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

by Jonathan Weiner

  This entrance to C-Level is constricted by refrigerators marked CAUTION—RADIOACTIVE and by an oak natural-history cabinet of the kind that was popular in Darwin’s century, full of dissections made by long-dead Princeton biology students: Head of a Rattlesnake, Lungs of a Hawk, Stomach of a Screech Owl, Skeleton of a Bat. The cabinet is topped with baseball trophies of young men waiting for the pitch.

  Down the corridor, just past an emergency eye-wash station, and an emergency shower, the way is half-blocked by a freezer chest. Open its lid, and clouds of vapor spill out onto the linoleum tiles of the sub-subbasement.

  At the bottom of the chest, stacked in hundreds of plastic vials, are the samples that Rosemary and Peter are collecting drop by drop in the Galápagos: the blood of Darwin’s finches.

  “You have most cleverly hit on one point, which has greatly troubled me,” Darwin wrote Huxley, after Huxley had read the Origin; “… what the devil determines each particular variation? What makes a tuft of feathers come on a cock’s head, or moss on a moss-rose?” Darwin never did find the answer. He was convinced that variations are the origin of species, but he did not know the origin of variations. “Our ignorance of the laws of variation is profound,” he confesses in the Origin. “Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied.”

  Darwin did predict that the reason would be discovered. Some kind of secret writing would one day be detected and deciphered in the bodies of living things. He pictured this code as a swarm of letters streaming through the blood, invisible characters that meet and unite in each fertilized egg: “and these characters, like those written on paper with invisible ink,” he wrote, “lie ready to be evolved whenever the organisation is disturbed by certain known or unknown conditions.”

  Like Belshazzar, king of Babylon, who saw the writing on the wall, Darwin knew the characters in the blood would turn out to be of the utmost significance. But Darwin could not see the writing himself, and he had no Daniel to read it for him.

  Today, biologists call Darwin’s invisible characters “genes,” from the Greek verb meaning “give birth to,” the same root as genius and generation; and they can read the code in the blood.

  “FIDDLY WORK TO DO in this kind of weather,” says Peter Boag. “Not very much fun. It’s a very finicky technique.”

  He stands at his laboratory bench on a muggy afternoon in August, fussing with a small plastic bag. He has already extracted molecules of deoxyribonucleic acid, DNA, from a drop of the frozen finch blood. He has bathed the DNA in a solution of enzymes, which chopped the DNA into millions of fragments, snipping only at selected points, like manic but intelligent scissors. Boag has sorted these snippets in a sieve made of electrically charged gelatin, and transferred them all onto a swatch of nylon. Now he has the nylon floating in a Seal-A-Meal bag. He is trying to get the bubbles out of the bag so that he can seal it up. Tiny bubbles keep clinging to the nylon.

  The liquid in the bag looks like plain water, but it is loaded with radioactive phosphorus, P32. A transparent screen like a TelePrompTer stands between Boag and the lab bench, protecting his chest from the radiation. “When in water the nylons reasonably well shielded,” he says. “But we do the fiddly parts behind the screen.” He wears the same gloomy look with which he once knelt in the hot dust and blazing sun of Daphne Major, sorting seeds of Portulaca, Cacabus, and Heliotropium.

  For Boag the drought on Daphne Major took place in another lifetime. He and Laurene Ratcliffe are both professors now at Queens University in Kingston, Ontario. They have a three-bedroom suburban house near the university, one slightly cranky car, three small children, a black Labrador retriever, and a Siamese cat. Boag watches Galápagos evolution now from this laboratory bench.

  He and the other alumni of the Finch Unit are among the first cohorts of evolutionists to come of age after Watson, Crick, and the revolution in molecular biology. Every year, thanks to this revolution, more and more startling manipulations of DNA become possible—not only possible but routine. Boag went molecular seven years ago, and so have many other former watchers in the Finch Unit, although Laurene has so far cheerfully abstained, and so have the Grants.

  (“It really is a foreign language,” Peter Grant says. “I suppose ours is too, but I do have the impression that theirs is harder”)

  Boag swishes the nylon around in the bag, seals it up, and sets it aside to sit overnight in its radioactive bath. The DNA fragments on the nylon are still invisible. Tonight a chosen few among them will turn hot. In the jargon, these fragments will have been tagged by the Ρ32 probe. Tomorrow, he will press the nylon against a sheet of X-ray film, and keep it there long enough for the hot spots of DNA to expose the film and make a picture.

  Using that X-ray as a guide, Boag can select a single fragment of special interest, multiply it a millionfold, and make another, much more detailed X-ray.

  “When that picture comes out of the film developer,” he says, “you’re the first one who’s ever looked at the DNA sequence of one of Darwin’s finches.”

  He holds up a sheet of X-ray film, one of dozens he and his students have already made. The film is covered with rows upon rows of little ghost-gray blobs.

  “This is a cactus finch, scandens,” he says.

  BOAG GETS SOME OF HIS BLOOD SAMPLES from the Grants, and some he collected himself on a brief trip to the Galápagos in 1988. The X-ray he is holding up is a portrait of a single gene from a cactus finch that he caught, bled, and freed again on the island of Santa Cruz: “Just a random individual from Academy Bay.”

  The gray smudges in the X-ray are aligned in four columns labeled G, A, T, and C. Boag scans these columns familiarly, reading from the bottom to the top. A Galápagos finch has about one hundred thousand genes, roughly the same number as a human being. The genes are spelled out in a total of about one billion letters, an average of ten thousand letters to a gene. The story is big but the alphabet is small: there are only four letters, named for the four chemical compounds that, as Watson and Crick discovered, make the treads in every spiral staircase of DNA, rather like the leaden letters in a printing press. Their chemical names are guanine, adenine, thymine, and cytosine: G, A, T, and C.

  The gene in this X-ray comes from the mitochondria of the cactus finch. Mitochondria are the dark, peppery granules in which each cell converts oxygen into energy. This particular gene codes for the enzyme cytochrome b, which plays a role in the oxygen-conversion process. For the past few years Boag has been isolating this same piece of the cytochrome b gene in species after species of Darwin’s finches, working with a Swedish postdoctoral student, Hans Gelter, of the University of Uppsala.

  If species were created once and for all, if they all came tumbling to life finished and polished, as Milton paints them in Paradise Lost, then each species of Darwin’s finches would possess a fixed, permanent, never-changing set of genes. But genes are not fixed. The one hundred thousand genes of Darwin’s finches are shuffled and cut in every generation, like a mammoth deck of cards. Each finch egg in each cactus tree receives a unique, absolutely unprecedented combination of genes. That is why every finch in the Galápagos is endowed with its own set of measurements of beak, wing, tarsus, and hallux, to say nothing of thousands of variant forms of submicroscopic enzymes like cytochrome b.

  A finch as it flips pebbles looking for Tribulus seeds is bombarded by cosmic rays from outer space, by ultraviolet radiation from the sun, by miscellaneous lost wandering molecules that thonk into its DNA strands like loose cannons, even by the thermal motion of its own atoms and molecules: the thousand natural shocks that flesh is heir to. These aerial and internal bombardments agitate the billion letters of its DNA. Every twenty-four hours, inside every living cell, about one hundred copies of the letter C are half-sprung from their spiral railings. Squadrons of enzymes travel day and night along the strands of DNA and mend the broken Cs. In effect these enzymes are proofreaders. Sometimes they put a C back wrong, and forever
after that particular C reads G.

  Proofreading mistakes introduce haphazard new variants into the finches’ DNA, adding, subtracting, and switching letters around. If a mutation occurs in the DNA of an ordinary cell, it can cause cancer. If it occurs in a sperm or an egg cell, it can be passed on to the next generation. If it is more or less neutral, or if it happens to confer a slight benefit, its bearers may survive and pass it on again and again.

  In Boag’s X-rays he finds slight differences in the sequence of letters between one finch and another. The X-ray he is holding in his hand, for instance, shows exactly three hundred letters in the cytochrome b gene from his cactus finch. This sequence does not quite match the corresponding three hundred letters in a tree finch. Three out of the three hundred letters are different.

  Ground finches and tree finches are closely related, since they arose on Darwin’s islands quite recently in evolutionary time. All of the Galápagos finches are close-set twigs on the same small branch of the tree. It makes sense that about 99 percent of their invisible characters should be exactly the same. None of these lines have had the time to accumulate many new mutations.

  Birds on other, more distant branches of the tree of life carry many more differences in their DNA: a few percent. The farther apart two species sit in the tree, the more differences there are in their DNA. But every living thing carries its code in the same invisible characters, always the same four letters, because ultimately every living thing on earth shares the same ancestor, about four billion years back, near the very birth of the planet.

  Small tree finches. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  By looking at DNA sequences, evolutionists are now gleaning more and more of the lost history of life. They are filling in the family tree from the youngest twigs at the top to the oldest forks at the bottom. Investigators looking at DNA have discovered, for instance, that crows evolved in Australia, that storks are close kindred to vultures, and that mushrooms and toadstools are more closely related to animals than to plants. Evolutionists can learn these sorts of genealogical secrets from mutations in DNA precisely the same way historians learn from spelling errors in old manuscripts.

  Historians are aware, for example, that Darwin’s personal spelling habits evolved during the course of the voyage of the Beagle. He was sending his journal home piecemeal and getting letters back from his family. On his twenty-fifth birthday, February 12, 1834, his sister Susan, whom he called “Granny,” wrote to him praising his journal: “and what a nice amusing book of travels it wd. make if printed,” she wrote; “but there is one part of your Journal as your Granny I shall take in hand namely several little errors in orthography of which I shall send you a list that you may profit by my lectures tho’ the world is between us. —so here goes.—

  WRONG

  RIGHT ACCORDING TO SENSE.

  loose. lanscape. higest

  lose. landscape. highest.

  profil. cannabal

  profile. cannibal.

  peacible. quarrell

  peaceable. quarrel.

  —I daresay these errors are the effect of haste, but as your Granny it is my duty to point them out.”

  A year and a half later, after his stop in the Galápagos, Darwin’s sister sent him another spelling lesson: “I cannot think how you cd. write such a collected account of your travels when you were Galloping so many miles every day.—When I have corrected the spelling it will be perfect, for instance Ton not Tun, lose instead of loose.—You see I am still your Granny.”

  The historian of science Frank J. Sulloway has now compiled a table of all of Darwin’s spelling habits during the voyage. (Granny’s nightmare!) Sulloway collated every variant spelling in the more than three thousand manuscript pages that Darwin had scribbled aboard ship. By noting the dates at which each of these errors drifted in and out of Darwin’s letters, journals, and field notes, the historian was able to zero in on the date when Darwin, sitting in his cabin, inspected the Galápagos mockingbirds and realized that they might “undermine the stability of Species.”

  Five words helped Sulloway pin down that fateful moment. There was a stretch toward the end of the voyage when Darwin often spelled occasion, occasional, and occasionally with a double s, and coral with a double l. He also sometimes misspelled the ocean he was floating on—the Pacifick. Because Darwin’s ornithological notes contain occassion, corall, and Pacifick, they had to have been written after November 1835 and before mid-September 1836. (From watermarks and other evidence, Sulloway narrows the date to June or July 1836.)

  In much the same way, Boag and his postdoc Hans Gelter are trying to sort out how Darwin’s finches have branched since the birds first arrived in the islands. Lack drew his chart of their family tree by comparing and contrasting the finches’ sizes, shapes, feathers, and beaks—especially their beaks. Lack assumed that the most distinctive forms among the birds had been diverging for the longest time. By this reasoning the warbler finch (which looks so little like a finch that Darwin mistook it for a true warbler) was the first species to split from the ancestral stock. Then the tree finches split from the ground finches. Then the tree finch line and the ground finch line each put out branches and twigs of their own.

  Boag and Gelter are preparing the equivalent of Sulloway’s table of spelling errors. A mutation in cytochrome b that appears in tree finches, but not in any of the six species of ground finches, is likely to have arisen after the tree finches and ground finches diverged and went their separate ways. A mutation that appears in the small, medium, and large ground finches, but in none of the other species of Galápagos finches, is likely to have arisen after those three diverged from the rest. Given a large enough table of data, a computer program can crunch the numbers, weigh the probabilities, and draw a most-likely phylogenetic tree of Darwin’s finches.

  Warbler finches. From Charles Darwin, The Zoology of the Voyage of H.M.S. Beagle.

  The Smithsonian Institution

  No one knows which species was the first to reach the islands—the founder of all these lines, the bird to put at the trunk of the tree. Over the years evolutionists have proposed many candidates for the honor. At the moment several species of continental birds are in the running, including Melanospiza richardsonii, which now lives only on a single island in the West Indies, but may once have had a much wider range; and the blue-black grassquit, Volatinia jacarina, a finch that is common throughout Central and South America, including the whole length of the Pacific coast. Boag hopes that by looking at the DNA of all these birds he can help sort out which is most closely related to Darwin’s finches. Which one, for instance, carries a gene for cytochrome b that most closely resembles the finches in the Galápagos?

  Boag has a reputation in the Finch Unit as the most careful, cautious, detail-minded watcher in the history of Daphne Major. He and Gelter have been working at their project for several years and many rolls of Seal-A-Meal bags.

  But now there is an untidy complication, because genes are not fixed and permanent. They are manuscripts that are still being written, and among Darwin’s finches the rate of writing and revision is accelerating.

  “WE HAVE GOOD REASON to believe,” Darwin wrote, when the secret of variation was still a tightly lidded black box, “… that changes in the conditions of life give a tendency to increased variability.”

  Studies of DNA in laboratory cultures of bacteria have borne out this point and thrown light on its causes. In times of stress, when the temperature shoots up or down, for instance, or the environment goes suddenly more wet or dry, colonies of bacterial cells in a Petri dish will begin to mutate wildly. This is known as the SOS response, for the international distress signal Save Our Souls, Save Our Ship. It increases the chance that at least a few of the cells in the Petri dish will survive the disaster of the new conditions.

  The SOS response has been observed in the DNA of maize when it is shocked by hot or cold tempe
ratures. Recently it has been discovered in yeast. Apparently many different kinds of living cells can switch up their mutation rate under stress and relax it again when the stress dies down. They can also make select portions of their DNA unstable in the extreme. It is almost as if cells keep a lid on their own evolution—and stress blows the lid.

  When stressed in a Petri dish, many cells of E. coli, whose normal habitat is the human gut, will even open pores in their membranes and take in DNA from outside their cell walls. Strands of naked DNA are always floating around these bacterial colonies like scraps of old newspaper. The living cells open up, take in the naked DNA, and patch some of it into their own genes. The process is known as transformation, and it can be stimulated by the stress of unfriendly chemicals or ultraviolet radiation.

  “The implications … are significant,” writes one molecular evolutionist, John F. McDonald; “at precisely those challenging moments in evolutionary history when major adaptive shifts are required, genetic mechanisms exist that increase the probability that the appropriate variants will be provided.”

  What the Grants are seeing now on Daphne implies that something like this is going on at this moment among Darwin’s finches. If the finches always bred like with like, they would keep their gene pools separate. Their stores of variation, their unique spellings of thousands upon thousands of genes, would be kept distinct. Each species would have its own set.

  But these gene pools are far from separate. Genes are continually flowing back and forth between them, by the crossing of species and the backcrossing of hybrids. The islands are a melting pot.

  If they never intercrossed, the birds would be much more stable and uniform in their DNA and in their beaks and bodies. Intercrossing raises their variability. In fact, this appears to be the secret of the remarkable variability of Darwin’s finches. The Grants have calculated that if only a single immigrant on Daphne were to inject its genes into each species of finch in each generation, that infusion of fresh genes would suffice to keep all of the finches’ gene pools from running low. One injection per species per generation would be enough to keep the birds as variable as they were when the Grants began watching Daphne in the 1970s.