costs to wearing black plumage and hidden benefits to wearing brown.

  While the females are flying around checking out each territory, the males are doing battle with all their neighbor males to establish and expand the boundaries of their piece of lava. Since a male in black gets into more fights than a male in brown, the most vigorous males are the black ones with a lot of land.

  A male that stays brown a while may be able to avoid getting into so many fights, and set up its territory inconspicuously. “It does seem quite a problem for a male to establish a territory,” Trevor says. “I once saw a young cactus finch wake up and give a few tunes at the corner of an old male’s territory. The old male came for him like an arrow and hit him with his beak in full flight.” Trevor saw one rather battered-looking male that held on to a small territory only as long as he was still in brown; as soon as he turned black, a neighboring male drove him away.

  We think of the plumage of birds—the red of the male cardinal and the brown of the female, the green head of the male mallard and the brown of the female—as fixed and permanent, or as constant as anything else in the living world. To us they seem to stay the same as time goes by, like stones in a stream, which stand still hour by hour as the water flows. But while some of the birds’ features are more or less set in stone, like the plan of their bodies—one beak, two wings, two legs—other features, permanent as they look, are the product of perpetually contending forces. They look solid, but they are as fluid as ripples on the stream. They are standing waves that grow or vanish, shift or disappear, with every change in the currents or the rocks.

  THIS IS A STRUGGLE of struggles, or war of wars, that Darwin could only imagine, a war in which the forces of sexual selection wrestle with the forces of natural selection, pushing and pulling a living form this way and that, down through the generations. John Endler, the author of Natural Selection in the Wild, has been observing this conflict for years, in one of the most elegant and precise demonstrations of evolution in real time.

  What the Grants are to Darwin’s finches, Endler is to guppies. His guppies are not the variety they sell in pet stores (he considers those trash fish). His guppies live in northeastern South America, in the small streams that zigzag down the mountains of Venezuela, Margarita Island, Trinidad, and Tobago, flashing through steep, undisturbed green forests and then the broad spreads of the old cacao and coffee plantations, on their way to the Caribbean Sea and the Atlantic.

  The male guppies wear black, red, blue, yellow, green, and iridescent spots in varying sizes, shapes, hues, and combinations. In fact their spots vary so much that they are like fingerprints: no two guppies are alike.

  These spots, like the beaks of Darwin’s finches, are heritable. Although the exact placement and arrangement of spots is unique, each guppy inherits its particular palette of colors, and also the general size and brightness of the ensemble, from its parents. The spots only show up on the males (they can be made to appear on the female guppies with testosterone treatments).

  Like minute variations in the beaks of finches, the spots on a guppy are the sorts of details that one might imagine are beneath the notice of natural selection. Nature may scrutinize the slightest variation, but there are some things even Darwin’s process cannot see. Design could not possibly govern a thing so small.

  In the 1970s, while Peter and Rosemary Grant were watching the finches of the Galápagos, Endler began watching the guppies of Venezuela’s Paria Peninsula, and Trinidad’s Northern Range. There the streams run down the mountains roughly parallel, as if in a series of vertical stripes. The streams are clear, swift, and clean, deeply shaded by tropical evergreens and punctuated by waterfalls. Their beds are lined with brilliant, many-colored gravel, much like the floors of the fish tanks in the pet stores.

  It is obvious to anyone who has ever tried to watch a school of these guppies against the parti-colored sands and pebbles of a streambed th
at the spots are excellent camouflage. In fact you could watch one of these clear streams for quite a while before you noticed the guppies at all, because they tend to swim close to the gravel while the sun is out.

  The fish need this camouflage because they have seven enemies: six species of fish and one freshwater prawn. All seven of these enemies hunt guppies from dawn till dusk. The most dangerous is Crenicichla alta, a cichlid fish, which eats about three guppies an hour; the least dangerous is Rivulus hartii, which eats one guppy in about five hours.

  Endler found guppies and at least a few of their enemies in almost every section of almost every stream, from the headwaters near the summit of each mountain to the plains and plantations below. Neither the guppies nor the guppy eaters can swim up a waterfall, and the population of each section of stream tends to stay put. (Sometimes a few fish get swept downstream, but none of them can get back up.)

  High up near the headwaters of each stream, the only enemy the guppies have is the comparatively mild-mannered Rivulus hartii. But moving downstream, section by section, the population of guppies lives and dies in the company of more and more of its enemies, until down near the base of each mountain, the stream is loaded with all seven of the guppy eaters. So a graph of risk and danger runs with the current. For the guppies, the higher in the stream, the lower the risk; and the lower in the stream, the higher the risk. In stream after stream the intensity of natural selection is graduated in the same way: gentle pressure among the guppies at the top, violent pressure among the guppies at the bottom.