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

by Jonathan Weiner

  The unit had measured all of the seeds these ground finches can eat, and many of the seeds they can’t or won’t eat. Dolph knew what kinds of seeds are found on each island, and in what season, and in what amounts. He entered that information into a computer.

  Dolph also knew how big a beak can crack how big a seed. The critical dimension here is beak depth: the deeper the beak, the bigger and harder the seed it can crack. He entered that relationship into the computer.

  Finally, he knew how many seeds it takes to keep a finch alive. He entered that relationship into the computer too: such-and-such a mass of seeds translates into such-and-such a mass of finches.

  “Now,” Dolph says, in the tone of a geometrician who is trying to arrive at first principles: to use just a few lines and points and the minimum of rules, and from that generate a globe, a world, a universe. Suppose a single species of finch lands on a single Galápagos island. Suppose it meets no competition, and it survives and multiplies. What size beak will it evolve?

  Dolph told the computer the range of beak sizes available to all of the ground finches on the islands, from the smallest to the largest ever measured. He also told the computer the range of seed sizes available on the islands, from the smallest to the largest, and how many of each size there are. Then he programmed the computer to calculate how many finches a hypothetical island could support, given finches with beaks of every possible depth, from the shallowest to the deepest finch beak on record anywhere in the world, at increments of a small fraction of a millimeter. He programmed the computer to make the calculation over and over.

  Dolph assumed that it would draw a peak corresponding to the best of all possible beaks, that is, the beak size that would produce the maximum number of finches on that idealized Galápagos island. The peak the computer drew would represent the best possible beak on the island. The valleys on either side of the peak would represent all the miscellaneous beak sizes that were relatively unfit.

  He set the computer going, and he was thrilled by what he saw. The computer did not draw a single peak. Instead it drew three peaks, with deep valleys between them.

  The computer had picked up something that is not obvious to the eye. Seeds in the Galápagos come in three types, for a finch: easy, medium, and hard. Grass seeds are among the smallest and easiest, Tribulus seeds are among the largest and hardest, the seeds of the passionflower are among those in between. There is a continuous range, but on the whole the seeds of the islands do cluster into these distinct groups. Accordingly, the computer had drawn three peaks, for three beaks, one above each of the three piles of seeds.

  In effect the computer had predicted three species of finches, each with a beak precisely the size to crunch the seeds in its pile. Three peaks meant there could be not one but three highly adaptive beaks for ground finches on that idealized island. There was more than one good beak. A single founding species alighting in that adaptive landscape could diverge into three.

  Taking advantage of the Finch Unit’s masses of data, Dolph programmed the computer to run a more realistic simulation. He asked it to calculate the beaks that should have evolved on a dozen islands of the Galápagos, given the seeds that are actually found on each island.

  With just three factors, beak size, seed size, and competition, the computer predicted correctly the divergent paths of evolution for the beaks of finches on every one of the islands.

  Once again Darwin’s finches are true to their destiny as paragons of Darwin’s theory. They are the best case of character displacement ever found.

  “THE STRUGGLE,” Darwin writes in the Origin, “will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.” Yet in principle Darwin’s process of character displacement can occur even between living things spaced far apart on the evolutionary tree, if they are thrown into competition. There can be competition between kingdoms: say between plants and animals, or plants and insects, or insects and bacteria. Two British evolutionists, Michael Hochberg and John Lawton, have made a calculation that they find “both instructive and astonishing.” Suppose there are three hundred thousand species of higher plants. Higher plants include ferns, conifers, flowering plants—essentially anything green that grows more than a few inches off the ground—and three hundred thousand species is a conservative estimate. If every one of those species is fed upon by just ten species of insects (also a conservative estimate), and if each of those insects is the host for one infectious disease and five species of parasitoids (also conservative), then there is room for about fifteen million different competitive interactions at this single busy intersection in the kingdoms of life.

  Almost no one has looked for divergence in action on this front, but Dolph Schluter has. He suspects that distant branches in the trees of life may be competing right now in the Galápagos.

  Some of Darwin’s finches drink flower nectar, which finches do not do on continents. They can get away with it in the islands because they have little competition for it: not many pollinating insects have crossed the Pacific and colonized the archipelago. Only one species of bee has made the journey so far, a large carpenter bee, named for the naturalist who first collected it: Xylocopa darwini. Darwin’s bees have not made their way to the northernmost islands, but they are found in most of the southern islands. There they compete for flowers with the finches.

  The finches on the northern islands get most of their nectar from the little yellow flowers of Waltheria ovata. In the dry season, when food is hard to find, small beaks and some sharp beaks will drink nectar from Waltheria. The sharp beak inserts its whole long beak to probe the flower, while the small beak sticks in only its lower mandible. Both of these finches have gotten very good at the work: they can probe as many as forty flowers a minute.

  On northern islands like Genovesa, where there are no bees, nectar makes up about 20 percent of these finches’ diet; it is an important dietary supplement to the small seeds they spend their lives scrabbling for. But on southern islands, where there are bees, nectar makes up less than 5 percent of the birds’ diet.

  Dolph has noticed that the nectar-drinking finches tend to be smaller than finches of the same species on islands where there are bees. For instance, the average wingspan of small beaks is almost 5 millimeters shorter on the islands of Pinta and Marchena (no bees) than on the islands of Fernandina, Santa Cruz, Santiago, Española, and Isabela (bees).

  Dolph once spent two weeks observing a spot on Marchena that was rich with yellow Waltheria flowers. He saw finches fighting to defend clumps of these flowers. Small beaks that owned a few clumps would defend them against other small beaks. Dolph netted and measured as many of these finches as he could. The average nectar drinker was significantly smaller, and weighed about a gram less than the average teetotaler. That is, even on Marchena, only the finches that were cut out for flower feeding by their small size were drinking nectar.

  Apparently, then, on islands where there are no bees, selection pressure has reduced the size of two species of Darwin’s finches, and these birds have squeezed into the bees’ niche. On islands where bees have arrived, the birds have been forced back out of the niche.

  If so, this is a case of character displacement not just between sibling species but between a vertebrate and an invertebrate, which goes far beyond what Darwin imagined. We never see them fight over a flower, but there is no peace between the birds and the bees.

  IS CHARACTER DIVERGENCE UNIVERSAL and powerful, as Darwin thought, or is it rare and weak, as some evolutionists argue today? “My own guess is, character divergence is likely to be quite common and important,” says Peter Grant, “but I think of rather small magnitude, in terms of the quantity of the evolutionary shift.”

  “You mean …?” asks Rosemary.

  “Take two species that come together. Say they differ by 10 percent. They would need to be about 15 percent different to coexist without serious competition. There ar
e two possible outcomes. One of the two may go extinct, or the two species may diverge until they are 15 percent different. That shift is not very great: only another 5 percent.”

  “I see. Okay,” says Rosemary.

  “Now if you agree with that, how easy would it be to see the evolution of that 5 percent shift? The answer is, not at all easy. You’d need a lot of detail. Character divergence could be very common but simply not be known.”

  In Peter’s view, most of the divergence among Darwin’s finches takes place when they are isolated, living on separate islands. Then, when they come together and share an island, competition drives their lines farther apart. This divergence is a simple consequence of the struggle for existence, just as Darwin imagined in his carriage. It pushes the finches one or two steps closer to the origin of species.

  Chapter 11

  Invisible Coasts

  Cleopatra’s nose, had it been shorter, the whole face of the world would have been changed.

  —BLAISE PASCAL,

  Pensées

  When Darwin was hobnobbing with pigeon-men it was the power of selection he wanted to hear about, not the power of crossing. “I sat one evening in a gin palace in the Borough amongst a set of pigeon fanciers,” he writes gleefully to a friend, “when it was hinted that Mr. Bult had crossed his Pouters with [larger] Runts to gain size; and if you had seen the solemn, the mysterious, and awful shakes of the head which all the fanciers gave at this scandalous proceeding, you would have recognized how little crossing has had to do with improving breeds.”

  “It has often been loosely said that all our races of dogs have been produced by the crossing of a few aboriginal species,” he writes in the Origin; “but by crossing we can only get forms in some degree intermediate between their parents. Did the Italian greyhound, bloodhound, bulldog, &c [exist] in the wild state?” No, of course not. Ergo, he writes, “the possibility of making distinct races by crossing has been greatly exaggerated.”

  Darwin was so eager to build a case for natural selection that he may have missed something here. Hybrids may have been more important to most breeders than he thought.

  At the turn of the century, the American geneticist Raymond Pearl, adopting a so-show-me attitude, cultivated and quizzed hundreds of breeders, just as Darwin had done. Pearl asked bantam breeders, for instance, if they could document a case in which a new bantam line was “created solely by selection of small-sized individuals of a large race, variety or breed of fowls, without any crossing in of bantam blood?”

  Pearl got his answer from the worlds leading authority on bantams, one J. F. Entwisle. “If such can be done,” Entwisle wrote, “then our thirty-odd years’ experience of bantam ‘manufacturing’ counts for very little. We have lived to see the manufacture of some forty varieties, and none without crossing, so far.”

  “Darwinian selection plays an extremely minor and unimportant part in the process as it is actually performed,” Pearl concluded, swinging the other way from Darwin.

  Today, unlike Pearl, the Grants know that Darwinian selection does work. They have seen it work. Now, as they contemplate the rise of the hybrid finches, they are beginning to suspect that selection and crossing work together as part of the same creative process.

  THERE ARE TWO KINDS of shorelines in the islands, the visible and the invisible.

  The visible shores are the black broken rocks and white broken waves where volcanoes rise out of the Pacific. They are the borders of the air, the sea, and the lava, wave-gnawed rings around the summits that have given Darwin’s finches their homes in the middle of nowhere. These shores are defined simply by the level of the sea.

  The Kicker Rock. From Charles Darwin, Geological Observations.

  The Smithsonian Institution

  The invisible shores are the borders between the birds themselves. These shores are more intricate. They are defined by the secret codes and unwritten rules that wrap each of the thirteen Galápagos finches in a kind of self-invented isolation. These boundaries hold each species apart from the rest (with the interesting exception of the hybrids), so that even though seven or eight of them may share the same volcanic summit, feed together in mixed flocks, scrape the same cinders for the same seeds, they still breed as much apart as if they were themselves an archipelago of enchanted islands.

  Darwin helped to map the visible coasts of rock and coral sand—the Beagle was a surveying ship. But his thoughts about the invisible borders between species were inconsistent and confused. In his first secret notebooks he described species as isolated by their sexual instincts and equipment. Later he became so absorbed in his principle of divergence that he neglected this aspect of his subject. What holds the new species apart? What are the barriers, and what makes the barriers harder or easier to cross? These are coasts that Darwin left more or less unsurveyed and unexplored.

  Evolutionists understand now that the isolation of species is not merely a matter of populations locked apart by mountains, canyons, or seas, as Darwin’s finches are held loosely apart by the visible shores of the Galápagos. The isolation of species consists chiefly in the invisible barriers that can carve off a corner of a population into a new island or carve a single large population into a set of scattered, more or less lonely gene pools.

  It is not just intersterility that produces these invisible shorelines, as between horses and donkeys. Nor is it physical or physiological incompatibility, as between elephants and their fleas. Darwin’s finches can interbreed and produce fertile young, but something keeps most of them from doing it.

  The barriers around the birds are invisible because they are created by the creatures’ own behavior. It is not anatomy but instinct that holds them apart. To inquire into the origin of species is to inquire into the formation of these instinctive, invisible barriers. Though Darwin himself had trouble with the subject, the secret of the origin of species lies somewhere along these viewless coasts, these shifting shorelines among living things.

  Clearly, once varieties begin to split off from one another, they need some way to keep themselves apart. Whether they diverge in isolation, or partway in isolation and partway in each other’s company, at some point they do have to learn to breed with their own kind—their own new kind. Otherwise a new line would blend with the lines around it and disappear. Darwin was impressed, for instance, by the work it took to maintain fancy lines of pigeons. If breeders crossed their birds carelessly their creations would soon revert to their starting point, the common rock pigeon. All the pigeon fanciers’ whimsical creations, all those pouters, fantails, laughers, and scandaroons, would be lost.

  What holds these breeds apart and prevents their collapse to the origin is the pigeon fanciers’ continual vigilance in arranging crosses. The best of the breeders in Darwin’s day, like the famous pigeon-man Sir John Sebright, could tell by eye a difference in beak of one sixteenth of an inch. Darwin writes (pointedly) that he had to work hard to persuade these exquisitely sensitive pigeon fanciers to make the kinds of crosses that would undo their lines. Such experiments went against the grain, he says—even in the name of science, even for a fee. Darwin had to cajole and wheedle, reminding the breeders that after all, even ugly birds are “still good for the pot.”

  It is Darwin’s argument that natural selection can arrange matches too, and better than the best human breeders. The selection process will give living things this gift, this talent, simply by choosing the fittest variants in the nest, the litter, and the seedbed. It will improve their ability to tell one another apart as naturally and inevitably as it improves their ability to eat or to fly. Selection will act this way because those individuals that make bad choices in their mates will suffer a disadvantage: their offspring will be less likely to prosper in the struggle for existence. Those individuals that make good choices will enjoy a corresponding advantage: their offspring will prosper.

  That is why hybrid finches should be rare on Daphne Major. In a population that is diversifying, evolving distinctive adap
tations, selection will tend to preserve a new and valuable invention because individuals in a gradually diverging line will have an adaptive advantage if they choose, as mates, others in the new line. If they choose a mate within the line, they stand to perpetuate the new adaptation. If they do not, they stand to lose the adaptation: they lose the family jewels. So there is selection pressure on creatures like Darwin’s finches to learn to discriminate. Those birds that evolve a sexual taste for the new line will do better than those that retain a taste for the old line.

  In this way, natural selection will ensure that the birds’ own powers of discrimination are finer than the pigeon fanciers’. It will make birds in the wild, as Darwin put it, “their own Sir John Sebright.” The power of natural selection will make birds of different lines sexually repugnant at mating time.

  Darwin saw some evidence of this kind of mutual repugnance even in the pigeon coops. In Natural Selection he reports that “the Dovecot pigeon, the ancestor of all the breeds, seems to have an actual aversion to the several fancy breeds.”

  But when Darwin describes the finches of the Galápagos (“probably the earliest colonists, having undergone far more change than the other species”), it is the visible, physical isolation of their islands that he stresses, not the finches’ invisible, instinctive isolation—which Darwin knew nothing about.

  AMONG DARWIN’S FINCHES, a mating season begins with all the black cocks on the island singing in the rain. Each male broadcasts from his singing post, and while he sings he scans his territory. If a female alights near one of the display nests that he has built, he darts from his singing post and flies to her. If she is one of his own kind, he sings and shakes his wings at her, makes them quiver tremulously. Then he flies to the nearest nest. (Any nest will do, even the nest of a rival if the rival is out.) He goes in and out, in and out of the nest, looking back at the female over his shoulder. Sometimes he picks up a bit of grass in his beak and puts it down again, rapidly, over and over, as if he were trying to catch her eye.