When Schluter put the sparrows under the microscope in this way he found that natural selection had been working quite ruthlessly among the sparrows.
Among the males, selection had worked to eliminate the outliers—the birds that deviated most toward large or small. This is what is known as stabilizing selection. This kind of selection pressure helps to explain why the sparrows on the island are so much less variable than the finches on Daphne.
Among the females, Schluter found oscillating selection, and the case was remarkably similar to the one on Daphne Major. There had been two tremendous population crashes in the course of the study, just as on Daphne. One crash was caused by bitter cold weather, high winds, and snowfall, during the winter of 1987–88. The watchers on Mandarte took a census before and after the cold spell. (It would have been tough to get out to the island in the middle of a cold spell: the ride out in a Zodiac takes about one hour, and in bad weather you arrive at the island covered with ice and snow.) When the sparrow watchers arrived after the cold spell was over, they found that they were down to eight birds (which was scary for Smith; his flock almost went extinct). The second population crash was not caused by a hard winter—in fact, the sparrow watchers still do not know what was killing their birds. But the females were pushed one way by the first crash, and the other by the second, again much as on Daphne Major.
“My result: lots of selection,” says Dolph merrily. “At least one event every year.” Yet when he summed all these changes over the lifetime of a generation of sparrows, he saw no selection at all, just as Smith had said.
“So we were both right,” Dolph concludes. Summed over years, the effects of natural selection were invisible. But at each stage of their lives and each year of their lives the sparrows on that little island had been “daily and hourly scrutinized” by the hand of natural selection, much as Darwin imagined, only in fast motion.
The population on Mandarte is still being pushed every year, first left, then right. Smith and his team have not progressed as far as the Grants in determining the causes behind those pushes. They have never tried to count the seeds and bugs on the island and match them with the numbers of the sparrows, for instance. (Mandarte is much more complicated than Daphne.) But year after year they are seeing fluctuating selection at different life stages—opposing selection, between young and old stages of life, just as in the finches. And they are seeing oscillating selection from one year to the next, also as in Darwin’s finches.
“You start to view species not as constant entities but as fluctuating things,” Dolph says. “A species looks steady when you look at it over years—but when you actually get out the magnifying glass you see that it’s wobbling constantly. So I guess that’s evolution in action. The world is not as stable as you think!”
What seemed most striking to Dolph as he studied the selection events hidden in Smith’s data was the boring uniformity of Smith’s birds. Compared with Darwin’s finches, the sparrows of Mandarte might have been turned out by a cookie cutter. They showed only the very slightest of variations from one bird to the next in length of beak and length of tarsus. Yet even these variations, trivial as they seemed, had helped to decide who lived and who died. “That is pretty amazing to me!” Dolph says. It means populations don’t need to be excessively variable in order to experience natural selection.
“Selection doesn’t happen just in the Galápagos,” Dolph concludes. “It happens in your backyard.”
WHEN DARWIN LOOKED at the fossil record, he saw it static and frozen for long stretches. In retrospect that is not surprising. If selection events do not show up among a tribe of sparrows when summed over the course of a single generation, consider how much less visible these events will be in the strata of rock beneath our feet, in which the generations have been summed for many millions of generations.
One of the most famous examples of “rapid” evolution in the fossil record is the ancestral lineage of the modern horse. That was the subject that Huxley chose for his lecture “The Demonstrative Evidence of Evolution.” The transmutation of an upper molar in Mesohippus from Hyracotherium is one of the most rapid changes whose sequences are preserved like a snip of flicker-frame silent movie in the rock record. This event took about one and a half million years, or about half a million generations.
Even that rate is molasses and stasis compared with the stutter step of live finches and sparrows. Compared with life above the ground, the record of life in the stones flows almost as slowly as stone itself. The apparent lack of action in the fossil record confirmed for Darwin his belief that evolution by natural selection must be a rare event, and that any and all action must be unendurably, geologically slow. It is because the changes in the record are invisible, and the changes around us were at the time invisible, that he says in the Origin we “see nothing of these slow changes in progress, until the hand of time has marked the lapse of ages.”
In 1949, the evolutionist J. B. S. Haldane suggested that the rate of evolution be described in terms of a universal unit, whether the change took place in the animal or vegetable kingdoms, among the living or the long extinct. This unit he named in the same spirit of tribute that we call continents and oceans after their discoverers. He called his unit the darwin.
To keep things simple, Haldane said, let us define the darwin as a change in the length of some character. Since we want a universal unit, and not one that depends on the size of the creature, let us use the percentage change. The beak of a duck-billed dinosaur might lengthen by a few meters in a million years, and a bird’s beak by a few millimeters in a million years, Haldane said, and yet both could represent a change of 10 percent. A percentage change will mean the same thing in myriad living and once-living forms, from the beaks of birds to the teeth of hyenas, from the skulls and cannon bones of horses to the inner whorls of ammonites. Haldane defined one darwin as a change of 1 percent per million years.
When Haldane looked at a few representative fossil records of evolution he found that the rate of change was very slow, on the order of 1 percent per million years, or a rate of change of a single darwin. Other investigators since Haldane have borne out his guess that this snail’s pace is typical of the fossil record.
Haldane concluded as Darwin had that the rate of evolution by natural selection in the world around us must be infinitesimally slow, far too slow to watch, that it could only be watched in the long, slow additions of the fossil record. Rates in the living world would have to be measured in millidarwins. In artificial selection, he said, you could get rates of thousands of darwins, but that is not something you would see in the wild: “Rates of one darwin would be exceptional in nature.”
“Such calculations are extremely rough,” Haldane said, “but they suggest the remarkably small order of magnitude of the selective ‘forces’ which are at work if natural selection is largely responsible for evolution, and the extreme difficulty of demonstrating them in action.”
Now it is possible to translate the evolution of Darwin’s finches in the drought and the flood into Haldane’s whimsically named unit, the darwin. In the drought the change was 25,000 darwins. After the flood the change was about 6,000 darwins.
So there is an enormous gulf between what we see when we take the time to watch the living world in action, and what we see when we look at the world recorded in stone. To set this discrepancy in a broad perspective, an evolutionist not long ago compiled more than five hundred cases of evolutionary change, from short and fast experiments in artificial selection (events that took months or at most a year and a half) to evolutionary experiments in the fossil record (events that took millions of years). The evolutionist, Philip Gingerich, translated all of these events into darwins. He discovered a simple pattern, a pattern that is just the opposite of what earlier evolutionists—the whole lineage of evolutionists from Darwin to Haldane—would have expected. The closer you look at life, the more rapid and intense the rate of evolutionary change. The farther back in time you stand, the
less you see.
In a single year, you can find rates of change as high as 60,000 darwins. But in the fossil record the average is only a tenth of a darwin.
The reasons for the discrepancy are not far to seek. If at any time in those millions of years a species changed swiftly, but the rest of the time it changed slowly, that start-and-stop motion would average out into a very sluggish movement. What is more, if the species changed first one way and then the other way, over and over again, as Darwin’s finches did in the first decade of the Grants’ watch, then the fossil record would register virtually no change, a near equilibrium. Yet the beak of the finch is in fact in so much evolutionary motion that as soon as people started watching closely enough, they saw it change right before their eyes.
“There’s quite a bit of wobble in the fossil record too,” says Peter Grant. “But you don’t normally see it. When you look at a series of fossils, the usual practice is to take 3,000 or 5,000 or 10,000 years as the unit. You take the average position at the beginning and the end, and average the rate of change in between.
“Even then, you see wobbles in the record. It’s one in a thousand wobbles, if you like. You rarely get successive generations preserved in fossils. It’s like looking at specimens of Darwin’s finches from 1874, 1932, and 1987. You may get some wobbles there—but you miss all sorts of action in between.”
The fossil record is just too primitive a motion-picture camera to capture the fast-moving life. Rapid motion disappears like the whir of a hummingbird’s wings. In such a record, the two wonder years of Darwin’s finches would disappear as surely as a wing-beat up and a wing-beat down, canceling out in the blur.
If we look at the billowing smoke of a volcano from close up, we see intense and rapid motion, enormous and dangerous turbulence. If we look at the eruption from far, far away (a safe distance that puts it almost to the horizon), the smoke seems to hang in the air almost motionless: we have to watch a long while to see any change at all. The evolution of life turns out to be rather like the eruption of a volcano. The closer you look, the more turbulent and dangerous the action; the farther your remove, the more the living world seems fixed and stable, hardly moving at all.
It is amazing to think of all these species around us not fixed but in jittery motion. It is like the difference between the old view of solid physical matter around us—the view in the time of Newton—and the view now, of infinite motion down to the level of each atom and molecule, and below, in the ceaseless assault and battery of elementary particles. The beak of the finch is an icon of evolution the way the Bohr atom is an icon of modern physics, and the study of either one shows us more primal energy and eternal change than our minds are built to take in. Yet like the vista of the atoms, the vista of evolution in action, of evolution in the flesh, has enormous implications for our sense of reality, of what life is, and also for our sense of power, of what we can do with life.
“This jittering is an aspect of all populations everywhere,” says Dolph Schluter. “It is demonstrating that populations are currently dynamic—still wobbling—so that a larger change in the environment at any moment might push them one way or the other.” If they weren’t jittering, that would suggest that the processes that brought them here had finished, that the creation was over, just as the universe would be moribund or dead if there were no motions to be found in its atoms. But these motions are to be found everywhere, Dolph says. “They are ever-present.”
Having seen even this much of the view from the rim of Daphne Major, we can no longer picture the story of life as slow and almost static, a world view for which the chief emblem of evolutionary change is a fossil in a stone. What we must picture instead is an emblem of life in motion. For all species, including our own, the true figure of life is a perching bird, a passerine, alert and nervous in every part, ready to dart off in an instant. Life is always poised for flight. From a distance it looks still, silhouetted against the bright sky or the dark ground; but up close it is flitting this way and that, as if displaying to the world at every moment its perpetual readiness to take off in any of a thousand directions.
PART TWO
New Beings on This Earth
We will now discuss in a little more detail the struggle for existence.
—CHARLES DARWIN,
On the Origin of Species
Chapter 8
Princeton
These birds are the most singular of any in the archipelago.
—CHARLES DARWIN,
Journal of Researches
… the more you look the more you see.
—PETER GRANT,
Ecology and Evolution of Darwin’s Finches
Mid-morning and mid-June in Princeton, New Jersey. Rosemary Grant is at work in her office, Eno Hall, Room 106. She wears an Icelandic sweater, a long blue Laura Ashley dress, and sandals. Sunlight slants in the bay window behind her, through the spreading green arms of a katsura tree with shaggy bark.
She is sitting on a Danish backless chair, at a table with wrought-iron legs. This table runs the length of the room and holds a Casper GM-1230 computer, which is an IBM clone; a Hewlett Packard LaserJet printer; a second printer; and a Macintosh II with a screen that would be large enough for a living-room television.
The Grants got back from the islands only a few weeks ago. Already they have transcribed every one of the numbers from their waterproof notebooks into the computers. Now they plan to spend a year without teaching, alone with all their years of records. “We won’t be working,” Peter tells people at Princeton. “Our purpose in taking this sabbatical is not to work with our hands but to creep away and analyze our data.”
“Why do you say we won’t be working?” Rosemary protests. “That’s work.”
“Yes. In some ways that’s the real work, and everything else is play.”
Rosemary runs her finger through the air above the tidy row of boxes that she and Peter keep shelved above the computer table. Charles Darwin kept his notes in thirty or forty big portfolios, which he stored in his study on specially built racks to the right of the fireplace. Thanks to his powers of organization and his capacious mind he managed to file and retrieve an extraordinary breadth and depth of information in those portfolios. But he could not have handled as much data as Peter and Rosemary keep in the rows of little boxes on this shelf, the archives of the International Finch Investigation Unit.
Rosemary chooses a box, slips out a diskette, and pops it into the Macintosh. “Okay,” she says, after a brief search. “Here’s 3425.” Number 3425 is one of the two rogues that Rosemary caught half a year ago on Daphne’s north rim. He was the first of the two cactus finches that hopped into her traps. The rows of numbers on the screen summarize what the Grants and their assistants have learned of the fortunes of 3425 since that morning in January.
“So far this year … he had two clutches,” Rosemary says, speaking slowly as she decodes the numbers and letters on the screen. “And he had the same female for both clutches, 5582. In the first clutch, there were three eggs. Two of them hatched, and only one fledged. In the second clutch, three eggs hatched … but again, only one fledged.”
The drought is over, which is why 3425 has been so busy. The first bout of rain fell on Daphne in February. Within days the Portulaca flowered, and the torchwood trees put out leaves and filled the air with the memorable odor of their greenish-white blossoms. Tall grass sprouted and covered all the paths. Even after all these years, the speed of Daphne’s metamorphosis still took the Grants by surprise.
The finches bred and bred. Peter and Rosemary and this year’s assistants ran up and down the sides of the volcano, trying to keep up with hundreds of nestlings—Darwin’s latest finches. Then the Grants climbed down to the welcome mat and into the panga of a little local boat called the Flamingo to visit the finches of Gardner, Floreana, Genovesa, and Española.
Just off the island of Floreana the Flamingo’s engine failed. An engine cable had broken, and the captain went belowdeck
s to repair it. The Flamingo drifted toward the resounding surf of a rock called Enderby. The swell was long and deep, and it pulled them closer and closer to the boiling surf and the rock. Rosemary, Peter, and Thalia stood at the railing waiting for the engine to start, watching the rock get closer and closer. At the last second the captain got the cable fixed and the engine started; another season of precious numbers made it to this shelf in Princeton.
Rosemary ejects the diskette from the Macintosh and loads another one, labeled “NESTOTAL76-91.” There is a long pause while the computer clicks and clacks, but the screen stays blank. “This is a big file,” Rosemary says while she waits: “5,575 kilobytes, I think it is.” A file that size could hold about a million words, or the complete manuscript of Darwin’s “Big Book,” Natural Selection, plus several editions of the Origin and the Descent of Man. That is how much information the Grants and their field assistants have compiled on the nestlings born to Darwin’s finches on Daphne Major from 1976 to 1991.
Abruptly, columns of numbers cascade down the screen and fill it from top to bottom. “That’s why it took so long,” Rosemary says, nodding at the dense little rows of numbers. “And this is just the start of the file.” She scrolls down through the streams and streams of numbers. “Okay, here he is again. Our 3425 is quite an old male. These are all the times that 3425 has bred in his life: one, two, three … ten times. For the first time in 1982. Then he bred”—she counts aloud—“eight times in 1983.” That was the wonder year, the year of the flood. “He bred again in 1984, once. That’s about as much as anybody did in ’84.… Then he did not breed again until this year.”
The Beak of the Finch: A Story of Evolution in Our Time Page 14
