The Beak of the Finch Page 7
Darwinism is often spurned by the devout as a branch or prop of atheism. Yet Paley inspired Darwin at least as much as he inspired FitzRoy, and it was precisely this tradition of natural theology that led Darwin to the most original and unconventional step in his argument, his theory of the importance of variations.
If living things are well made, Darwin argued—if they are admirably adapted to their places in nature, contrivances more elaborate than watches—then even the slightest variations must make a difference to the individual animals and plants that are saddled with them. Some variations must help living things run better, some worse, and some—a very few variations, arising only once in thousands of generations—might help them fit into an entirely new spot in the economy of nature.
The beak of a bird makes a natural test of this step in Darwin’s argument, not only because the beak is so easy to measure, but because it is so obviously vital to the life of the bird. Darwin’s finches can’t put food in their mouths with their wings. They can’t use their claws either, any more than we can dine comfortably with our feet. They have to use their beaks. Beaks are to birds what hands are to us. They are the birds’ chief tools for handling, managing, and manipulating the things of this world (manus meaning hand).
The shape of a bird’s beak sets tight limits on what it can eat. Although the bones and the horny sheaths of the mandibles are a little more flexible than they look—a woodcock can poke deep down into the mud, part its beak at the very tip, and grab an earthworm—still, these are not many-jointed and articulate instruments like our hands. Each beak is a hand with a single permanent gesture. It is a general-purpose tool that can serve only a limited number of purposes. Woodpeckers have chisels. Egrets have spears. Darters have swords. Herons and bitterns have tongs. Hawks, falcons, and eagles have hooks. Curlews have pincers.
There are about nine thousand species of birds alive in the world today, and the variety of their beaks helped confirm Paley’s belief in an inventive God. Flamingos’ beaks have deep troughs and fine filters, through which the birds pump water and mud with their tongues. Kingfishers’ beaks have such stout inner braces and struts that a few species can dig tunnels in riverbanks by sailing headlong into the earth, over and over again, like flying jackhammers. Some finch beaks are like carpentry shops. They come equipped with ridges inside the upper mandible, which serve as a sort of built-in vise and help the finch hold a seed in place while sawing it open with the lower mandible.
But plain or fancy, each beak can do only so much. The flamingo’s beak is good for filtering pond water. The hawk’s is good for ripping up a rabbit, a fox, or another bird. If the flamingo and the hawk ever tried to trade jobs, the hawk would drown in the pond scum, and the flamingo would get its eyes poked out.
Darwin extrapolates from big variations like these to the much smaller variations between individuals. According to his theory, even the slightest idiosyncrasies in the shape of an individual beak can sometimes make a difference in what that particular bird can eat. In this way the variation will matter to the bird its whole life—most of which, when it is not asleep, it spends eating. The shape of its particular beak will either help it live a little longer or cut its life a little shorter, so that, in Darwin’s words, “the smallest grain in the balance, in the long run, must tell on which death shall fall, and which shall survive.”
None of Darwin’s readers doubted that the hooks, swords, spears, and pincers of the world’s birds are of adaptive value. That was the pious, conventional, and commonsense view of Paley. But many readers did doubt that individual variations mean as much as Darwin says. He himself never actually saw a slight variation help or hurt the chances of an animal or plant in its struggle to survive.
Darwin does give one passage in the Origin the promising heading “Illustrations of the Action of Natural Selection.” “In order to make it clear how, as I believe, natural selection acts,” this passage begins, “I must beg permission to give one or two imaginary illustrations. Let us take the case of the wolf.…” Then he gives a few quick sketches to show that hypothetically, in a hard winter, when there is almost nothing else for a wolf to eat but deer, the fleetest and slimmest wolves would be expected to do best. Darwin makes similar arguments about nectar-bearing flowers and honeybees, all logical and hypothetical. That is the end of the section.
These sketches are so vivid that they enter straight into the minds of Darwin’s readers. For years, those who accepted and those who rejected them felt no compelling need to go beyond them—and certainly not Darwin’s bulldog, Huxley. “But the question now is:—Does selection take place in nature?” he asks, rhetorically, in one of his defenses of Darwinism. “Is there anything like the operation of man in exercising selective breeding, taking place in nature?” In answer he asks us to imagine what it must be like for a species of animal in nature, surrounded by fifty or one hundred others, with “multitudinous animals which prey upon it, and which are its direct opponents,” and others preying on those, and still others indirect helpers, etc. He concludes that “it seems impossible that any variation which may arise in a species in nature should not tend in some way or other, either to be a little better or worse than the previous stock.…”
Even after half a century, Darwin’s point about variations was still being defended with imaginary illustrations, and attacked for being imaginary. “The whole question [of the struggle for existence] has been discussed very largely from the a priori standpoint, throughout the whole period since the appearance of the Origin of Species,” wrote the geneticist Raymond Pearl in 1911. “The ‘rabbit with his legs a little longer,’ the ‘fox with the little keener sense of smell,’ the ‘bird of dull colors which harmonized with the background,’ et id genus omne, have been made to do valiant service.”
Darwin himself never tried to produce experimental confirmation of this particular point. It is at once extremely logical and extremely hard work to prove. Certainly he could not prove his case with his finches. He never learned much more about the details of their struggle for existence than he did during that first glimpse on San Cristóbal, when he watched the birds hopping together under the bushes, “scratching in the cindery soil with their powerful beaks and claws.” If anything, those mixed flocks argued against his case. He saw finches with long thin beaks and short fat parrot-like beaks all hopping on the same lava, eating identical bird food. If beaks with such widely different shapes could handle and crack the same seeds, then what could it possibly matter if, among the parrot-beaked finches, one bird’s beak was a little fatter than another’s, or if, among the sharp-beaked finches, one was a little sharper?
On Daphne Major, for instance, the beak of the average magnirostris is 14, 15, and 16 millimeters in width, length, and depth. The beak of the average fuliginosa on Daphne is only about 7, 8, and 7 millimeters—that is, less than half as big. Yet Darwin saw both species eating the same food. If those two tools can do the same work, then what is the point of the Grants’ measurements of two neighboring cactus finches, one with a beak 14.9, 8.8, and 8 millimeters, and the other 15.8, 9.7, and 9 millimeters? Variations that small would seem to mean nothing.
Natural selection is supposed to scrutinize the slightest variations in nature, “daily and hourly.” But as far as Darwin could say after his five weeks in the Galápagos, natural selection is blind to the beak of the finch. No wonder he left them out of the Origin.
An ornithologist named Osbert Salvin looked over some museum specimens of Galápagos finches in the 1870s, four decades after Darwin collected them. It was Salvin who noticed how variable the finches can be, one from the next, in the length of their legs, in their wingspans and their weights, and especially in their beaks.
Salvin felt the Galápagos was “classic ground” (even in the 1870s). Having discovered that the islands’ finches represent an extraordinary range of variations, he must have been disappointed that these variations seemed to have so little influence on the survival of the fittest. “The me
mbers of this genus,” he wrote, “present a field where natural selection has acted with far less rigidity than is usually observable.” Of course, the action of natural selection had never been observed at all.
Most of the series of scientific pilgrims who made their way to Darwin’s islands arrived in the wet season, or what passes for a wet season in these desert islands. All of the bushes and trees were in leaf and flower, and there were plenty of seeds on the ground. The scientists watched closely, and they saw exactly what Darwin had seen. Most of the ground finches were hunting and pecking together beneath the half-naked bushes. All those different beaks were cracking the same birdseed.
One after another, this series of careful ornithologists concluded that the shape of the beak of a ground finch makes no detectable difference in the food it eats. “To look at the bills of these birds in the hand, we would conjecture wholly different diets,” wrote the biologist and explorer William Beebe, who sailed out to Daphne Major through clouds of yellow butterflies in the wet season of 1923. “The small, delicate mandibles of fuliginosa would seem adapted to insect food, or at least small, rather soft seeds,” he said. “At the other extreme, the huge beak of magnirostris, almost as large as the entire head, would be equal to the hardest of acorns.” But both birds were eating identical foods. “What a mad country for birds and butterflies!”
It seems unbelievable now; but the action of natural selection is so easy to miss that Darwin’s finches were considered by generations of ornithologists to be an exception, or even a counter-illustration, to Darwinism. In 1935, the hundredth anniversary of Darwin’s visit to the islands, the ornithologist Percy R. Lowe gave a commemorative lecture before the British Association on the birds of the Galápagos. Lowe confined his talk to the Galápagos finches. He called them—apparently for the first time—Darwin’s finches. But having read the by-now-extensive literature, Lowe declared his belief that the birds are not separate species at all, but “hybrid swarms.” He thought their extraordinary variations would prove to be as meaningless as the varieties of coats in the stray mutts and cats in an alley. The beak of the finch offered “no scope for Natural Selection.”
(“Yes, he really said that, didn’t he,” Peter Grant says now. “ ‘No scope for natural selection.’ Which was a wonderful way of stimulating people to go out and try to disprove him.”)
Three years later, another British ornithologist anchored off San Cristóbal, Darwin’s first island. Like Darwin he was a young man in his twenties. Lowe’s lecture had piqued his interest, and although the Galápagos Islands had seemed “impossibly remote,” he had been encouraged to make the trip by Julian Huxley, a grandson of Darwin’s bulldog.
David Lack stayed through almost all of the wet season—one of the wettest wet seasons in the Galápagos in this century. The wet season is the finches’ breeding season, and Lack saw a lot of breeding. He saw that the thirteen species of finches in the islands rarely interbreed. He even built aviaries in the long, hot, humid afternoons and tried to get the birds to hybridize inside them, but they would not cooperate. They seemed to be very particular in choosing their mates. So Lowe had been wrong to call them “hybrid swarms.”
Still, even though the birds were not breeding together, most of the ground finches were feeding together, eating the same seeds. Lack had to agree with Lowe that the beak of the finch offers “no scope for natural selection.” “In fact,” Lack concluded, “there is no evidence whatever, in any of the island forms of Geospizinae [Darwin’s ground finches], that their differences have adaptive significance.” He wrote up this result in a monograph, but its publication was delayed by the outbreak of World War II.
It was some time after Lack got home to England that, like Darwin before him, he did a double-take. As he looked over his data he noticed that the species of finches whose beaks are most nearly identical do not live together on any of the islands in the archipelago. The cactus finch (Geospiza scandens) breeds on Daphne Major, for example, and also on all of the biggest islands in the archipelago except Fernandina; the large cactus finch (Geospiza conirostris) breeds on Genovesa and Española. Lack had never seen breeding colonies of both cactus finches on any one island. What is more, if two finch species with rather similar beaks do share an island, their beaks are more divergent in their measurements on that island than they are elsewhere. That is, the longer beak is longer than average, and the shorter beak is shorter than average, almost as if they were consciously trying to get out of each other’s niches.
Lack found these patterns in case after case, not only in his own data books, but also in measurements of the thousands of museum specimens that had been collected since Darwin. Like Darwin, he could not see evolution in action, and he assumed it would be too slow to watch; but he could infer, looking back at the Galápagos, that something must be going on.
Lack was influenced in part by other biologists’ studies of microcosms smaller than the Galápagos. Place two different species of Paramecium in a test tube, and come back in a few days. One species will have conquered the top of the test tube, and the other owns the bottom of the test tube, and the border in the middle is a no-cell’s-land. Likewise with barnacles: one species takes the high tide line and another takes the low tide line.
Such experiments seemed to show that no two species eating the identical foods in identical ways can coexist peaceably in the same test tubes, on the same rocks, or on the same islands without one species driving the other to extinction. This is just the sort of competition and conflict that Darwin had imagined might lead to the extinctions of many branches and twigs on the tree of life; the branches in the middle would die off, and the survivors would bend and twist and diverge to either side as if to minimize competition by making themselves as different as possible.
Lack made charts of the beaks and their distributions in the archipelago. On island after island, Darwin’s process seemed to him to have exterminated one or the other of two like-beaked species, or else to have pushed the survivors far enough apart to coexist. Whenever species with very similar beaks try to colonize the same island, Lack decided, they are thrown into competition. The struggle grows so bitter that one or the other species of finch is driven to extinction. But occasionally two like-beaked species evolve enough local differences that the intensity of their competition is reduced. Then both species survive.
Lack turned a classic negative case into a classic positive case, and helped to end the eclipse of Darwinism. In 1947 the very title of his monograph, Darwin’s Finches, had a triumphant ring to it. Darwin’s finches really are Darwin’s. There is scope for natural selection in the beak of the finch.
The book had a powerful influence on specialists and on the general public, even though Lack had not actually seen natural selection in action any more than Darwin had.
DURING THEIR FIRST FIELD SEASON in 1973, the Grants and the Abbotts measured not only finch beaks but also finch behavior. They staked out eight sites of twenty-three thousand square meters. At each site they marked off a grid of reference points by tying red flagging tape to hundreds of cactus bushes and torchwood trees. Each morning they would crisscross one of the grids with binoculars, notebooks, and stopwatches, and see what the finches ate for breakfast.
The Grant team discovered that the ground finches were concentrating on about two dozen different species of seeds. So the members of the team put each of these two dozen kinds of seeds between the points of a vernier calipers and measured them as carefully as they measured the birds’ beaks. They also measured the seeds’ hardness with the McGill nutcracker. This is a gadget that Peter Grant designed with the help of an engineer at McGill University, in Montreal, his first teaching post. The McGill nutcracker is a pliers with a scale attached. Squeeze a seed with the pliers, and the scale shows how much force it takes to crack the seed open. Modern physicists measure force in a unit they named for the founding father of their science: the newton. To crack a grass seed, which is a speck about the size of a poppy seed
, takes very little force, less than 10 newtons. A big cactus seed, the size of a peppercorn, takes more than 50 newtons. Cracking the toughest seeds in the Galápagos requires a force of 250 newtons, which is enough force to lift more than a thousand cactus finches into the air.
Peter Grant combined the measurements of seed size and seed hardness and rated each kind of birdseed as the finches might themselves, in a sort of Struggle Index. The small soft ones of Portulaca score the lowest on this index, only 0.35. The big hard seeds of Cordia lutea score highest, almost 14. Any of the finches can handle Portulaca in its beak, but very few are up to Cordia.
The Grant team also kept a census of the numbers of each kind of seed on the lava. To do this objectively they used a random-number table to select a single plot of lava, one meter square, somewhere in each grid. Then they counted every single fruit and seed they could find on that square of lava, whether it was dangling from the top of a cactus tree or lying in the middle of a cactus patch. Next they chose a much smaller plot within that square meter, again at random, and they sifted the hot cindery soil, collecting every fruit and every seed they found. Finally they withdrew to their tents and spread out their trophies on white trays to count one by one. And they repeated the whole routine fifty times.
“Most miserable piece of data we did,” says Peter Boag, who, with his wife, Laurene Ratcliffe, joined the Grant group early on.
“Sift the dirt!” groans Ratcliffe. “Count every seed, every single seed! That’s Portulaca. That’s Rynchosia. That’s Setaria, Acalypha, Mentzelia, Heliotropium … aaargh!”