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

by Jonathan Weiner

  “Maybe we should all be grateful that Mother Nature is a bit slovenly when it comes to reproduction, for this may ultimately permit the unfolding of the bountiful diversity of life on Earth.”

  The old vision of the tree of life was plain, neat, stark; this view is softer, messier, more tangled, and more alive. In a way it is also more sympathetic. Clearly the lineages of Darwin’s finches do compete: they struggle and push one another apart according to Darwin’s principle of divergence; they play endless games of King of the Mountain. But at the same time the birds on their separate islands and lonely peaks are not as solitary as they had seemed. They are full of fissions and fusions, competition and cooperation, like brothers and sisters in a nuclear family, bound by a thousand nuclear ties and tensions; or like the old royal families of Europe, exchanging princes and princesses to link their lines. The birds pass invisible messages back and forth, swapping genes as casually as good neighbors exchange recipes, tools, or limericks. They are secret sharers, communing on their long voyage, open to suggestions. Their lines come together and come apart, and in this way the birds are created and re-created, again and again.

  The apparent fixity of species once seemed the greatest argument against evolution, just as the apparent fixity of the earth once seemed a commonsense argument against Copernicanism. Now the satisfying and reassuring sameness that once encouraged Aesop and other fable spinners to speak of The Fox, The Owl, The Wolf, The Whale, and The Crow seems more illusory than ever before. “All is flux,” said the Greek philosopher Heraclitus; “everything flows.” The forms and instincts of living things, the invisible borders among them, and the very coasts and landscapes they inhabit are all more fluid and in more flux than even Heraclitus could have imagined.

  Chapter 14

  New Beings

  Hence, both in space and time, we seem to be brought somewhat near to that great fact—that mystery of mysteries—the first appearance of new beings on this earth.

  —CHARLES DARWIN,

  Journal of Researches

  A Victorian gentleman contemplates the skeleton of a stork. He has a yardstick, a divider, a set of calipers, a stack of notes. With a cloth ruler stretched between his hands and a pencil clamped in his mouth, like a tailor, he stares up in the general direction of the bird’s beak as if to say, “What do I do now?”

  This portrait of the naturalist at work was painted in 1879 by Henry Stacy Marks, a member of the Royal Academy of Arts. Marks called it Science Is Measurement.

  “When you can measure what you are speaking about and express it in numbers you know something about it,” declared the Victorian physicist Lord Kelvin, in 1883, “but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be.”

  The painting and the Kelvin quotation are reproduced together on the frontispiece of Album of Science: The Nineteenth Century, a book of pictures edited by the historian L. Pearce Williams. This sabbatical, one of the Grants’ many friends at Princeton brought them a copy of the book; opening its covers to the frontispiece gave Peter a moment of high glee. He read the Kelvin quotation aloud to Rosemary.

  “Your knowledge is of a meagre and unsatisfactory kind!” Rosemary repeated, in the voice of a Scottish hanging judge.

  Science Is Measurement, by Henry Stacy Marks.

  The London Graphic, 1879

  “That makes one look pretty small, doesn’t it!” cried Peter.

  “Oh, that’s lovely!”

  “This is interesting,” Peter said, and read the historian’s commentary:

  The painting illustrates the puzzlement of the natural scientist, whose subject offers little opportunity for significant measurement. He was faced with the question of whether all of natural history was to be excluded from “science”—or whether the close observer of nature was as near to scientific “truth” as the mathematical analyst.

  “Yes,” Peter said. “I think for much of this century too there was a prejudice against what we now call ecology, on the grounds that you can’t measure anything with precision—and if you can it’s probably not very interesting.

  “That was a prejudice among people who could do very precise measurements in laboratories—physicists and physiologists and so on.”

  The Grants’ measurements make a wonderful reply to Marks’s painting, and to the doubts of Darwin’s century. They are as quantitative and rigorous as even Lord Kelvin would have wished. While working on their hybrids this sabbatical, for instance, the Grants extracted a small sample from their shelves of data and carried out a test of the predictive power of Darwinian theory. They took the mean length, width, and depth of scandens beaks and fortis beaks on Daphne in the year 1984. They inserted each of these numbers into a succinct mathematical formula, together with an assortment of key variables (including the separate heritabilities of beak depth, width, and length; the ways each of these characters affects the others; the ways seed abundances on the island went up or down between 1984 and 1987; and the separate ways that seed abundances affect beak depth, width, and length). Then they let the formula predict the mean length, width, and depth of scandens beaks and fortis beaks on Daphne in 1987. Finally they compared the results their equations predicted with the actual results of Darwinian evolution on Daphne between 1984 and 1987.

  Mean fortis beak width, for instance, was 8.86 millimeters in 1984. The formula predicted that by 1987, give or take a minute fraction of a millimeter, the mean width should have dropped to 8.74 millimeters. The actual width of fortis beaks on Daphne in 1987 was 8.74 millimeters.

  They were “spot-on,” as Peter says. Every single one of the numbers was right.

  “Those who criticize the theory of evolution by natural selection often do so on the grounds that it is impossible to test it by making quantitative predictions,” the evolutionary biologist Jeremy J. D. Greenwood notes in a recent commentary in Nature. “Rosemary and Peter Grant now show this view to be wrong.… Their predictions have been precisely correct.”

  If the Grants’ measurements were not as precise as this, they would have missed virtually all of the action they have seen in the Galápagos in the last twenty years. Certainly they never would have caught a glimpse of the invisible pendulum that they now see swinging across the decades in the Galápagos. “We only discovered that by doing an analysis of the numbers,” Peter says. “It doesn’t shriek out from the data. It’s a subtle matter of probabilities shifting by a few percent.”

  Their discovery of the rise and fall of the mixed breeds takes nothing away from the power of natural selection. On the contrary. It shows more vividly than ever before that Darwin’s finches are new beings on this earth. The intense selection pressures that shape and reshape the beaks of the finches also keep all these beaks from disappearing. Darwin’s process created the many out of one, and Darwin’s process is in the act of their creation even now. If natural selection did not go on working hard on each island, in each generation, the many would very soon vanish into one again.

  Darwin’s finches are not like Michelangelo’s Adam, who raises his finger languidly to meet the down-stretched finger of God: the first man, molded of clay, half-raised from earth, created in an instant. These birds are more like Michelangelo’s Prisoners, the famous statues he left half in and half out of the marble, so that looking at them today we can almost see and hear the sculptor’s chisel at work. The birds are alive and breathing, but they are unfinished; in the Galápagos the sculptor is still at work, measurably and demonstrably. The Grants’ discovery this sabbatical makes the action of the chisel only more dramatic to contemplate. The more the birds are able to fuse, the more impressive the work of the sculptor: the faster the chisel must be flying, to keep them all apart, as if it were not sculpting in stone at all, but writing in water. Roughly one out of ten of the finches born on the desert
islet of Daphne Major now are hybrids, and the hybrids are doing better than any of the others on the island. In a blink of evolutionary time, all of Darwin’s finches could run together and congeal, and the sculptor’s art would be lost. As the evolutionist Ernst Mayr once pointed out, the tendency toward fusion, the “successful leakage of genes from one species into another,” is “a self-accelerating process.” Each case of introgression weakens the invisible barriers between two species and leads to increased frequency of hybridization, a process that if unchecked will spill downhill faster and faster, “until ultimately the two species are connected by a continuous hybrid swarm.”

  The finches are not yet carved completely apart from one another or from the ancestral stock, from the line of birds that first colonized the islands millions of years ago. If the chisel were not flying fast, the work of the carver would soon disappear without a trace, fusing back into the block of the living stone.

  This same tension between fission and fusion runs through all the kingdoms of animals and plants. Everywhere hybrid swarms are rare; good, solid, more-or-less-separate species are common. Yet in many of the birds flying overhead in many of the fish in the sea, and in almost all of the green things growing around us the genes are intermingling. The chisel is hard at work daily and hourly in every landscape on the planet.

  Measuring the length of a beak.

  Drawing by Thalia Grant

  WHY ARE THERE SO MANY KINDS of animals? Adaptive radiations like Darwin’s finches are the essence of the answer. They are in progress all over the Galápagos (the mockingbirds, the cacti, the sharks, the tortoises, the torchwood) and all over the planet. In the Hawaiian Islands, the lineage of a single finch has radiated into more than forty species, with forty beaks, including seed crushers, bug catchers, nectar sippers. Their beaks are even more divergent than the beaks of Darwin’s finches. The akiapolaau has one of the strangest. It strips bark with its lower mandible, like a knife; then it pries out bugs by poking the wood with its very long, thin, overgrown upper mandible, like a needle. Two tools in one, a double-bladed pocketknife.

  Also in Hawaii, a few lost fruit flies that blew in over the Pacific millions of years ago have now radiated into somewhere between five hundred and one thousand species. More than a third of all the species of fruit flies on earth belong to this single adaptive radiation. There are predatory flies and parasitic flies; nectivorous, detritivorous, and herbivorous flies. Some of them are the size of pinheads, and some are as long as a child’s thumb; a few, according to an evolutionist who has stared at them face to face, “have strange broad heads with eyes positioned far apart like those of hammerhead sharks.”

  In the Great Lakes of East Africa a group of cichlid fish has achieved a whole series of these starbursts. In Lake Victoria alone about two hundred species of cichlids have evolved from a single ancestral stock within the last 750,000 years. Some feed in the water column, some at the bottom; some eat snails, others eat fish, still others eat fish scales. One species, more gruesome than the vampire finch of the Galápagos, plucks out and eats fish eyes.

  All of these radiations are displays of evolution in action, success stories in the contemporary history of life. Any evolutionist can rattle off half a dozen more of them: Hawaiian silverswords, leiognathid fish, heliconid butterflies, and so on, and so on.

  At every moment in the history of life, including our own moment, adaptive radiations like these are in progress all over the earth. They decorate the map of every part of the globe in every era like the compass roses of the old cartographers, or cross sections through the branches of a growing tree.

  The history of these adaptive radiations is the history of life, from the explosive radiations of the bizarre fauna of the Cambrian, 540 million years ago, to the radiation of the first jawless vertebrates, the Agnatha, in the Ordovician, 500 million years ago; the radiation of fish in the Devonian; amphibians and insects in the Carboniferous; dinosaurs and mammals, beginning in the Triassic; angiosperms and yet more insects in the Cretaceous; and in the Pleistocene, a few million years ago, radiations of herbs and human beings.

  “You had better stay with Darwin’s finches or else there is no end to your labors,” Peter says. “Stay with Darwin’s finches or the finches will seem at last like a little momentary excitement in your youth.”

  THE GRANTS’ SABBATICAL YEAR is already over; sabbaticals never last as long as one would like. But Rosemary and Peter are trying to pretend that this one is still going on. They are sitting together over a brown-bag lunch in a corner of Peter’s office, wedged between classes and seminars, planning their next trip to the islands and puzzling together about the origin of species.

  With nearly 99 percent of their finches, the As still mate As, and the Bs mate Bs. “They are definitely species,” he says.

  “They’re undoubtedly species,” says Rosemary. “They differ in song, size, and shape. It’s easy for us to tell them apart, and they tell each other apart.”

  And yet, when Peter and Rosemary consider the rise of the misfits, the 1 or 2 percent of As that choose Bs and Bs that choose As, the finches do not look the same as they did one year ago.

  “The most intriguing feature,” says Peter, “is the possibility of speciation. By putting genes together like this, you make combinations that could take off, that could be the starting point of a new evolutionary direction not easily within reach of either species.”

  Evolutionists have talked about this notion for decades, but it never seemed as compelling to Peter and Rosemary as it does after this sabbatical. They see now how much mixing is going on all the time on Darwin’s islands, and how often fate seems to smile on the oddballs, which to their experienced eyes are very strange birds and queer ducks indeed. The Grants are wondering whether some of these mixed-bloods on Daphne Major could be “potential escapees,” in Peter’s phrase, from the constraints that bind the genes of their kind; whether birds like these could be the beginning of a really new departure, “the starting point of a new evolutionary lineage.”

  “An evolutionary response could follow,” says Rosemary. A new variant among the broods of novelties could go off along a new road. But the window of opportunity might be very small, she thinks. “There’d have to be very strong selection.” Without a selection pressure that favored a novelty-shop beak, promiscuous interbreeding would soon erase it, she says, “because you’d constantly get backcrossing.”

  “Undoubtedly it’s a complicated matter,” says Peter. Not for the first time, he begins sketching the future of a lucky hybrid line on his paper napkin. Rosemary jumps up and heads for the blackboard, crying, “Let’s see if …”—if their visions match.

  “If you start off with hybrids, they’re at a terrific numerical disadvantage,” Peter says, drawing a few scattered dots between two thick clouds, representing As and Bs. In his sketch, the hybrids are floating here and there in the space between the two clouds, and they are vastly outnumbered. Because they are in a minority, they are more likely to mate with one of the old lines than to mate with another hybrid. “So their offspring are halfway back. A dilution of novelty.” He adds an X-axis and a Y-axis to the graph, plotting beak length against beak depth.

  Rosemary stops sketching on the blackboard, darts back to the table to see what Peter is doodling on his napkin, then adds a few finishing touches at the blackboard. She and Peter are both thinking the same way. She draws a long thin arrow that shoots off between her clouds of As and Bs. That is the narrow path an evolutionary novelty might follow to escape in a new direction.

  “A dense cloud of birds,” Peter says playfully, studying Rosemary’s chalk drawing. “It’s all preliminary.…”

  “Could be wrong,” says Rosemary.

  “Speculative,” says Peter.

  “Much better off with numbers,” says Rosemary.

  “We have ourselves at the forefront of pure ignorance.”

  “We’ve got to do more reading,” Rosemary concludes. “Reading, thinking.…
”

  “Measuring,” says Peter.

  PART THREE

  G.O.D.

  The river is moving.

  The blackbird must be flying.

  —WALLACE STEVENS,

  “Thirteen Ways of Looking at a Blackbird”

  Chapter 15

  Invisible Characters

  Immediately the fingers of a man’s hand appeared and wrote on the plaster of the wall of the king’s palace, opposite the lampstand; and the king saw the hand as it wrote.… The king said to the wise men of Babylon, “Whoever reads this writing, and shows me its interpretation, shall be clothed with purple, and have a chain of gold about his neck, and shall be the third ruler in the kingdom.”

  —Daniel 5:5

  The Grants have one more Galápagos archive, which they keep not far from their offices in Eno Hall; and this is the one that would really have astonished Darwin. The shortest way there is through Princeton’s Natural History Museum, past the black-boned Allosaurus, the extinct Irish Elk, the remains of a Neanderthal from France, and an early Homo sapiens from Israel; past broken jaws and speechless fragments, including bits of Eohippus, Pliohippus, Dinohippus, and Equus, the sequence of fossils that Huxley used more than a century ago in his lecture “The Demonstrative Evidence of Evolution.”

  One floor below the museum there is a long drab basement corridor. Toward the far end, a steel stairway drops sharply as if into the lower level of a ship. This stairway leads to a subbasement, or rather a sub-subbasement: C-Level of the building next door to the museum, the George M. Moffett Biological Laboratory.

  Moffett C-Level throbs like an engine room. The din of compressors, fans, and generators; the drone of fluorescent tubes; and the narrowness of the passageway combine to give the place a bowels-of-the-beast feeling. The air reeks of formaldehyde and chemicals less familiar.