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

by Jonathan Weiner

  But now the flood of fresh genes is pouring in at many times that rate, and the rate is still rising. The triumph of the mixed-bloods on Daphne implies that the DNA of Darwin’s finches is becoming more rich and strange than anyone ever imagined. The hybrids are passing their motley collections of alien genes throughout the quasi-separate species on Daphne. It is as if the finch watchers were collecting the manuscripts of not one but thirteen young voyagers, all of whom are still writing and still learning to spell, and many of whom are copying from each other, with sentences and even whole paragraphs and pages flying back and forth.

  All this flying DNA is rapidly raising the finches’ variability, like the SOS response of E. coli in a Petri dish. It is even possible that the influx of new genes is increasing the birds’ mutation rates. Such an effect has been observed in laboratories, in the DNA of hybridizing Drosophila.

  Times are changing: the birds are under stress, and they are eroding and breaking down their invisible coasts, taking in new genes, growing more variable. The flood of 1982 has set off a flood of genes, a sharing of secret messages, a flying shuttle of invisible characters, a wild mixing party, a huge upsurge in the amount of hidden variation in Darwin’s finches.

  When the first finches alighted in the Galápagos millions of years ago, the stress of settling the young volcanoes must have been extreme. The intensity of natural selection, the rate of interbreeding, and the speed of evolution must all have been extraordinary. Now something rather like that seems to be happening again.

  Chapter 16

  The Gigantic Experiment

  Man therefore may be said to have been trying an experiment on a gigantic scale; and it is an experiment which nature during the long lapse of time has incessantly tried.

  —CHARLES DARWIN,

  The Variation of Animals and Plants under Domestication

  From his first hour on San Cristóbal, when he shoved a hawk off a branch with the muzzle of his gun, and saw a finch swatted dead with a hat, Darwin knew that the birds of the Galápagos had no experience with human beings. When he mentions the Galápagos birds in his journal, it is almost always to exclaim at their appalling innocence.

  “I saw a boy sitting by a well with a switch in his hand, with which he killed the doves and finches as they came to drink,” Darwin writes, after visiting the prison colony on Floreana. “He had already procured a little heap of them for his dinner; and he said he had constantly been in the habit of waiting there for the same purpose.”

  “One day,” Darwin writes, “a mocking-bird alighted on the edge of a pitcher (made of the shell of a tortoise), which I held in my hand whilst lying down. It began very quietly to sip the water, and allowed me to lift it with the vessel from the ground. I often tried, and very nearly succeeded, in catching these birds by their legs.”

  Darwin had been reading the second volume of Lyell’s Principles of Geology, which he received in his mail at the port of Montevideo. The whole volume is an argument against the possibility of evolution. But Lyell does write about the changes that an invader might bring to an island. When the first polar bears landed in Iceland on an iceberg, for instance, the chaos must have been “terrific,” Lyell argues. The bears must have feasted on the deer, the foxes, the seals, even some of the birds. With fewer deer on the island, the local plants would have prospered, and also the insects that fed on the plants. With fewer foxes, the ducks would have multiplied, thinning the schools of fish. In this way “the settling of one new species,” Lyell wrote, might alter “the numerical proportions of a great number of the inhabitants, both of the land and sea … and the changes caused indirectly might ramify through all classes of the living creation, and be almost endless.”

  For Darwin, the tameness of the Galápagos birds became a symbol of Lyell’s lesson. That is how Darwin closes his chapter on the Galápagos in his Journal of Researches, after describing the tameness of the birds. “We may infer from these facts,” he writes, “what havoc the introduction of any new beast of prey must cause in a country, before the instincts of the aborigines become adapted to the stranger’s craft or power.”

  Darwin elaborates the point in a famous passage in the Origin, a passage that helped establish the science of ecology. A “web of complex relations” binds all of the living things in any region, Darwin writes. Adding or subtracting even a single species causes waves of change that race through the web, “onwards in ever-increasing circles of complexity.” The simple act of adding cats to an English village would reduce the number of field mice. Killing mice would benefit the bumblebees, whose nests and honeycombs the mice often devour. Increasing the number of bumblebees would benefit the heartsease and red clover, which are fertilized almost exclusively by bumblebees. So adding cats to the village could end by adding flowers.

  For Darwin the whole of the Galápagos archipelago argues this fundamental lesson. The volcanoes are much more diverse in their biology than their geology. The contrast suggests that in the struggle for existence, species are shaped at least as much by the local flora and fauna as by the local soil and climate. Why else would the plants and animals differ radically among islands that have “the same geological nature, the same height, climate &c”?

  “This long appeared to me a great difficulty,” Darwin writes toward the end of the Origin, “but it arises in chief part from the deeply-seated error of considering the physical conditions of a country as the most important for its inhabitants; whereas it cannot, I think, be disputed that the nature of the other inhabitants, with which each has to compete, is at least as important, and generally a far more important element of success.” When each seed and bird arrived on one of these islands, he says, “it would have to compete with different sets of organisms … and it would be exposed to the attacks of somewhat different enemies. If then it varied, natural selection would probably favour different varieties in the different islands.”

  Again, Darwin never actually saw this happen. But he concluded that the introduction of a new species anywhere on earth might be an evolutionary event: that small invasions can have far-reaching consequences. The invader itself evolves rapidly as it adapts to its new home; meanwhile, everything already living there either adapts to the invader or becomes extinct. The result is an expanding pulse of evolution and extinction, a general acceleration of the pace of change.

  In the past, the pace of travel was generally slow. The finch had to wait for the freak wind. The polar bear had to wait for the iceberg. The union and disunion of continents were limited by the pace of continental drift, which is measured in centimeters per year.

  Today invasions on this scale are happening every day, and virtually every spot on the surface of the planet is being assaulted by varied, new evolutionary pressures. Invaders ride in the puddles of old car tires, the bilge water of ships, the pressurized cabins of airplanes; in suitcases, on pant legs, and on mud-caked shoe soles. These are Darwin’s old jelly-jar experiments replayed in earnest. The consequence is an intensification of Darwinian pressures not only in the Galápagos but everywhere we look.

  THE YANKEE EVOLUTIONIST Hermon Carey Bumpus witnessed one small step of an invasion at the close of Darwin’s century. On the last day in January 1898, Providence, Rhode Island, was struck by a howling blizzard. The storm blocked the street cars and the railroad trains, snapped the telegraph and telephone lines, and threw sparks from dangling wires onto the shingled roof of Walter H. Willis’s furnishings store. (According to the front-page story in the next day’s Providence Journal, the fire was put out with snowballs by “a crowd of merry makers.”)

  Bumpus was teaching biology at Brown University. His path to work each morning led up College Hill and past the Providence Athenaeum, one of the oldest libraries in the United States. The morning after the storm, as he made his way through the snow, he happened to notice a large number of English sparrows lying dead or exhausted in the drifts beneath the Athenaeum. The sparrows had been wintering in the ivy that covered the library, and the
gale had overwhelmed them.

  These sparrows were newcomers to New England, as Bumpus knew: Old World birds in the New World. One of the first pairs had been released in New York’s Central Park in 1851, the decade before Bumpus was born, by an eccentric bird lover who wanted to import every one of the birds in Shakespeare’s plays to the United States. So the birds were lying in the snow that morning in part because Shakespeare had written, “There is a special providence in the fall of a sparrow.”

  Bumpus collected as many of the sparrows as he could and brought them to the Anatomical Laboratory. In the warmth of the lab, seventy-two of the sparrows revived; sixty-four died. He recorded the sex, body length, wingspans, and weight of both the living and the dead; he also measured the length of the head, the humerus, the femur, the tibiotarsus, the skull, and the sternum. When he tabulated all these results he found that most of the survivors were males, and that among the males the survivors tended to be shorter and lighter than average, with “longer wing bones, longer legs, longer sternums, and greater brain capacity.”

  At the time, English sparrows were multiplying and overrunning the continent. Bumpus believed that “the English sparrow, since its introduction into this country, has found life so easy that the operation of natural selection has been practically suspended, and that the American type consequently has become degenerate.” The birds that had died in the storm were those that had diverged most from the original type. It was an episode of what is now called stabilizing selection.

  In the early 1970s, just before his first trip to the Galápagos, Peter Grant reread Bumpus’s paper and studied the tables of numbers. The paper had long since become one of the most famous studies ever conducted of evolution in action. Peter reanalyzed the data using more powerful statistical tools. He concluded that Bumpus had actually seen not one but two kinds of natural selection. For the female sparrows the storm was stabilizing. The event killed the largest and the smallest but preserved the mean, just as Bumpus had said. In the males, however, the pressure of the storm was directional, pushing the birds toward smaller size.

  The reanalysis of Bumpus’s classic data helped to inspire the Grants’ first trip to the Galápagos. There was a providence in the fall of those sparrows. If you could demonstrate natural selection under those freakish conditions, Peter and Rosemary wondered what they could see under more normal conditions. They might not be able to see it at all.

  Today English sparrows are still evolving and adapting to North America (and to South America, South Africa, Hawaii, Australia, and New Zealand) through Darwin’s process. So are barnacles that invaded the Salton Sea during World War II, riding in with the U.S. Navy. So are fruit flies that invaded Chile in the 1970s. With these species and many others, biologists are now tracking evolution in action, quite confident—as the Grants’ and other studies pile up—that the process can be watched. Each of these creatures, now that we have put it down in a new place in the world, is in the middle of a crisis, an epochal event, like the first finches to land in the Galápagos, and their evolution, like the finches’, is turning out to be rapid.

  WHENEVER WE INTRODUCE AN ALIEN to a new country we also change life for the natives—Lyell’s point in the parable of the polar bear, and Darwin’s in his vision of cats helping flowers. This kind of chain of events is going on everywhere too, and it too is being watched.

  The evolutionist Scott Carroll of the University of Utah is proving the point with the beak of the soapberry bug. This is a bug with a long needle of a proboscis—which is, in effect, its beak. The bug pokes its beak through the walls of fruit, pierces the walls of the seeds, and then liquefies and sucks up the nutrients, as if it were sipping cider through a straw.

  Carroll, who is one of the tallest biologists in the world, has spent many of his working hours in the last few years stooping over soapberry bugs from Yavapai County, Arizona, to Key Largo, Florida. They are New World bugs, rather pretty-looking things. Carroll paints black numbers on their backs to keep track of individuals and watches them eat. At the start of his study, he says, he assumed they would be like little wind-up automata, but they are not: privately he has come to believe as he watches them that they are each different, with personalities. But that is off the subject of his quest.

  In the south-central United States, soapberry bugs live on the soapberry tree. In southernmost Texas they live on the serjania vine. In southern Florida they live on the perennial balloon vine. These are its native hosts; the bug and these plants go back thousands of years.

  Besides these three native plants, the bugs also steal seeds from three species that have been introduced to their turf quite recently: the “round-podded” golden rain tree and the “flat-podded” golden rain tree, which have been imported from Southeast Asia as ornamentals, and the heartseed vine, which grows wild in Louisiana and Mississippi.

  The beak of the soapberry bug is easy to measure, and the meaning of the variations is obvious. Someone once asked Abe Lincoln how long he thought a man’s legs should be. Abe is said to have replied, “Just long enough to reach the ground.” Likewise the beak of the soapberry bug should be just long enough to reach the seeds.

  Now, the fruit of the balloon vine, which is native to South Florida, has a radius of a dozen millimeters. The beak length of the soapberry bugs that feed on it is a little more than 9 millimeters, long enough to reach the seeds, which cluster around the center of the fruit. But the fruit of the flat-podded golden rain tree, which is new to Florida, has a radius of less than 3 millimeters. The soapberry bugs that eat these fruits do not need such long beaks, and accordingly their beaks are growing shorter: they now average less than 7 millimeters long. That is rapid evolution, since the golden rain tree was not planted in any significant numbers in Florida until the 1950s (several thousand generations, for the bugs). With time their beaks are likely to grow shorter yet.

  Meanwhile the fruit of the native soapberry tree has a radius of only 6 millimeters, and the beaks of the bugs that dine on it are 6 millimeters long. But the fruit of the newly introduced heartseed vine is almost 9 millimeters. Soapberry bugs that have switched to heartseed now have beaks almost 8 millimeters long. That is even faster evolution, for heartseed did not become abundant in the territory of soapberry bugs until about 1970. With time their beaks will grow even longer.

  Carroll has also measured specimens of these bugs in state and county natural-history museums. In case after case, he finds a change in beak length at about the right time—that is, about the time the new plants were introduced in their territory.

  In every case the beaks of the soapberry bugs are evolving to obey Lincoln’s rule: just long enough to reach the seeds. And because the dates of introductions of the new plants are known so well, and are so recent, it is clear that this is yet another example of evolution in action around us, with the driver and the driven both plainly in view. Carroll calls this “natural history with the history.”

  DARWIN SUSPECTED that this kind of change in habits might sometimes lead in surprising directions. In the Origin he notes that there are species of British insects that feed exclusively on British farmers’ crops. Meanwhile, the insects’ ancestral line goes on eating the native weeds in the woods and hedgerows around the farmers’ fields. Darwin concludes that “within the same area, varieties of the same animal can long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from varieties of the same kind preferring to pair together.” He thought that many new and “perfectly defined” species might have been formed this way, not on islands but side by side in the fields and hedgerows of rural England.

  One of the Origin’s first readers in the New World was a British graduate of Cambridge from the same year as Darwin. Benjamin Walsh had studied for the church in Cambridge, and like Darwin he had discovered that he preferred bug collecting. Instead of setting up in a country estate outside London, as Charles and Emma did, Walsh and his wife sailed for the United States.

>   In Illinois Walsh “built a mud-plastered log cabin with a fireplace that took a log that had to be rolled in with oxen,” as he later wrote to Darwin at Down:

  I was possessed with an absurd notion that I would live a perfectly natural life, independent of the whole world—in me ipso totus teres atque rotundus. So I bought several hundred acres of wild land in the wilderness, twenty miles from any settlement that you would call even a village, and with only a single neighbor. There I gradually opened a farm, working myself like a horse.…

  By the time Darwin published the Origin, Walsh had caught malaria, lost two farms, built a lumber business, built ten brick tenements, stormed into and out of local politics, and settled back at last into entomology. In 1864 he sent a letter to England:

  More than thirty years ago I was introduced to you at your rooms in Christ’s College … and had the pleasure of seeing your noble collection of British Coleoptera. Allow me to take this opportunity for thanking you for the publication of your Origin of Species.… The first perusal staggered me, the second convinced me, and the oftener I read it the more convinced I am of the general soundness of your theory.

  Walsh applied Darwin’s ideas to what he saw happening in the fields and woods around him. There is a species of native American fruit fly, commonly known as the haw fly, tha lays its eggs on wild hawthorn. In Walsh’s day, farmers had begun planting and cultivating apple orchards in the Hudson River Valley. Some haw flies had left the haws and begun eating the apples.