And the more we learn of the nature of things, the more evident is it that what we call rest is only unperceived activity; that seeming peace is silent but strenuous battle. In every part, at every moment, the state of the cosmos is the expression of a transitory adjustment of contending forces; a scene of strife, in which all the combatants fall in turn. What is true of each part, is true of the whole.
—THOMAS HENRY HUXLEY,
Evolution and Ethics
The next watch fell to Trevor Price. He landed on the island early in 1979, and made camp in the craterlet on the east rim, the same small flat sandy spot where the Grants, and the Abbotts, and Peter Boag and Laurene Ratcliffe had camped before him.
Because he was standing watch after the others, Trevor could stand on their shoulders. He inherited all their painfully accumulated evidence demonstrating that variations in the finches’ beaks are important, and that natural selection moves swiftly and surely among them. As Peter Grant’s latest graduate student he also inherited the cooperation of the Charles Darwin Research Station, the routine of the pangas and chimbuzos, the names of local fishing captains, and (on buggy nights) the martial arts of the tent. Camping in the islands, he and his field assistant Spike Millington swatted mosquitoes beneath their tattered canvas with “Comparative Ecology of Galápagos Ground Finches,” by Abbott, Abbott, and Grant, published in the Ecological Monographs of 1977, and already a classic.
After Trevor’s first year on the island, seven finches in every ten were banded, and after the next year, nine out of ten. Trevor was the first who could recognize every single finch on the island on sight. Ever since Trevor, the finch watchers have been able to recognize at a glance not only the banded birds but even the handful of rogues that are still at large. When a Finch Unit graph shows 1,250 finches on the island in a given month of a given year, it is not an estimate. The watchers have made a head count, like shepherds in a fold.
Trevor was able to follow Darwin’s finches more closely than anyone else before him, not only because he had the benefit of so much data, and because so many of the finches on Daphne were banded, but also because after the drought of 1977 there were only a few hundred birds left. He could spot every one of their nests before its dome was woven. He had time to peer into it, mark its crook in the cactus tree with red flagging tape, and check it often on his rounds—around and around the desert island. Very few young fledglings hit the lava without one of Trevor’s bright-colored bands around their ankles, the bands that would identify them as surely as rank, file, and serial numbers—or given, middle, and family names.
During Trevor’s watch the flocks on Daphne Major became known so comprehensively and microscopically that the whole island seemed as small—for a brief interlude, at least—as a Petri dish. He moved toward the kind of near omniscience we expect in a laboratory. Now he could begin to see what happens to Darwin’s finches when they are squeezed by not one but several conflicting selection pressures at once. For there is more than one force at work at a time in evolution, and their collisions are unruly.
Trevor measured all the finch chicks on Daphne when they were eight days old. He measured them again as eight-week-old juveniles, and as eight-month-old adults. He could see that the beak of the finch is full grown, or very nearly so, at eight weeks. If a young fortis has a beak depth of 9.45 millimeters at eight weeks, the bird will still have a beak of 9.45 millimeters at eight months, and at eight years, if it lives that long.
However, looking over his data, and the data of finch watchers before him, Trevor noticed something peculiar that the others had missed. When he looked at each generation of fortis, he saw that the average beak depth did not stand still as the birds grew up. In 1976, for instance, the average beak depth of the juvenile fortis on Daphne had been 9 millimeters. Six months later, the average beak depth of that same cohort of birds had dropped to 8.73 millimeters.
The individual birds had not changed. But the cohort as a whole had changed, because the smallest of the young birds, the ones with the shallowest beaks, were surviving, while the biggest, the ones with the deepest beaks, were dying. Trevor saw the same thing happen to generation after generation of young Darwin’s finches. Not every small young finch survived, and not every big young finch died, but the small ones were the most likely to succeed.
After some thought, Trevor figured out why. At that tender age, the young finches’ skulls and beaks are still soft, like the skulls and jaws of human babies; the bony puzzle pieces are not yet firmly sutured together. So even the biggest birds cannot crack the big hard seeds that some of them will learn to manage later on. During their first dry season, as the small seeds on the island grow scarce, and as the biggest of the adults on the island begin to eat the biggest seeds, the juveniles still have to hunt and peck only small soft seeds. Even the very biggest juveniles will not pick up a Tribulus seed.
Big birds need more food than young birds, and big juveniles need the most food of all, because they are still growing. But because they are young, their big soft beaks do not help them get it. For a finch, being young and big is all liability and no opportunity. After a while, as each dry season drags on, some of these big juveniles get so thin and sluggish that the finch watchers on Daphne Major can reach down and pick them up with their bare hands.
So bigger is not always better. While these birds are young, natural selection forces them in the direction of small size. When they get older, selection can force them in the direction of large size. Trevor measured these small conflicting waves of natural selection as they passed through each generation, like ripples bouncing back and forth across the face of a pond, pushing each cohort first one way and then the other.
LIFE IS NOT SIMPLE, even for a flock of birds on a desert island. Just staying alive from one life-stage to the next is a full-time job. And of course survival is only the first step. After the birds get a little older they also have to meet, mate, and raise families—while continuing to survive. Sex adds a whole new set of struggles to the struggle for existence, and the pressures of sexual selection sometimes conflict with the pressures of natural selection.
Darwin mentions sexual selection in the Origin, but he writes about it at greater length in The Descent of Man, published in 1871. In fact the subject fills half the book, whose complete title is The Descent of Man, and Selection in Relation to Sex.
In one way the process that Darwin called sexual selection is less harsh than natural selection. The worst penalty in the game of natural selection is death. The worst penalty in sexual selection is life without a mate. But then, a failure to mate is equivalent to genetic death.
In the dry season, natural selection metaphorically scrutinizes these birds, “daily and hourly,” as they strive to keep body and beak together. Some birds make it, and some don’t. In the wet season, which is also the breeding season, the survivors are scrutinized daily and hourly by one another, not metaphorically but literally, as males begin jousting for territory, building nests, and singing from the highest cactus in their territories, while females troop by and inspect the males’ nests and plots of lava and listen to their songs.
In other words, as soon as nature stops selecting among these birds, the birds start selecting among one another. Again, some make it and some don’t.
Darwin was convinced of the power of sexual selection, just as he was convinced of the power of natural selection. But he never saw evolution happen through either process, and his sexual selection theory went into a long eclipse after his death. It began to reemerge only after The Descent of Man was reprinted in a centennial edition in 1971.
The process has now been demonstrated in action many times and in many places, and one of the most dramatic demonstrations, of course, was the aftermath of Boag’s drought on Daphne Major. There, because the females on the island chose only the largest males with the largest beaks, the process of sexual selection worked in the same direction as the process of natural selection and magnified it.
The skewed sex ratio on the island, a legacy of the drought, lasted a long time. During Trevor’s watch on the island, the male fortis outnumbered the female fortis by two to one, or even three to one. This provided Trevor with an ongoing demonstration of the power of sexual selection. In fact it turned the island into as dramatic a theater for the comedy of sexual selection as the drought of 1977 had been for the tragedy of natural selection.
No females went without a mate in those years, and some of the females also had a second mate on the side. But males all over the island languished through the wet seasons without a mate, building nests in the cactus trees on their territory and singing from the highest point and winning nothing. Not one of the males on the island ever had two mates at a time, as far as Trevor could tell—and he was watching as closely as a Washington reporter.
Once again, Darwin’s finches were not only meeting but surpassing Darwin’s expectations. Darwin had assumed that the pressures of sexual selection would be higher in polygamous species than in monogamous species. Among the sea lions of the Galápagos, for instance, one male has a harem, and all the rest are bachelors and out in the cold—so the selection pressure among the males is enormous. Among more or less monogamous birds like these finches, on the other hand, Darwin assumed (reasonably enough) that the pressure of sexual selection would be milder, because there would usually be about equal numbers of males and females to pair off in each generation. Yet because of the remarkably skewed sex ratio after the drought the pressure of sexual selection on Daphne Major approached the intensity it attains among sea lions, where males play winner-take-all, losers-take-nothing. Year after year, many of the males remained bachelors, while other males mated and fathered many chicks.
Trevor could not tell, by eye, why one finch was winning round after round of sexual selection and others were losing. But after he left the island he entered into his computer the weight, wingspan, beak length, beak depth, and beak width of all the finches on the island in 1979, 1980, and 1981.
In two of those years, 1979 and 1981, the finches were breeding after having survived a terrible time, first the drought of 1977, and then the more moderate drought of 1980. During those breeding seasons, Trevor saw that it was the biggest males with the biggest beaks that were winning mates. The females were choosing the very features that had allowed the birds to pull through those hard times.
In one of the years of his study, Trevor also measured the sizes of the males’ territories and their wealth—how many fruit- and seed-bearing trees there were in each male’s territory. Here too he found a pattern. Males with bigger territories were more likely to win females than males with smaller territories.
Plumage mattered too. For a jet-black fortis male on Daphne Major, the chances of finding a female in 1979 and 1980 were better than fifty-fifty. But for a male that was not yet in full black feather—a male that still looked a little immature—the chances were less than one in three.
Black plumage indicates age and experience, and age and experience can make a difference in the number of offspring a couple can turn out. On the island of Genovesa, for instance, the nests of first-time fathers get dive-bombed by owls significantly more often than the nests of experienced breeders. Females with experienced, jet-black males are more likely to breed early in the wet season than females with first-time males. Experienced couples can sometimes lay not just one but two clutches of eggs before the breeding season is over.
If black males win more females, why don’t all males turn black as fast as they can? Why do some of them turn black in their first year while others linger for years in plumage of such unattractive brown, in spite of the powerful sexual selection pressure that Trevor measured? The huge amount of variation in the finches’ plumage suggests that there are hidden costs to wearing black plumage and hidden benefits to wearing brown.
While the females are flying around checking out each territory, the males are doing battle with all their neighbor males to establish and expand the boundaries of their piece of lava. Since a male in black gets into more fights than a male in brown, the most vigorous males are the black ones with a lot of land.
A male that stays brown a while may be able to avoid getting into so many fights, and set up its territory inconspicuously. “It does seem quite a problem for a male to establish a territory,” Trevor says. “I once saw a young cactus finch wake up and give a few tunes at the corner of an old male’s territory. The old male came for him like an arrow and hit him with his beak in full flight.” Trevor saw one rather battered-looking male that held on to a small territory only as long as he was still in brown; as soon as he turned black, a neighboring male drove him away.
We think of the plumage of birds—the red of the male cardinal and the brown of the female, the green head of the male mallard and the brown of the female—as fixed and permanent, or as constant as anything else in the living world. To us they seem to stay the same as time goes by, like stones in a stream, which stand still hour by hour as the water flows. But while some of the birds’ features are more or less set in stone, like the plan of their bodies—one beak, two wings, two legs—other features, permanent as they look, are the product of perpetually contending forces. They look solid, but they are as fluid as ripples on the stream. They are standing waves that grow or vanish, shift or disappear, with every change in the currents or the rocks.
THIS IS A STRUGGLE of struggles, or war of wars, that Darwin could only imagine, a war in which the forces of sexual selection wrestle with the forces of natural selection, pushing and pulling a living form this way and that, down through the generations. John Endler, the author of Natural Selection in the Wild, has been observing this conflict for years, in one of the most elegant and precise demonstrations of evolution in real time.
What the Grants are to Darwin’s finches, Endler is to guppies. His guppies are not the variety they sell in pet stores (he considers those trash fish). His guppies live in northeastern South America, in the small streams that zigzag down the mountains of Venezuela, Margarita Island, Trinidad, and Tobago, flashing through steep, undisturbed green forests and then the broad spreads of the old cacao and coffee plantations, on their way to the Caribbean Sea and the Atlantic.
The male guppies wear black, red, blue, yellow, green, and iridescent spots in varying sizes, shapes, hues, and combinations. In fact their spots vary so much that they are like fingerprints: no two guppies are alike.
These spots, like the beaks of Darwin’s finches, are heritable. Although the exact placement and arrangement of spots is unique, each guppy inherits its particular palette of colors, and also the general size and brightness of the ensemble, from its parents. The spots only show up on the males (they can be made to appear on the female guppies with testosterone treatments).
Like minute variations in the beaks of finches, the spots on a guppy are the sorts of details that one might imagine are beneath the notice of natural selection. Nature may scrutinize the slightest variation, but there are some things even Darwin’s process cannot see. Design could not possibly govern a thing so small.
In the 1970s, while Peter and Rosemary Grant were watching the finches of the Galápagos, Endler began watching the guppies of Venezuela’s Paria Peninsula, and Trinidad’s Northern Range. There the streams run down the mountains roughly parallel, as if in a series of vertical stripes. The streams are clear, swift, and clean, deeply shaded by tropical evergreens and punctuated by waterfalls. Their beds are lined with brilliant, many-colored gravel, much like the floors of the fish tanks in the pet stores.
It is obvious to anyone who has ever tried to watch a school of these guppies against the parti-colored sands and pebbles of a streambed that the spots are excellent camouflage. In fact you could watch one of these clear streams for quite a while before you noticed the guppies at all, because they tend to swim close to the gravel while the sun is out.
The fish need this camouflage because they have seven enemies: six species of fish and one freshwater prawn. All seven of these enemies hunt guppies from dawn till dusk. The most dangerous is Crenicichla alta, a cichlid fish, which eats about three guppies an hour; the least dangerous is Rivulus hartii, which eats one guppy in about five hours.
Endler found guppies and at least a few of their enemies in almost every section of almost every stream, from the headwaters near the summit of each mountain to the plains and plantations below. Neither the guppies nor the guppy eaters can swim up a waterfall, and the population of each section of stream tends to stay put. (Sometimes a few fish get swept downstream, but none of them can get back up.)
High up near the headwaters of each stream, the only enemy the guppies have is the comparatively mild-mannered Rivulus hartii. But moving downstream, section by section, the population of guppies lives and dies in the company of more and more of its enemies, until down near the base of each mountain, the stream is loaded with all seven of the guppy eaters. So a graph of risk and danger runs with the current. For the guppies, the higher in the stream, the lower the risk; and the lower in the stream, the higher the risk. In stream after stream the intensity of natural selection is graduated in the same way: gentle pressure among the guppies at the top, violent pressure among the guppies at the bottom.
Endler saw that the streams would make a wonderful natural laboratory for the study of natural selection. He developed standardized methods of measuring guppy spots, as careful and ritualized as the Grants’ methods with Darwin’s finches. He learned to anesthetize and photograph each guppy he caught. (Like Darwin’s finches, the guppies have met very few human beings, so they are easy to catch.) From the photographs he recorded the color and position of each spot of each and every male guppy, dividing each guppy into dozens of sectors to make a standardized guppy map that is easy to read, to tally, and to enter into a computer.
When Endler analyzed his surveys he discovered a pattern. The spots on each guppy look chaotic, but the spots of all of the populations of guppies in a stream, taken together, from the headwaters down to the base, have a kind of order. The spots on each population of guppies bear a simple relationship to the number of guppy eaters in their part of the stream. The more numerous the guppies’ enemies, the smaller and fainter the guppies’ spots. The fewer their enemies, the larger and brighter their spots.
The lucky guppies in the headwaters wear sporty coats of many colors, and each color is represented by big clownish splotches. Many of their spots are blue. These blue spots are iridescent, like the Day-Glo patches that cyclists wear; they flash as the fish swim, and they can be seen a great distance through the clear water.
Meanwhile the guppies downstream tend to wear conservative pin dots of black and red. The spots are almost vanishingly small. Most wear only a tiny amount of blue.
Endler looked at his data from stream after stream. In every one of them, the size and number of spots ran steeply downhill. And Endler drew the same sort of conclusion that Lack did when he noticed the patterns of beaks in the Galápagos. Endler thought he could see the hand of natural selection at work among the guppies. The greater the pressure from their predators, the more camouflage they wear; the less the pressure, the slighter the camouflage.
Of course, that interpretation did not explain why guppies are colorful at all. If they are in some danger everywhere, even in the headwaters, then why doesn’t natural selection favor the best-camouflaged guppy everywhere?
The answer is that a male guppy has more to do in life than merely survive. It also has to mate. To survive it has to hide among the colored gravel at the bottom of its stream and among the other guppies of its school. But to mate it has to stand out from the gravel and stand out from the school. It has to elude the eyes of the cichlid or the prawn while catching the eyes of the female guppy.
The gaudier the male, the better his sex life. He is more popular among females, and he gets many chances to pass on his gaudy genes—as long as he lives. In a quiet spot near the headwaters of the stream his life is likely to be long and happy and he may father innumerable gaudy children. But in a spot near the base of the stream he may not father a single guppy before he vanishes down the gullet of a cichlid.
The quieter the colors of a male, the less luck he has in courting females. On the other hand he is likely to have more time to try, because the less he stands out among his own kind, the less he stands out among his enemies.
This is not just a problem for Trinidadian guppies. Wherever males court females, or females court males, whether the signals are a bright splash of color, as in guppies or red-winged blackbirds, or loud far-carrying songs, as in frogs and crickets, their broadcasts are always in danger of being intercepted by the enemy. Strong colors or loud calls can attract a mate from one side and a predator from the other. Every bullfrog calling in the night is in the dangerous spot of Romeo calling out beneath the balcony of the house of Capulet. A few species have found ways to finesse this problem. Among fish, some wrasses change color only very briefly, to flash a sexual signal in dangerous waters—the equivalent of a sexy whisper, pssst!
Looking at his guppy data, Endler read into them a struggle between two contending forces. Everywhere in the stream the gaudier fish produce gaudier young—pushing the next generation toward loud colors and self-advertisement. And everywhere in the stream the quieter fish produce quieter young, pushing the next generation toward modesty. In the relative safety of the headwaters the gaudier guppies live long enough to win many females before they are eaten, so the population evolves in the direction of greater and greater gaudiness, and almost every male wears a coat of many colors. But in the dangerous waters at the base of the mountain the gaudy guppies live such a short time that they are outreproduced by the modest guppies. So the whole population evolves in the direction of greater and greater drabness. Males court females at distances of 2 to 4 centimeters, and from there the little spots are visible; but from farther away the males blend into the gravel. So the small-spotted guppies can blend into the background in the eyes of their predators, Endler says, “yet still be visible and stimulating to females.”
When Endler first began studying the guppy streams, he was in the same position as David Lack after the Galápagos. Endler could see patterns that strongly suggested the forces of selection at work. He did not actually see selection shaping the patterns, but the closer he looked the more he was sure that the hand that shaped the patterns really was the hand of natural selection. Within the broad patterns he kept finding curious subsidiary patterns. For instance in a few of the headwaters there are prawns. In these headwaters the guppies favor red spots. This red shift makes sense because although guppies and other fish see more or less the same colors that humans do, prawns and shrimp are red-blind—they cannot see the last band in our rainbows. So in those particular headwaters, male guppies with big red spots can show off to female guppies while hiding from the prawns.
Back in the 1940s, Lack made his selectionist argument about Darwin’s finches without trying to measure it in the field to see if he was right. But Endler went the extra step: he decided to test the predictions of his theory by trying to detect these processes in action. He built ten ponds in a greenhouse at Princeton University. Four of the ponds were about as wide, deep, and long as the low-water territories of Crenicichla alta. The other six ponds were about the size of the headwater streams with the comparatively mild-mannered Rivulus hartii. Endler put black, white, green, blue, red, and yellow gravel in the bottom of his artificial ponds and pumped water through them to give them a current, like the streams in the wild.
Meanwhile, Endler collected guppies from up and down a dozen streams in Trinidad and Venezuela. In some places he took guppies that lived with just one predator, in some places guppies that lived with two predators, and so on up to the maximum, seven. He wanted stocks of wild guppies that had evolved under the whole spectrum of guppy menace, that were coping in the wild with every level of danger. He bred each stock in a separate aquarium.
When the artificial streams were ready for his guppies, Endler took five pairs at random from each stock and put them all together into two of his ponds to let them breed and mingle and get used to their new homes. Guppies can give birth at the age of five or six weeks, and a female guppy can spawn a lot of baby guppies, so it did not take long for the populations to double. After a month he took guppies from those two ponds and used them to stock two more ponds. A month after that, he had enough guppies to seed each of his ten ponds with two hundred fish per pond.
What he had done, in effect, was to shuffle and reshuffle the deck. He now had a highly heterogeneous assortment of guppies. They had all kinds of spots, and their spots were completely random with respect to the gravel at the bottom of their homes.
He let these guppies breed in their new streambeds for months. Then he added a few of their natural enemies to the streams, according to a careful plan. The evolutionary experiment had begun.
According to his prediction, the guppies should now evolve rapidly. The guppies in each tank should begin to look more like guppies that live with that same set of predators in the wild, they should come to look more like the gravel in their particular stream, and those in the most dangerous tanks should come to mimic the gravel more closely than those in the safer tanks.
After five months, Endler took his first census. He drained each stream, counting every male’s spots and noting their position, anesthetizing them, photographing them, as he had done in the wild, and then starting up the stream again. Nine months later he took a second census. By that time nine or ten generations had passed in the lives of his guppies.
Some of the guppies were safe, with no enemies. These guppies got gaudier between the foundation of the colony and the first census, and they got gaudier still by the time of the second census. The males evolved more and more spots, bigger and bigger spots, wilder and wilder palettes of spots.
Meanwhile males in tanks with the dangerous cichlids evolved fewer and fewer, smaller and smaller spots. They were still visible to females, but they got less and less visible to cichlids, who strike from 20 to 40 centimeters away. These guppies mostly dropped the blue and the iridescent spots, their Day-Glo patches, just like guppies that live with cichlids in the wild. Endler measured these differences as meticulously as the Grants measure finch beaks. “Spot height, spot area, total area, and total spot area relative to body area also decreased significantly with increased predation intensity,” he reports. The fish themselves changed size, too. Full-grown guppies in the dangerous tanks were smaller, while mature guppies in the safe waters were larger—again, just as in the wild.
Each tank had a different bottom: different mixes of gravel colors and different gravel sizes. In the pools with no predators the guppies did not change their spots to match the gravel—the opposite. Their spots evolved to be smaller than the big gravel and larger than the small gravel, making the males easier and easier to see, like chameleons in reverse. They carried more iridescent spots, and a wider palette of colors per fish, and generation after generation they looked less and less like their background, all of which is just what we would expect if they were competing for attention. Sexual selection was operating to make males as different from the gravel bottom as possible.
If only one force or the other had been operating, just natural selection or just sexual selection, the guppies would not have evolved in this remarkable way. Without natural selection all of the fish would have gotten gaudier. Without sexual selection none of them would have gotten gaudier. But the safe ones did get much more colorful, adding, in particular, blue spots. It is probably not a coincidence that guppies’ retinas are exquisitely sensitive to blue. Almost all males carry some blue somewhere, even in the most dangerous waters—it may be a sine qua non of courtship.
The fish had evolved in Endler’s greenhouse until they replicated the patterns that they display in nature, and they had done so in a very short time. Of course, Endler’s streams were artificial. He had not seen natural selection in the wild. A skeptic could still argue that Endler was wrong about his explanation for the pattern in the wild. So Endler figured out a way to run the same sort of evolutionary experiment in nature.
Early in his fieldwork he had found a Trinidadian stream that contained the guppy eater Rivulus hartii, but no guppies. About 2 kilometers away was a second stream that contained both guppy eaters and guppies. Endler took a random sample of about two hundred guppies from one of the high-danger zones in the second river. He measured each and every one, as usual, and then he transferred them to the safe place in the first river. He took a sample of their descendants more than a year later, after a passage of fifteen generations.
The males in the safe stream were now much gaudier than their immediate ancestors, who were still living in the stream next door and coping with many enemies. The immigrant males wore bigger spots, and more of them, and each male sported a wider assortment of colors. Natural selection had acted just as predicted. Evolution had run as fast in the wild as in the greenhouse.
Everywhere in those streams, daily and hourly, natural selection in the form of cichlids and prawns is not just metaphorically but literally scrutinizing the male guppies. The result of enemy predations on each generation keeps pushing the males to blend in with the stream bottom. At the same time, daily and hourly, sexual selection in the form of female guppies is scrutinizing those same males. The result of their choices is that generation after generation of males is pushed to stand out.
Now it is clear why there is such virtually infinite range of variation in the way each individual male guppy is spotted. Many different random patterns of splotches will be equally good camouflage, because the streambed patterns are random too. It would not help the guppies to sport the same pattern as all the others, and in fact it would hurt them. If the guppy males all looked alike, their enemies could develop a search image—an inner template. They would search for that pattern, as we search for the face of a friend in a crowd. The rare misfit would have a great advantage. Meanwhile the females would go for the unusual males, too, and that would drive more and more diversity of patterns. So in this respect natural selection and sexual selection cease to oppose each other and push in the same direction: toward almost infinite diversity.
We see here an example of what Darwin saw in the wide world. He understood that his simple process can lead to the most bewildering and chaotic-looking diversification and variety—but underneath, the driver is as simple and plain and commonsense as ever, “small consequences of one general law leading to the advancement of all organic beings,—namely, multiply, vary, let the strongest live and the weakest die.” The guppy experiments suggested to Endler what Darwin’s finches were suggesting at about the same time to the Galápagos finch watchers: that natural selection can be swift and sure. The process is flowing along, all around us, much faster than Darwin ever dreamed.
Endler’s study is leading him into deeper and deeper waters. He now suspects that the guppies’ spots, their mating habits, and their color vision are all evolving simultaneously, with change in any one of these factors driving change in all the others. To measure variations in the guppies’ retinas, Endler is collaborating with physiologists. These “hard science” types often remind him how “soft” the science of evolution is perceived to be by the outside world—even by biologists. “I was talking with someone in vision physiology the other day,” Endler says, “and he told me, ‘Wow, I had no idea that the subject was so rigorous. I had no idea that you actually did experiments.’
“We have a serious public-relations problem,” Endler says. “People don’t realize this is real science.”
AS TREVOR PRICE’S WATCH DREW to a close, the members of the Finch Unit had logged almost a decade on Daphne Major. When they looked back, they saw this: in 1977, evolution by natural selection had made the birds bigger. In the wet season of 1978, evolution by sexual selection made them even bigger. Then one or the other or both pressures acting together made the birds on the island bigger yet in ’79, ’80, ’81, and ’82. The sum total of all the varied pressures of life on Daphne Major seemed to be pushing the finches in a single direction. At this rate, the birds would go on shooting into the future like a searchlight, growing larger and larger.
So the results of the finch watch presented a paradox. If there is strong selection for large size, year after year, why don’t all the small finches turn into large ones? Why is there a small ground finch, a medium ground finch, and a large ground finch? Why doesn’t every small become a medium and every medium a large? Or were they doing just that—had the course of evolution taken off in a new direction on this island just as the Grants and their students arrived to watch?
It did not seem likely that the trend would keep going like this. Surely the Grants had not begun to watch just at the moment that Darwin’s finches were in the middle of a radical transformation. That would have been like training a new telescope on a distant star and watching it explode into a supernova before your eyes.
Something had to happen soon. The trend had to break. Some force had to whirl down upon the birds and force them back in the other direction.
Trevor Price was positive that with the next heavy wet season he would see something new. In the next solid rain he would detect the pull to oppose the push, the event that would reverse the trend he had seen in the run of dry years.
In 1980 Trevor had two bouts of rain. But that was not enough. It did not trigger much breeding. He needed just one good breeding bout.
Price got as desperate for rain as Boag had been on the watch before him. He paced the island for months on end, waiting for rain. Then he’d come into town—the fishing village of Puerto Ayora on the island of Santa Cruz—half-crazy because it hadn’t rained. He’d be ragtag and barefoot, wearing an old striped shirt, checked pants, a beard that had never known a comb. He had a wild and friendly manner. He spoke atrocious Spanish with an impossibly strong British accent. He would hang out in town for months, making friends and having a good time. Everyone in Puerto Ayora knew him.
But he suffered through each drought convinced in his bones that in one good bout of rain he could make the discovery of his career. The finch watchers posted on the other islands of the archipelago were all rooting for him.
“That’s the difference between field biology and physics,” says one of Trevor’s friends, philosophically. “You can be stuck waiting for rain. You’ve got a beautifully planned research program, and all you need is the rain.”
During Trevor’s last year, 1981, he brought a new field assistant to Daphne Major, a Turkish mathematician named Ayse Unal. She put up with him mooning around for months waiting for the rain. And when rain came at last, in March, she watched him dance in it for two solid hours praising the heavens, a dervish of tangled hair and beard and ripped clothes, hollering into the rain.
But even then it was too little too late. Not many finches bred. Trevor still thinks about that last rain in the small hours, the iguana hours, the hours of what-might-have-been. If only that bout of rain had come in February, or January. If only his watch had lasted just one more year. “It would have been an amazing thesis then.” (“It was an amazing thesis anyway,” says Peter Grant.)
But it was time for another changing of the guard. Lisle Gibbs, a Canadian graduate student of Peter Grant’s, took the next watch. He went down in December 1981 with a field assistant, to be sure to be there when it started to rain. They sat there, two men on a desert island, and waited half a year for the gray clouds to shower on them. Some rain fell—but not enough. “It was like waiting for Godot,” Gibbs says now.
They went home to Michigan. And late in 1982, as Christmas approached, Lisle got a postcard from the village of Puerto Ayora. “It’s raining.”