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

by Jonathan Weiner

  A superspecies is just the kind of messy, borderline case that fascinated Darwin. Here is a situation in which, as Dobzhansky once wrote, “the process of species splitting has, on our time level, reached the critical stage of transition from race to species. D. paulistorum is one species; it is also a cluster of species in statu nascendi; it bespeaks the correctness of Darwin’s opinion that ‘each species first existed as a variety.’ ”

  Within the paulistorum cluster, Dobzhansky and Pavlovsky explained, female fruit flies will mate freely with males of their own incipient species, but seldom with males of another. That is, an Amazonian female will mate with an Amazonian male but not often with a Guianan, or an Orinocan, or any of the other incipient species in the complex. When flies in the paulistorum cluster interbreed, which they do more often in the laboratory than in the wild, they tend to have fertile daughters and totally sterile sons.

  All these flies look alike. To diagnose an unknown strain, Dobzhansky and Pavlovsky had to cross it with what they called “testers,” a series of flies representing the whole array of incipient species in the paulistorum complex. The unknown strain would yield partly sterile progeny, or none at all, with most of the testers. It would give completely fertile hybrids with only one set of testers: those of its own kind.

  “On March 19, 1958, a sample of Drosophila was collected at Chichimene, south of Villavicencio, in the Llanos of Colombia,” Dobzhansky and Pavlovsky reported in the Proceedings. From these flies the evolutionists reared a strain called Llanos-A. (Llanos are the wide grassy plains that still cover much of South America, as in the days when Darwin rode over them on his long inland wanderings from the Beagle.) Because the Llanos-A strain yielded fertile hybrids with most Orinocan strains, Dobzhansky and Pavlovsky classified Llanos-A as a member of the Orinocan incipient species. The evolutionists kept Llanos-A alive in their laboratory at Rockefeller University, in New York City, as well as strains from all of the other flies in the paulistorum complex.

  Five years later, as part of an unrelated study, the investigators began testing all these strains again. By now, generations of fruit flies had come and gone within their separate population cages. But all of the investigators’ strains yielded the same results as before—all of the strains, that is, but one. “The Llanos-A strain now behaved in a totally unexpected manner; namely, it failed to give fertile hybrids with any strain other than itself,” Dobzhansky and Pavlovsky reported. With Llanos-A, no matter which testers they tried, Dobzhansky and Pavlovsky could not make a single fertile cross. Llanos-A was no longer compatible even with Orinocan strains, the very same strains with which it had produced fully fertile hybrids only a few years before.

  “Conclusions—Llanos-A is a new race or an incipient species having arisen in the laboratory at some time between 1958 and 1963. It has diverged from its progenitor, Orinocan.…”

  No one was watching in the interval, so no one saw it happen. Why did that strain diverge? Dobzhansky and Pavlovsky suspected that the flies had become infected with a bacterium. The infection might have rendered the flies infertile with any strain but their own. This probably is just what happened: infections of that kind have recently been detected and confirmed by drosophilists in several laboratories. The infection sweeps through the population cage fast, like cholera or flu, and it can carve invisible borders around a population, wall it off from all the rest of its kind, like a new and invisible coastline or an invisible cage, virtually overnight. Most of these infections can be cured with antibiotics, some not. In principle the same thing could happen to our own kind, or make all but a small, lonely pocket of the human population sterile, as in Kurt Vonnegut’s wonderful novel Galapagos. If the divergence of Llanos-A was driven by a bacterial infection, then it may have happened literally overnight.

  AS WITH THE ORIGIN OF SPECIES, so with the origin of adaptations. A recent experiment suggests that this process need not be as gradual as Darwin imagined. It also shows how tightly the origin of species and adaptations are linked together.

  The origin of adaptations is, like the origin of species, one of the deepest problems in Darwinism. How do novel adaptations arise from small and gradual beginnings? The ancestral line of Darwin’s finches was all of a kind. One set of finches blew into the islands, and today we have a finch that perches in trees, making and wielding toothpicks against grubs, and another finch that perches on the backs of boobies with its long, sharp beak dipped in their blood.

  How did blind creation make so many new kinds of tools? How do evolutionary inventions and innovations like these get started, if their raw material is merely random individual variations? As one of Darwin’s early critics writes, it is hard “to see how such indefinite oscillations of infinitesimal beginnings can ever build up a sufficiently appreciable resemblance to a leaf, bamboo, or other object, for Natural Selection to seize upon and perpetuate.”

  No one has ever put this problem more forcefully than Darwin himself, in the Origin. “To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree,” he writes. But if we look at the whole tree of life, Darwin says, we can find innumerable gradations from extremely simple eyes consisting of hardly more than a nerveless cluster of pigment cells, which are rudimentary light sensors, to the marvels of the human eye, which are more impressive pieces of work than the human telescope.

  Darwin argues, essentially, that all the sophistications we see in the eagle’s or the human’s eye could have arisen gradually, by stages, across geological spans of time, each stage conferring somewhat clearer vision than the one before. “We must suppose each new state of the instrument to be multiplied by the million; each to be preserved until a better one is produced, and then the old ones to be all destroyed.… Let this process go on for millions of years; and during each year on millions of individuals of many kinds; and may we not believe that a living optical instrument might thus be formed as superior to one of glass, as the works of the Creator are to those of man?”

  Darwin was emphatic that all complex adaptations arise by the gradual agency of natural selection. He even makes the point a sort of test case: “If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.”

  Richard Dawkins defends Darwin’s position vigorously in his book The Blind Watchmaker, which is framed as a reply to the Reverend Paley’s parable of the watch on the heath. Dawkins argues that selection on even the slightest nubbins and rude beginnings can build up instruments as complicated as watches, telescopes, or human eyes. As long as each stage in the evolution of a complex adaptation is adaptive for its own sake, it is likely to be preserved within each generation and be embellished by the next, by Darwin’s process of natural selection. The process does not look ahead. The watchmaker is blind. Yet blind selection can build an eye. Suppose, Dawkins writes, a few nerve endings in a simple organism gave it a rudimentary sense of light and dark. Even rudimentary vision is better than no vision at all. The variant individual that carried that adaptation would be likely to be preserved; and so would those that inherited those first dim eyes. “Vision that is 5 percent as good as yours or mine is very much worth having in comparison with no vision at all,” writes Dawkins. “So is 1 percent vision better than total blindness. And 6 percent is better than 5, 7 percent better than 6, and so on up the gradual, continuous series.”

  The evolutionist Stephen Jay Gould notes that a big change in an organism can sometimes arise from a small change in its genes. So adaptations may sometimes evolve by big steps as well as small steps. Suppose a random change in the genes is sizeable, but not so big that it prevents the animal from breeding with others of its kind. “Suppose also,” Gould writes, “that this large
change does not produce a perfected form all at once, but rather serves as a ‘key’ adaptation to shift its possessor toward a new mode of life.” If it began to make a living in a new way, it would find itself subject to a whole new constellation of selection pressures, just as if it had been picked up by a storm and dumped down on a desert island.

  In his recent book Darwin on Trial, the lawyer Phillip E. Johnson speaks sarcastically of “all this supposing.” “Gould supposes what he has to suppose, and Dawkins finds it easy to believe what he wants to believe, but supposing and believing are not enough to make a scientific explanation,” Johnson writes, adding, “The prevailing assumption in evolutionary science seems to be that speculative possibilities, without experimental confirmation, are all that is really necessary.”

  There is now a simple experimental confirmation of this point. The experiment was published the same year as Johnsons book, 1991. It was carried out by two evolutionists at the University of British Columbia, in Vancouver, working in a borrowed corner of the laboratory of Dolph Schluter.

  There is a genus of finches with peculiar mandibles that cross over at the tips. There are about twenty-five species and subspecies of these birds in North America, Europe, and Asia. They are called crossbills, or crossbeaks. According to legend, they twisted their beaks when they were trying to wrest the nails from Christ’s cross. The red on the breast of the male is the blood of Christ.

  Darwin was intrigued by the beak of the crossbill. In Natural Selection he notes how variable they are “in length, curvature & the degree of elongation of lower mandible.” He notes further that some small curvature of this kind had been reported in many species of birds, as a deformity, and that in good times some of these deformed birds survive.

  The peculiar mandibles of these finches are adaptive. As Lack notes in Darwin’s Finches, there is a small-beaked crossbill that feeds chiefly on the soft cones of the larch, a medium-sized crossbill that feeds on the harder cones of the spruce, and a heavy-beaked parrot crossbill that feeds on the still harder cones of the pine. The twisted beak allows the bird to pry open closed cones. The connection between the beak and the food is so obvious that it was accepted by evolutionists even at the time Lack wrote, when his colleagues believed that most differences between sibling species have no adaptive significance at all.

  The evolutionists Craig Benkman and Anna Lindholm performed their experiment on seven captive red crossbills. The red crossbill lives in coastal forests from Alaska to California, and its beak is specialized for the cones of western hemlock. What Benkman and Lindholm did was to uncross the beaks of these birds by trimming the crossed part of the mandibles with an ordinary nail clippers. This did not hurt the birds, because they have no nerve endings in their beaks: it was as painless an operation as trimming claws or fingernails.

  Three crossbills and their special interests. Top, a parrot crossbill with a pinecone. Middle, a common crossbill with a spruce cone. Bottom, a two-barred crossbill with a larch cone. From Ian Newton, Finches. Courtesy of HarperCollins Publishers Limited.

  Library, the Academy of Natural Sciences of Philadelphia

  The birds with uncrossed bills turned out to be just as good as ever at extracting seeds from dry, open pinecones. But they could no longer handle closed cones. Of course they still tried, the way a declawed cat will still try to climb a tree, but with their straightened beaks they could not get anywhere. Day by day, as the twist in their beaks grew back, the birds did better and better with more and more recalcitrant cones. After a month, their beaks were completely re-grown, and they were back in business.

  What is striking about this little experiment is that Benkman and Lindholm could measure the value of an adaptation from its very beginnings to its final form. If crossed mandibles were useful to these birds only when fully formed, then it really would be a puzzle how they could have arisen by natural selection. The cross would have to appear all at once, as what the geneticist Richard Goldschmidt called “a hopeful monster.” It would be the kind of problem before which Darwin felt his theory would “absolutely break down.” But the finches began to get better at opening pinecones when the cross in their beaks was still too small to be visible to the eye. Even a slight crossing of the mandibles confers a small, incremental benefit, making more and more tightly closed cones accessible. So it is easy to see how the crossbill’s crossed bill could have arisen gradually, by selection, over generations, each generation doing a little better with closed cones than the generation before. The press of competition in the woods would have made the novelty of a crossed beak more and more desirable, because it would allow its possessor to eat food no one else could eat; the same competitive pressure would favor each new twist. New worlds kept opening around the birds: pinecones, spruce cones, hemlock cones, fir cones. Today, however, there is no profit to a sparrow or bunting in a deformed, twisted bill, because the crossbill niche is taken.

  One new adaptation opened a new way of life, and led to a host of other adaptations, including refined instincts for cone hunting and strong muscles to operate the peculiar beak. Today these crossbills are so skilled at opening cones and so committed to their twisted way of life that they eat almost nothing else. In good years their specialty sets them apart and gives them a diet that is closed off from all the other birds in the woods. But when a crop of pinecones fails, they often starve to death.

  A small, simple variation in a beak has led to an adaptive radiation that is, counting by the number of living species and the number of individual birds, much larger than the divergence of Darwin’s finches. The finches in the forests did not have to discover a new archipelago. Two roads diverged, they took the road less traveled by, and that has made all the difference.

  IT WOULD BE EVEN MORE SATISFYING to watch the process of divergence in action, in real time, from beginning to end. “Wouldn’t it be lovely,” Dolph Schluter asks, “if you could actually introduce fuliginosa to Daphne—and track the changes for the next one hundred years? That is inconceivable, of course. Or, alternatively, if you could remove fuliginosa from islands that have both fuliginosa and difficilis, and track the evolution of beak size for the next one hundred years and more. That would be a wonderful thing!”

  No one will ever do that, because no one can wait one hundred years, and also because the finch watchers and the directors of the National Park Service of Ecuador have too much respect for the birds. As Dolph puts it, “These populations are so unique that we have no desire at all to mess with them.”

  So Dolph has begun a new study, not far from Mandarte Island, in the Strait of Georgia. The Strait of Georgia is full of three-spined sticklebacks. These sticklebacks live along most of the ocean shorelines of the Northern Hemisphere. They swim into the mouths of rivers and streams, thousands upon thousands of inlets and arms of the sea, wherever the tide of salt water meets the current of fresh water, to breed.

  Sometimes these small salt-water fish ended up invading fresh water and coming to stay. In southwestern British Columbia, as the ice melted from the land at the end of the last ice age, sticklebacks swam into some of the new glacial lakes at the edge of the sea, and got stranded there. “All of the lakes are younger than 13,000 years. That sounds like a long time, but its—lightning,” Dolph says, laughing. The lakes were cut off from the sea about 12,500 years ago. For 12,500 years now the fish have been evolving in those lakes like finches marooned on islands.

  Most of the lakes in that part of British Columbia hold just one species of stickleback apiece. But in the last few years, Schluter and John Donald McPhail (a longtime student of sticklebacks) have discovered that lakes on the islands of Texada, Lasqueti, and Vancouver each hold a pair of species.

  These pairs are so new to science that they have not yet been named, but they fall into two general types. In each pair, there is a species that feeds along the bottom, and a species that feeds above it, up in the water column. Dolph and others call the two kinds of fish “benthics,” from the Greek word ben
thos, the depths of the sea, and “limnetics,” from limnos, marsh, which by modern usage refers to life in the open waters of lakes, away from the shore.

  These are wilderness lakes, and the sticklebacks in them are almost as free and wild as the finches of the Galápagos. The finches’ only regular enemies are owls and hawks; the sticklebacks’ only enemies are cutthroat trout. So what Dolph has here is a whole new set of test tubes for the same experiment.

  “I want to do it again!” he says.

  Dolph has looked at the pairs of sticklebacks in Enos Lake, on Vancouver Island; in Hadley Lake, on Lasqueti Island; and in Paxton, Priest, and Emily lakes, on Texada Island. The largest of the lakes is 44 hectares, which is about the size of Daphne Major, and the smallest is 5 hectares, about the size of Darwin’s old property at Down House, not counting the Sandwalk. For a few years now Dolph, Don McPhail, and their team have been sampling these lakes with minnow traps and seines, and measuring the sticklebacks. The team measures body length, body depth, and width of the gape, and the length and number of gill rakers.

  The gill rakers are finger-like digits that filter the food the fish swallows. Their size is strongly related to the size of the fish and the size of the food it can eat. Coming from the Galápagos, Dolph says, he likes to think of the gill raker as the beak of the fish.

  Like Darwin’s finches, these fish are variable; and like the finches, two of their characters are extremely variable: their body size and the length of their gill rakers.