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

by Jonathan Weiner

  Something has changed since the flood. Something is happening. Strange as it seems, these hybrids are the fittest finches on the island. If the Grants’ hunch is right, this is a missing piece of a puzzle, a puzzle they have been struggling with for the last twenty years: “that mystery of mysteries,” the origin of species.

  Chapter 9

  Creation by Variation

  The Galápagos seems a perennial source of new things.

  —CHARLES DARWIN,

  letter to Joseph Hooker

  “Evolution happens the whole time,” Peter Grant tells his classes at Princeton. “You look up in surprise. But evolution is always happening. Completely contrary to Darwin’s view that very, very slowly, very intermittently, life evolves.

  “Geneticists will tell you that. Evolution is always happening. What they mean is that the genes of this generation are not precisely what they were the preceding generation. Nor will they be precisely the same in the next. And evolution is that change. And it is almost a certainty, a mathematical certainty, that the genes will never be the same.

  “Darwin was right in a sense: you won’t see it. You look out at the maples on this campus, or the robins, or the gray squirrels, and year after year they look the same. They aren’t. They’re different. But you can’t see it, the differences are too subtle.”

  Peter sometimes thinks about the time that Darwin himself almost saw it happen. Darwin was a man of excruciatingly regular habits. Year after year, twice a day if health allowed, Darwin used to leave his house, cross to the farthest end of the kitchen garden, and go out through a wooden door in the back hedge. Then he would stride down a fenced path between two lonely and—in the wintertime—desolate meadows to a strip of land that he rented from his neighbor, a wealthy astronomer.

  Back there, with the astronomer’s permission, Darwin had planted trees and laid out a trail among them. This was his Sandwalk, his Thinking Path. The ground was littered with flints, which looked to him when he first saw the place “like great long bones.” At the start of his walk, Darwin would collect a heap of these flints and set them on one side of the path. Then as he went around and around he kicked away a flint or knocked it aside with his iron-tipped stick, to count the laps.

  Year after year, day after day, the Sandwalk was evolving around him, though Darwin did not see it. One cold and dreary March, for instance, when the whole household was sick with “coughs & colds & rheumatism” and “Hooping cough,” Darwin noticed that the birds in his trees had been decimated by the cold. In the third chapter of the Origin, “Struggle for Existence,” he estimates that the freezing weather had killed about four-fifths of the birds on his grounds: “and this is a tremendous destruction,” he writes, “when we remember that ten per cent. is an extraordinarily severe mortality from epidemics with man.”

  This is the event that intrigues Peter. By then—it was the winter of 1855—Darwin was already drafting his “Big Book,” Natural Selection. Yet Darwin did not dream he could see his process in action among the birds in his own backyard, so he did not try.

  “I really do believe that if he had been thinking along the lines that selection can be observed, he would have done something about it that winter,” Peter says. “I believe he came very close to documenting the process.”

  If the weather was bad enough to have knocked birds out of the trees, which sometimes happens in England, and New England, then all Darwin had to do was get his gardener to collect some of the dead blackbirds.

  “If he had done,” Peter says, “Darwin might have noticed that some were big, or fat, or long in the beak, or long in the leg, or weird in some way.” Then he could have asked his long-suffering butler Parslow to shoot some of the birds that had survived the winter, so that Darwin could compare and contrast the measurements of the dead and the living.

  “He might not have found anything,” Peter says. “But it’s coincidental that about four-fifths of the birds died. That’s how many disappeared the first time we were watching—85 percent in the 1976–77 long drought.

  “Possibly those blackbirds could have done it for him.”

  This is what evolutionists have been doing ever since Darwin. They have been walking around and around his Thinking Path, kicking the flints, looking for ways to test and extend the ideas that Darwin worked out on those quarter-mile laps. Darwin’s followers are always thrilled when they can confirm what Darwin saw, and they are even more thrilled when they can see what Darwin missed. Probably no other major branch of science today is so haunted, dominated, and driven by the thoughts of one man.

  The Grants are retracing the path of Darwin’s thought by beginning where Darwin began—in more ways than one. Not only do they gather their data in the center of Darwin’s islands. They gather it in the same simple, concrete, objective units that Darwin considers the origin of all evolution: individual variations, measured in millimeters. Every evolutionist appreciates the value that Darwin attached to variations, but not many people in the field are as excited by variation as Peter Grant. When he lectures, Peter sometimes sounds even more interested in variation than in evolution itself. “Evolution is change in variation,” Peter says. “By studying evolution in action we can better understand the variation.” He draws a bell curve on the blackboard and then gestures, as if to lift the curve off the board into the air with his hands, trying to suggest to his students the dimension of time—coaxing the curve off the wall with his hands and warping it eloquently as it floats out into the air. “This is what we have to explain,” he says, pointing at the curve on the blackboard. “This is variation. And its change through time”—pointing with his eyes at the path of the curve through the air—“is evolution.”

  WHAT MAKES new species? How, exactly, does variation lead to creation? Why does a line of life keep more or less the same shape and more or less the same habits for thousands, tens of thousands, sometimes millions, of years, and then branch out into new lines, new shapes, new habits? This is what Darwin wanted to know, what the Grants want to know, what Darwin’s followers have been debating ever since Darwin.

  It is one thing to demonstrate, as the Grants have done, that natural selection leads to evolution. It is another and much more complicated thing to demonstrate precisely how this evolution leads to new species; and despite the title of his greatest book, Darwin himself never spells out the details.

  Darwin calls the Origin “one long argument,” and this is the step in the argument that many of his readers find the hardest to follow, the step that feels like a leap of faith: from slight individual differences in one nest, or one seedbed, or one family album, to the striking differences between species. They can accept that Darwin’s mechanism, natural selection, can refine adaptations. They can understand that Darwin’s process might play, in this way, a sort of supporting role on the stage of life, as an editor of beaks and bodies, an improver of lines. But they cannot believe the process can create something new.

  Not even Darwin’s friends were satisfied that natural selection leads to the origin of species. Hooker, the botanist, wrote to him diplomatically:

  You certainly make a hobby of Natural Selection, and probably ride it too hard; that is a necessity of your case. If the improvement of the creation-by-variation doctrine is conceivable, it will be by unburthening your theory of Natural Selection, which at first sight seems overstrained—i.e., to account for too much.

  Huxley, Darwin’s standard-bearer, managed to give a sort of victory speech for evolution, “The Coming of Age of the Origin of Species,” without mentioning natural selection once.

  After Darwin’s death, many biologists found it easy to accept evolution and impossible to accept Darwin’s chief explanation for it. Evolution, yes; selection, no. William Bateson, the founder of modern genetics, wrote an elegy for Darwinism in 1913, calling it “so inapplicable to the facts that we can only marvel … at the want of penetration displayed by the advocates of such a proposition.”

  Nordenskiöld’s
History of Biology dismissed Darwinism forever in 1924:

  To raise the theory of selection, as has often been done, to the rank of a “natural law” comparable in value with the law of gravity established by Newton is, of course, quite irrational, as time has already shown; Darwin’s theory of the origin of species was long ago abandoned.

  And Singer’s A Short History of Biology killed Darwin with kindness in 1931:

  … natural selection by the survival of the fittest, is certainly far less emphasized by naturalists now than in the years that immediately followed the appearance of Darwin’s book. At the time, however, it was an extremely stimulating suggestion.

  In 1981, as the centennial of Darwin’s death approached, the staff of the British Museum’s Natural History Building in South Kensington unveiled a permanent exhibit entitled “The Origin of Species,” a lavish effort in eleven sections, with diagrams, displays, a natural-selection computer game, and a continuously running film loop on which a narrator intones:

  The Survival of the Fittest is an empty phrase; it is a play on words. For this reason, many critics feel that not only is the idea of evolution unscientific, but the idea of natural selection also.… The idea of evolution by natural selection is a matter of logic, not science, and it follows that the concept of evolution by natural selection is not, strictly speaking, scientific.

  The film loop prompted an indignant editorial in Nature: “Darwin’s Death in South Kensington.” The editorial in turn drew letters from scientists and philosophers in Manchester, Chicago, Brussels, and Odense, Denmark. All that year the journal was full of clashing commentary, for and against: “How True Is the Theory of Evolution?” “Darwin’s Survival,” “Evolution’s Waterloo,” “Fit for What?”

  Even Darwin himself admitted twinges of doubt. He asks in the Origin: “Can we believe that natural selection could produce, on the one hand, an organ of trifling importance, such as the tail of the giraffe, which serves as a fly-flapper, and, on the other hand, an organ so wonderful as the eye?” And though he answers in the affirmative, the question is more than rhetorical, for in a letter to a friend, Darwin confesses:

  I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of the complaint, and now small trifling particulars of structure often make me very uncomfortable. The sight of a feather in a peacock’s tail, whenever I gaze at it, makes me sick!

  Can the Darwinian process really produce something as marvelous as an eye, a wing, or a feather—let alone a flying bird, a thinking human being? Without being allowed to watch, without the spectacle actually before them, scientists have found it hard to picture how Darwin’s process could lead again and again to such magnificent results. The mind’s eye simply cannot see that far, as the evolutionist George Williams writes:

  I believe that modern opposition, both overt and cryptic, to natural selection, still derives from the same sources that led to the now discredited theories of the nineteenth century. The opposition arises, as Darwin himself observed, not from what reason dictates but from the limits of what the imagination can accept.

  Feather of a peacock. From Charles Darwin, The Descent of Man, and Selection in Relation to Sex.

  The Smithsonian Institution

  Watching natural selection in action is one way to get beyond the debates and abstractions that have wrapped this subject in a century and a half of philosophical fog. The Grants can look and see. And this year, with the help of the hybrids, they hope to see a little more than they have seen before.

  A Galápagos finch rides a Galápagos tortoise.

  Drawing by Thalia Grant

  THE STEP FROM INDIVIDUAL VARIATIONS to new species, Peter Grant wrote recently, “will be exercising the minds of evolutionary biologists well into the next century.” However it happens, clearly it must happen a lot. Peter often thinks of the way his last teacher, G. Evelyn Hutchinson of Yale, used to put the question: “Why are there so many different kinds of animals?”

  In the Galápagos alone there are not only thirteen species of finches, found nowhere else in the world. There are also Galápagos penguins, Galápagos sharks, Galápagos hawks, and Galápagos doves; Galápagos flycatchers, martins, centipedes, butterflies, bees, rats; to say nothing of the celebrated Galápagos mockingbirds and tortoises, and the Galápagos iguanas, Darwin’s “imps of darkness.”

  Why are there so many different kinds of animals? Or plants? There are more than seven hundred different species of plants in the Galápagos, and new ones are still being discovered and described. Almost two hundred of these species are found nowhere else on earth. There is one species of (very variable) Galápagos tomato. There are six species of prickly-pear cactus. There are thirteen species and subspecies of Scalesia trees, which, as Peter Grant says, puts them on a par with Darwin’s finches. Scalesia belongs to the daisy family: a garden-lawn daisy that has grown to the size of a tree.

  “The Galápagos seems a perennial source of new things,” Darwin wrote to Hooker, after Hooker had sorted through Darwin’s Galápagos plant specimens. And of course it is not only the Galápagos that poses the mystery of mysteries. On almost any patch of earth the variety of animals and plants is staggering. Once, as a botanical demonstration, Darwin chose a small fallow field in Kent. It was a poor field without water, its soil “of heavy very bad clay,” Darwin writes in Natural Selection. On this land in the course of a year a friend of his collected plants belonging to 108 genera. Darwin knew of another botanist “who says that he covered with his hat, (I presume broad-brimmed) near to Lands End six species of Trifolium, a Lotus & Anthyllis; & had the brim been a little wider it would have covered another Lotus & Genista; which would have made ten species of Leguminosae …!”

  Darwin himself went one better. One February, he relates in the Origin, he took “three table-spoonfuls of mud from three different points, beneath water, on the edge of a little pond; this mud when dry weighed only 6¾ ounces; I kept it covered up in my study for six months, pulling up and counting each plant as it grew; the plants were of many kinds, and were altogether 537 in number; and yet the viscid mud was all contained in a breakfast cup!”

  All told there are somewhere between two million and thirty million species of animals and plants alive on the planet today. Something like a thousand times that many species—about two billion, by the most conservative guess—have evolved, struggled, flourished, and gone extinct since the first shelly fossils were laid down in the Cambrian explosion, about 540 million years ago. And the great question for the evolutionist is, Why?

  Darwin did not claim that natural selection is the exclusive agent in the origin of species. But he did maintain that his mechanism is one way of producing new species, and that it is probably the main way. He stakes this claim in the title of his most important book: On the Origin of Species by Means of Natural Selection.

  In his first secret notebook he sketched the origin of species as a few rough diverging lines, calling the sketch at first the Coral of Life. One species splits into two, two into four, growing and radiating into branches that will fork again. Later the image was elaborated by Darwin’s followers, such as the German biologist and philosopher Ernst Haeckel, into great gnarled oaks with hundreds of species names neatly lettered at the twig tips. That is how David Lack drew his chart of the family tree of Darwin’s finches.

  Lack’s chart represents a close-up of one branch on the tree of life. The species the Grants have chosen for their life’s work are half a dozen twigs on this branch. They are the six species that Lack placed in the center of his diagram, the ground finches: three diverging twigs on the left, three diverging twigs on the right.

  Darwin’s finches, of course, are a classic model of adaptation: generations of textbooks have used their celebrated tool kit of thirteen beaks to illustrate the process. Darwin’s finches are also a classic model of speciation: again, they figure in virtually all of the textbooks, very often as the central illustration. That is why thes
e birds have become such a universal symbol of Darwin’s process, so that their beaks now represent evolution the way Newton’s apple represents gravity, or the apple of Adam and Eve represents original sin.

  The standard textbook description of speciation sets the story in the dim past, like a scientific book of Genesis. The textbook diagrams and charts suggest that the flora and fauna of the Galápagos Islands are end points, products of a process of creation that went on once, “in the beginning,” and is now more or less complete.

  From David Lack, Darwin’s Finches. Courtesy of Cambridge University Press.

  Library, the Academy of Natural Sciences of Philadelphia

  But in Darwin’s islands the forces of creation are still at work, in plain view, with “the manufactory still in action.” And since Darwin’s mechanism for the refinement of a finch beak is also his mechanism for the manufacture of a new species, the Grants’ data base allows them to test the power of both processes at once. They can analyze not only the forces that shape adaptations but also, at the same time, the forces that put new beings on this earth.

  When Darwin thought about the way the branches of life can grow and split, his mental image was first and always what he imagined had happened in the Galápagos archipelago. Darwin assumed that the summits of these lonely volcanoes were settled by a few chance colonizers that blew or drifted out from the coast of South America. He reasoned that the first immigrants to thrive on the naked lava must have been plants, since any animals that arrived before the vegetation would not have lasted long. A seed-eating finch cannot last a day without seeds.