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

by Jonathan Weiner

  He drops it in the bottom of a clear plastic vial. It looks out of place lying there, surrounded by the whirling centrifuges and chirping Geiger counters of the laboratory, like an African mask on the wall of a sleek modern art gallery. Taylor fusses with his preparations. “It’s very—hmmmm—tedious,” he mutters. He picks up a glass rod, to grind a bit of the moth in the bottom of the vial.

  “Not yet resistant to the pestle,” he says.

  Chapter 19

  A Partner in the Process

  Creation is not an act but a process; it did not happen five or six thousand years ago but is going on before our eyes. Man is not compelled to be a mere spectator; he may become an assistant, a collaborator, a partner in the process of creation.

  —THEODOSIUS DOBZHANSKY,

  Changing Man

  In the last decade, Rosemary and Peter have witnessed two of the most extreme years of the century in these islands: the wettest and the driest. In the wettest, more rain fell in a single day than falls in a normal wet year. In the driest, not a single drop of rain fell. As Peter says, you can’t get a drier year than that.

  At any other time, the flood and the drought would have been seen as acts of God or freaks of nature. But today, when the Grants think about those Malthusian years, they wonder.

  “The idea that organisms evolve was transformed during the last century from conjecture to fact,” they write in Noticias de Galápagos, the journal of the Charles Darwin Research Station. “As the present century draws to a close, we are experiencing another transformation. The conjecture that the world’s temperature is gradually rising has become widely accepted as a demonstrated fact.”

  Most earth scientists now agree that during the last one hundred years, the planet’s surface temperature has risen, with hesitations and reversals, by about half a degree centigrade. This now-notorious global warming began around the time of Darwin’s death, in the 1880s, and it became strongly marked by the close of the 1980s, which was, as the Grants write, “undoubtedly the warmest decade of the century.”

  Meanwhile the amount of carbon dioxide in the atmosphere has also risen, with fewer hesitations and no reversals. Other gases have accumulated too. All of them are by-products of human industry and agriculture: carbon dioxide, carbon monoxide, and nitric oxide are billowing invisibly from our fires, smokestacks, and exhaust pipes; methane from our vast fields of cattle, sheep, and rice. The gases trap heat, which is why they are called greenhouse gases, and most earth scientists expect them to trap more and more heat in the next century.

  The forecasts are highly uncertain; the crystal ball is cloudy. “Nevertheless,” the Grants write, “we should be thinking about the implications of global warming for Galápagos.”

  Global warming is of special interest here because in these islands the round of the seasons is driven by ocean currents. Half the year the archipelago is bathed in cool waters, the other half in warm. The cool waters come from the South Equatorial current, and the warm waters from the North Equatorial current. The difference between these currents is often 10° C, sometimes as much as 20°, which is more than enough to bring distinct seasons to the islands.

  If it were not for these alternating currents the islands would have no seasons at all, since they lie exactly on the equator. Nor would they carry their bizarre flora and fauna. It is because they are a meeting place of waters from north and south that the passenger list of the islands is so diverse, including not only tropical lizards but also fur seals, not only tropical flamingos but also penguins—the only penguins on the equator.

  A warming of even half a degree centigrade can cause a change in global circulation patterns, and make trade winds and ocean currents veer away on new tracks. Because they depend on winds and currents for their very seasons, the Galápagos Islands are particularly vulnerable to changes like these. That is why the finch watchers on Daphne Major always put quotes around the word “typical” when they describe a typical wet season or dry season. The currents are so variable that no two years are alike.

  What is more, the Galápagos lie near one of the key pressure points in the global circulation system: the birthplace of El Nino. The arrival of El Niño every few years has the effect of heightening and lengthening the contrast between the warm bath and the cool bath. Though no two years are alike, the Niño years are more different than the others. Each Niño turns life upside down coming and going.

  If the winds and currents and hence the seasons were not so variable, the Galápagos finches would not need such variable beaks. Indeed it must have been a freak of these variable currents that first swept the finches to the islands. These winds and currents helped make Darwin’s finches what they are and they are still shaping the finches today.

  It is no exaggeration to say that a lasting change in the ocean currents—especially a change in the intensity or the frequency of El Niño—would change the course of evolution in Darwin’s islands. And the islands sit in a spot where even a slight global warming could make an enormous and early difference.

  A few years ago, when they got back from Daphne and began their annual catch-up with the world’s news, Rosemary and Peter were interested to read a short article that had appeared in the journal Science. The article was written by a climatologist at the U.S. National Oceanic and Atmospheric Administration, Andrew Bakun, who described the implications of his study as “uncertain but potentially dramatic.”

  If present thinking about global warming is correct, Bakun argued, and Earth’s atmosphere is growing warmer, then the surface of the planet should be responding unequally. The land areas should be warming faster than the sea. (In the seas, cold water is always being churned up from below.)

  If the continents are warming faster than the seas around them, Bakun reasoned, then the contrast in temperature between coastal lands and coastal waters should be increasing. One consequence might be an acceleration of offshore winds, because such winds are driven precisely by this difference in temperature between the coastland and the sea.

  Bakun collected wind-stress records for the northern and southern coasts of Peru, and also for California, the Iberian Peninsula, and Morocco. He found that along every one of these coastlines, wind-stress has risen significantly since mid-century. The waters off Peru, the birthplace of El Niño, present an extreme case, according to Bakun; in the middle of this extreme case, of course, sit the Galápagos Islands.

  So the freak weather the Grants have seen in the past ten years may have been something more than chance. It may, just possibly, have been influenced by global warming, and if so it may be only the beginning.

  IN THE Origin Darwin asks readers to imagine, hypothetically, “the case of a country undergoing some slight physical change, for instance, of climate.” If the country were part of a continent, its borders might be flooded with immigrants, Darwin writes, with endless trains of evolutionary consequences. Even on a lonely island the inhabitants would adapt to the altered conditions, “and natural selection would have free scope for the work of improvement.”

  No islands are too remote for these experiments, not even the Galápagos. In fact, with Darwin’s islands once again nature seems to have arranged a particularly dramatic case, a demonstration of the power of slight physical changes to push Darwin’s process in surprising directions. A rise in global temperature of half a degree centigrade is probably less than what Darwin has in mind when he speaks in this passage of “a slight physical change.” A rise in carbon-dioxide concentration of one part per million per year might have seemed to Darwin almost too slight to consider, too small to propel his process. But changes even as small as these can propagate through Earth’s climate system and through Darwin’s “web of complex relations” until they literally change the face of the world.

  The Grants’ argument about the fission or fusion of Darwin’s finches, for instance, depends on the islands’ climate staying more or less as they have seen it in the last twenty years. That is, when they project the fate of D
arwin’s finches, Rosemary and Peter have always assumed there will be no net change in the seasonal cycle of the archipelago. They have assumed that the pendulum of ocean currents will go on swinging annually but erratically between wet seasons and dry seasons. But this is no longer a safe assumption, the Grants write: “Global warming alters the argument.”

  If global warming did give birth to the Wild Child of 1982, then more warming, if it comes, may well bring more Niños like that one. The Niño of 1982 has often been described as the strongest of this century. Some climatologists now believe it was the strongest of many centuries, perhaps the strongest in the second half of this millennium.

  Before that flood year, on Daphne Major, Darwinian selection was keeping Darwin’s finches apart and distinct. After the flood, selection pressures on Daphne began forcing the birds together. If global warming does bring more extreme Niños, then the selection pressures on the islands may take a very long time to return to what they were before the flood. The conditions on the islands may not return to what they were for many decades, the Grants speculate, “perhaps a century, in which the prospect of three species fusing into a single population becomes more likely.”

  The small, medium, and large ground finches, those bicycle-pump productions, are extraordinarily responsive to changes in the weather of the islands. If Niños were to begin to come harder and faster in the next century, then it would take only about two hundred years for the finches to fuse together and undo all the evolutionary work that has carved them apart.

  On the other hand, it might not take Darwin’s process very long to separate the birds again: to turn a group of fuliginosa into a fortis, or a fortis into a magnirostris. Trevor Price has calculated that it would take about twenty selection events as intense as the drought of 1977 to turn a fortis into a magnirostris. And if the starting point were not Daphne but one of the islands where fortis is larger, then the change would take only a dozen droughts. “Trevor worked out that in a relatively small time you could get from A to B,” says Peter Grant. “I hadn’t, when we started this work, even thought that would be possible.”

  In other words, in the present climate of the Galápagos, it would take only a thousand years of not unlikely weather to create a new species of Darwin’s finches on the islands. And if the climate were to change and inflict a series of grim droughts or floods at just the right intervals, without missing a beat, it could create a new species in a single century.

  FOR THE MOMENT the link between Galápagos weather and global warming remains speculative. But the case suggests the kinds of Rube Goldberg-like and ultimately unpredictable local events that global warming may inflict in our lifetimes, even on some of the most isolated islands in the world.

  And our power to drive Darwin’s process, like the power of the process itself, is not hypothetical. The industrial revolution was changing environments and, with them, the course of evolution even before Darwin published the Origin. That is the message of the single best-known evolution watch in history.

  In 1848, a lepidopterist in Manchester by the name of R. S. Edleston put a pin through a moth, a rare form of the species Biston betularia. The normal form of the moth was whitish in color, with a peppering of fine black lines and spots. But Edleston’s specimen was almost as black as coal, from which it got its scientific name, carbonaria.

  Collections feed on rarities. In the second half of Darwin’s century the black moth was prized by every butterfly and moth hunter in the British Isles. It was so ardently sought after that evolutionists in our century have been able to use Victorian lepidopterists’ records and collections to trace the spread of the black mutant across England. In 1860 a specimen of carbonaria was netted in Cheshire, in 1861 in Yorkshire, in 1870 in Westmorland, in 1878 in Staffordshire, in 1897 in London. Soon after the first sighting the black mutant became more common in each of these places, until the white form at last grew rarer than the black.

  Black mutants conquered the Continent too. In 1867 a pair of them were caught copulating on an elm tree in the Netherlands, in the province of North Brabant. In 1884 the black mutants were reported in Hanover, and in 1888 in Thuringia. From there they seem to have made their way up the Rhine Valley.

  The black mutants swept up through the moth populations wherever the air was black with the soot of the industrial revolution. Their numbers did not rise in rural parts of Cornwall, Scotland, and Wales. In rural Kent, Darwin’s adopted county, the black form of the moth was not recorded during his lifetime; but by the middle of this century, nine out of ten Biston betularia were black in Bromley, and seven out of ten in Maidstone.

  Manchester, of course, was one of the grimy hubs of the industrial revolution. At about the time that Edleston pinned his carbonaria there, the novelist Elizabeth Gaskell was describing a family’s first sight of the city as they rode in on a train: “Quickly they were whirled over long, straight, hopeless streets of regularly built houses, all small and of brick. Here and there a great oblong, many-windowed factory stood up, like a hen among her chickens, puffing out black ‘unparliamentary’ smoke, and sufficiently accounting for the cloud which Margaret had taken to foretell rain.”

  That black cloud was “unparliamentary” because air-pollution laws had been passed even then. But the laws had no bite. Soot was blackening the trees around Manchester, and around every other dark mill town in England. In the twentieth century, experiments by H. B. D. Kettlewell of Oxford showed that this soot made a mortal difference to moths that landed on the trees. Kettlewell filmed hedge sparrows, spotted flycatchers, yellowhammers, robins, thrushes, and nuthatches eating moths. On the pale bark of rural birches and beeches the birds were quicker to spot the black moths. But on blackened bark around cities, the birds were quicker to pick off the white moths.

  The difference between the black form and the white is a single gene. Before the industrial revolution the black form was under strong negative selection pressure and the mutation stayed rare, except in forests with mostly black-barked trees. Factories reversed the selection pressure because the rare moths looked like soot themselves. The case of the peppered moth gave evolutionists one of their first inklings of the speed of Darwin’s process. Suppose in the year they were first reported, 1848, the incidence of black mutants around Manchester was one in a hundred (almost certainly a generous estimate). Fifty years later, in 1898, ninety-nine out of a hundred were black. From these figures, the British evolutionist Haldane calculated that the advantage of each black moth over each white moth must have been running as high as 50 percent throughout the second half of Darwin’s century. That is, generation after generation, a black moth was 50 percent more likely than a white one to pass on its genes.

  In the middle of the twentieth century, however, Britain enacted strong clean-air legislation. The city air began to clear, and so did the bark of the trees outside the cities. By 1966, Manchester, one British Biston expert has written, “was still distinctly satanic in appearance but was being cleaned and rebuilt.” As environmental legislation was passed in the rest of Western Europe, more and more white moths began cropping up again. In West Kirby, in northwest England, where nine out of ten Biston were black in 1959, their incidence was down to five out of ten in 1985, and fewer than three out of ten in 1989. In the Netherlands, in the province of North Brabant, where the first two black mutants on the Continent were seen fomenting revolution in 1867, the frequency of the black mutant dropped from seven in ten to fewer than one in ten.

  Today carbonaria is declining rapidly virtually everywhere in Britain. So are dark forms of ladybirds and dozens of other British insects whose bodies had blackened with the Industrial Revolution. Evolution has reversed itself. Finch watchers have seen it reverse from drought to flood and flood to drought, and moth watchers have now seen it reverse from the industrial to the postindustrial age. At present rates, carbonaria will be as rare as it was before the Industrial Revolution by about the year 2010.

  CARBON DIOXIDE IS really no more e
soteric than soot. It is a product of the same process of combustion, and it comes out of the same smokestacks. But the gas is invisible, and its influence is global rather than local. If it does have the impact on our next century that earth scientists are now warning us to expect, then this global effect makes it far more important than soot: the rise of this gas will prove to be the single most important physical change to take place on our planet in a long time. If present thinking is correct, temperatures in the next hundred years may be higher than they have been in the last several million years, and the change may come as much as ten times as fast as it did in those millions of years, a shocking evolutionary experiment.

  Meanwhile, genetic engineers are quite consciously manipulating and accelerating evolution. Some genetic engineers go so far as to call their work the Generation of Diversity: G.O.D. And this is precisely what they are doing: generating diversity. Their laboratories of evolution are only a few years old, but they hope to grow more prolific than the Galápagos. They are creating new corn and rice, new bacteria, new guinea pigs, a patented Harvard Mouse. With the same tools and techniques they could start re-engineering human beings, if we let them.

  What the genetic engineers and their G.O.D. are doing now can make your hair stand on end. Not long ago, two engineers reported in Nature that they had built two new molecules that fit in the spiral of DNA: new rungs in the twisted ladder, new steps in the spiral stair. That is, beside Darwin’s invisible characters, T, A, C, and G, they had added two new letters to the alphabet of life, which they called K and X. Writing in the same issue of Nature, a colleague of theirs wondered exuberantly how many more letters we can discover (twelve seems possible) and what new messages and new creatures we can compose with this new alphabet.

  Our wild acceleration of life’s central process makes the study of evolution excruciatingly timely. We are changing the environments of life faster and faster, and we are changing their genetic machinery faster and faster. All of this is G.O.D., the Generation of Diversity—as well as the Generation of Destruction. Greenhouse and field studies are being conducted of the forced evolution of oak seedlings, wheat, corn, butterflies, moths, and aphids due to the extra carbon dioxide our species is adding to the air. Microcosm and open-sea studies are being conducted of the accelerated evolution of Antarctic plankton under the ozone hole. A study of plankton at Anvers Island in the Antarctic in 1987 and 1988 found that UV B radiation is penetrating as deep as 20 meters down into the sea beneath the ozone hole, so there is potential for a great deal of damage, and also of evolutionary response. A recent study found that as the ozone hole sweeps by overhead the higher levels of UV-B cut the numbers of plankton in the water by 6 to 12 percent, for as long as they were under the hole. Fortunately the rays did not hit all species as hard as others. The UV-B inhibited the growth of species in the genus Phaeocystis much more strongly than they affected the diatom Chaetoceros socialis. Here we see again the power of variation to mitigate the unforeseen and unforeseeable. The mix of plankton in the sea is extremely variable and so is its vulnerability to UV-B. This variation is helping strains of plankton and krill in the Southern Ocean to evolve and adapt in the invisible spotlight beneath the ozone hole.