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 these 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.
Conventional wisdom said seeds would be killed by salt water. So Darwin at Down House tried soaking seeds from his garden, including lettuce, carrots, and celery, in little bottles of brine. Then he planted the seeds in glass dishes on the mantelpiece of his study, and was tickled to see them sprout, even after forty-two days in salt water. In forty-two days the average current in the Atlantic could have carried these seeds 1,400 miles. They could have lasted the length of an ocean voyage.
That was the point of Darwin’s pottering about with mud from the edge of a pond. “Wading birds, which frequent the muddy edges of ponds, if suddenly flushed, would be … most likely to have muddy feet.” They could carry mud, and seeds with it, from one place to another.
Darwin was not above gathering bird droppings and picking out undigested seeds with a tweezers. He planted these seeds, and they germinated too. So the first birds to fly over an island could help make the islands fit homes for more birds.
Sometimes his experiments misfired. Darwin complains in one letter that “the fan-tails have picked the feathers out of the Pouters in their Journey home—the fish at the Zoological Gardens after eating seeds would spit them all out again—Seeds will sink in salt-water—all nature is perverse & will not do as I wish it.”
But other days his unorthodox experiments left him cackling with glee. He fed oats to sparrows, and fed the sparrows to an eagle and a Snowy Owl at the zoo. Then he waited around a few hours, collected their pellets, and planted them. A seed came up. “The Hawks have behaved like gentlemen,” he wrote to Hooker. “Hurrah!”
In the Galápagos, experiments like these are still going on all the time. The archipelago is so young that not even the beginning is over. The very colonization of the islands is in progress, and members of the Finch Unit can watch it unfold before their eyes. When they fly to the islands from the Ecuadoran city of Guayaquil, they look down from their plane windows at the great Guayas River. The Guayas debouches into the convergence of two major ocean currents, the South Equatorial and the Humboldt. Anything rafting down the river can get swept out to sea and westward to the Galápagos. From the plane the Grants often see dozens of natural rafts on the river. The muddy water twists and turns, and long green strings and strands of matted greenery go drifting along with it. Anyone who takes off from or lands in Simón Bolívar International Airport, in Guayaquil, can look down at these fateful rafts: green mats in the dark green delta, perpetually floating out to sea.
In Darwin’s view, this was the first step in the origin of Galápagos species—colonization, immigration—and the Grants can watch even this step in action. During their first years in the islands they were surprised to discover half a dozen flame trees growing on the far-northern island of Wolf. Flame trees are common on parts of Santa Cruz and on Santiago, but the Grants could not imagine at first how the trees’ seeds had gotten all the way from these islands, which are in the center of the archipelago, to Wolf, at the farthest northern edge of the archipelago, where their red flowers look as striking and deliberately placed as a carnation in a black buttonhole. The Grants found another odd outlier, a single flame tree, old, solitary and flamboyant, on the west side of Genovesa, about 400 meters from the sea, the only flame tree on the island.
On Genovesa, red-footed boobies and frigatebirds often pick up bits of brightly colored plastic from the beaches and drop them far inland or along the tops of cliffs. The Grants, walking along the steep cliff of Darwin Bay, above the landing called Prince Philip’s Steps, have often found bottle caps, combs, streamers, and gaudy drift plastic of all kinds. The flame trees’ seeds are as red as their flowers, and the Grants wondered if the birds might have picked one up from the beach and carried it inland. To test this idea, Peter, Rosemary, and their younger daughter, Thalia, put thirty flame-tree seeds in a jar of salt water. It was the same sort of experiment that Darwin tried at Down, only the Grants were carrying it out right in the Galápagos, in their camp on Genovesa. Whenever they were back in camp from finch watching they stirred the water in the jar. After three days, most of the seeds were still afloat.
Toward the end of the great Niño, the Grants have reported, they found almost one hundred of these seeds strewn on the southern beaches of Genovesa, where there is not a single flame tree. They found even more seeds of the poison apple, which does not grow anywhere on the island. Apparently these seeds floated to Genovesa from another island, swept there by El Niño, “washed down to the sea,” the Grants write, “in the torrents and temporary rivers” that flowed so often that year on Santa Cruz and Santiago. “It is even possible they were carried all the way from the South American continent.”
On a typical voyage between islands, the Grants, sitting on a bench at the bow of their boat, may see a green sea turtle, a shark, frigatebirds, dolphins, manta rays, whales, and other lively interisland traffic, including rarities like the Hawaiian dark-rumped petrel, elegant white with black-rimmed wings, beautiful and endangered. Young and immature masked boobies will circle the boat, looking closely at the finch watchers each time they cross the bow, and coming closer and closer with each pass to Peter’s outstretched hand until he withdraws it, absentmindedly, talking away. The boobies hover and stare at the humans on deck. The birds’ brown heads and bodies as sleek as seals—seals with wings. Young boobies are such strong fliers, they do it so well, that they are a pleasure to watch, and they seem to take pleasure in their motion themselves.
So there are endless streams of wanderers between the islands. When Trevor Price was standing watch on Daphne with his field assistant and childhood buddy, Spike Millington, they kept a bird list, just as they did when they were boys growing up outside London. They spotted twenty-two different species of vagrants on Daphne Major. One spring they heard a Hawaiian petrel calling nightly for almost a week in March, and for another week in April. Sometimes just offshore they saw white-vented storm petrels and an immature great blue heron. They also saw peregrine falcons, yellowlegs, a ruddy turnstone, a willet, an offshore flock of about two thousand northern phalaropes, several species of warblers and gulls, and a dark-billed cuckoo.
Even among the finches on the island, the permanent residents, the citizens of Daphne Major, some are conspicuously more restless than others. In their camp at mealtimes the finch watchers sometimes see what looks like a little bug, growing larger and larger, winging over the crater. It is a finch whose territory is on the other side of the volcano, but who has acquired a taste for exotic travel. Meal after meal, there he is again, a born tourist, hopping in the alleys between the chimbuzos, cadging another morsel.
Darwin thought his finches were all more or less marooned, each on its own island, but these birds are still wandering and straying across their islands, all across the archipelago. The Rothschild expedition at the turn of the twentieth century spotted a single specimen flying many kilometers out at sea. Price and Millington found a few tree finches that landed on Daphne but did not stay. They also found a few immigrant fuliginosa, fortis, magnirostris, and scandens. Every year some of these species are spotted on the island, usually the ones they call “immatures”: the young and the restless. Sometimes after a breeding season a hundred or more immatures will land on an island and visit a while, but invariably, Peter says, they all leave or die before the next breeding season.
So every year some of Darwin’s finches go island hopping, landing in new places and new corners, with new neighbors. During the great Niño, quite a few finches from other islands visited Daphne Major, including some magnirostris. “The immigrants came in late 1982 and started breeding,” says Peter. “Five or six decided to stay. And some of their offspring are still on the island. Dispersal is not unusual. Breeding is.” The agitations of the super-Niño shoved these strange finches together, and at the same time, the hybrids began their rise.