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.
Chapter 10
The Ever-Turning Sword
… and he placed at the east of the garden of Eden Cherubims, and a flaming sword which turned every way,
to keep the way of the tree of life.
—Genesis 3:24
In his earliest secret notes, Darwin wonders if the lines of life in the Galápagos might have diverged simply by adapting to the strange new place in which they had alighted. They might have speciated simply by virtue of their isolation, as they altered further and further over time. In those first notes Darwin imagines that enough small changes accumulating over enough geological time would be able to accomplish almost anything.
Darwin implies as much in a famous scribble he made soon after the meeting with Gould over Darwin’s finches: “Let a pair be introduced and increase slowly, from many enemies, so as often to intermarry—who will dare say what result. According to this view animals on separate islands, ought to become different if kept long enough apart, with slightly differ[ent] circumstances —Now Galápagos tortoises, mocking birds, Falkland fox, Chiloe fox.—English and Irish hare—”
Without question this process of adaptation can and does go on. This is the process the Grants have documented in such detail in Darwin’s islands. The adjustments they saw the birds make after the drought, and the flood, are the kinds of changes the finches’ ancestors would have made when they landed in these islands for the first time. Each year Daphne is a new island, and its finches adapt to it from generation to generation. The work that began when their ancestors first landed on the island is never finished.
But Darwin’s finches are not marooned, each species to its own island. On average there are seven or eight species to each island in the archipelago. Beyond that there is the constant traffic of visiting finches. The birds may have diverged—or have begun to diverge—in isolation, but they are not in isolation now. What happens when lines of life that have begun to diverge in isolation meet up again?
Darwin has an answer, and it is one of the most original steps in his argument.
“I CAN REMEMBER the very spot in the road, whilst in my carriage, when to my joy the solution occurred to me,” Darwin recalls in his memoirs, “and this was long after I came to Down.” By then more than ten years had passed since Darwin had seen the Galápagos. He had already written out a “very brief abstract” of his secret theory, 35 pages in pencil. He had expanded this abstract into a longer sketch of 230 pages and copied out the whole draft in a fair hand. He had kicked innumerable flints. Yet it was not until that moment, riding on the road, that Darwin felt he really understood the branching of the tree of life.
What drives the branches to diverge again and again? “How, then, does the lesser difference between varieties become augmented into the greater difference between species?” Suddenly Darwin perceived that adaptation to isolated specks of land is not the whole answer. He saw a way that natural selection acting on local varieties, “species in the process of formation,” in Darwin’s view, “incipient species,” would drive wedges between these varieties and push them apart, everywhere on earth.
For Darwin it was like the eureka in the legend, when Columbus peers at a butterfly perched on an egg, imagines the earth in the palm of his hand, and suddenly perceives the obvious. It was as if Darwin had taken a few giant mental steps back and, craning his head, stared up for the first time at the whole tree of life.
His vision had nothing to do with star-crossed lovers and the meetings and matings of hybirds. It was more like a vision of war. What Darwin realized is that two varieties living side by side are thrown into competition—competition “in a very large sense,” like the contest the Grants are now watching among ground finches on Daphne Major. Individuals in each of two neighboring varieties will usually find themselves going after the same thing as their neighbors, precisely by virtue of their similarities, like two big-beaked ground finches hunting for the same Tribulus seed.
In the struggle for existence one variety or species must often squeeze another. Nearest neighbors, closest cousins, will pinch each other very hard, generation after generation. They collide because they are so much alike in equipment, instincts, and needs. Again, like finches scrabbling through the cinders of Daphne Major for the last Tribulus seed, the more alike the varieties the more frequently they will find themselves going for the same seed, the same nook, the same niche, at the same time. Competition through kinship.
An exceptional individual will benefit under these conditions. Any escape, however partial, from that oppressive competition will be an enormous release—almost like finding a fresh island. The lucky individual that finds a different seed, or nook, or niche, will fly up and out from beneath the Sisyphean rock of competition. It will tend to flourish and so will its descendants—that is, those that inherit the lucky character that had set it a little apart. Individuals that diverge from the madding crowd will tend to prosper, while the rest will be ground down.
Selection will act in this way on all neighboring varieties, not just on islands but everywhere on the planet, and the effect will be continually to move varieties apart and repel them. Even if they never actually jostle and joust, never once collide physically over a Tribulus seed or a nesting site in a crook of cactus, natural selection will gradually magnify their differences.
At last the two varieties will move so far apart that competition will slack off. It will slack off when the two varieties have evolved in new directions: when they have diverged. Natural selection will have led in effect to another adaptation—the mutual adaptation of two neighbors to the pressures of each other’s existence. And the result of this sort of adaptation would be forks in the road, partings of the ways, new branches on the tree of life: the pattern now known as an adaptive radiation.
In Darwin’s view, then, each line of life is forever reaching for its spot of sun. Competition among slightly divergent forms everywhere on the planet leads continually to new branches, radiating outward in all directions like a compass rose or the arms of a medieval sun. Darwin called this his principle of divergence.
Again, Darwin never actually saw it happen, though he argued that it could happen, had to happen, and had happened. He pointed to his pigeons. Pigeon fanciers enjoy novelty and they often cultivate the extremes. With tumbler pigeons, breeders had spent several centuries “choosing and breeding from birds with longer and longer beaks, or with shorter and shorter beaks,” Darwin notes in the Origin. As a result the breeders had created two subbreeds out of one, a long-beaked and a short-beaked tumbler.
In Darwin’s view, natural selection too would tend to make nature “more & more diversified.” Here the motor is not whim, taste, or love of novelty, as with the pigeon fanciers, but something more basic. The great thing is efficiency—or what the economists of Darwin’s day were already calling the “division of labor.” “It is obvious,” Darwin writes in Natural Selection, “that more descendants from a carnivorous animal could be supported in any country” if some were adapted “to hunt small prey, & others large prey.” Likewise with herbivores, “more could be supported, if some were adapted to feed on tender grass & others on leaves of trees … & others on bark, roots, hard seeds or fruit.” In other words, as varieties and species ramify they will become better and better consumers of the world around them, like Jack Sprat and his wife, who between them licked the platter clean.
The advantage of divergence “is, in fact, the same as that of the physiological division of labour in the organs of the same individual body,” Darwin writes in the Origin. There are certain relatively simple animals in which the stomach ingests both food and air and carries out both digestion and respiration. But a stomach specializing in digestion and a pair of lungs specializing in respiration can each do better work.
No wonder Darwin remembered the exact spot in the road for the rest of his life. It is an extraordinary vision. Natural selection literally organizes life. The process of evolution by natural selection works right up the tree, from individuals to varieties, varieties to species, onward and upward, branching and branching, always diverging, helping to create all the myriad life-forms on the planet. Whole lines die out as inevitably as
individuals die out, but the result is always something new and alive: the tree grows.
In this view natural selection is even more powerful than Darwin first imagined. It is both beautiful and terrible, an agent of creation and destruction, like the flaming sword at the gates of Eden, “which turned every way, to keep the way of the tree of life.”
DARWIN’S PRINCIPLE OF DIVERGENCE has fascinated generations of biologists. But the process is not easy to observe and measure in action, because it erases its own traces. In fact, Darwin’s thesis predicts the general absence of competition. In his view, competition drives neighbors far enough apart that they are no longer locking horns, or locking beaks.
The Grants, for instance, can see that fortis and magnirostris on Daphne Major are competing for Tribulus seeds. But how can they measure competition among the species that are no longer competing, like the cactus finches and the fuliginosa? If the Finch Unit finds the six species of ground finches living more or less harmoniously now, how do they know it was competition that drove them into relative harmony? Was the agent Darwin’s principle of divergence? Where can they catch the gleam of the ever-turning sword?
Again, this is one of the most contested questions in all of Darwin’s theory, and Darwin’s finches have been at the center of the controversy ever since David Lack published Darwin’s Finches in the middle of this century. The book created so much excitement because Lack announced that the beaks of the finches were in fact produced by Darwin’s principle of divergence.
All of the low-lying islands in the archipelago, for example, have either the small ground finch or the sharp-beaked ground finch—either fuliginosa or difficilis. The low islands never have both. The taller islands in the archipelago do hold both species, and there the two species keep more or less apart. Fuliginosa tends to gather around the base of the island, difficilis near the top.
The Beak of the Finch: A Story of Evolution in Our Time Page 17
