Although this species of fly “exists both in the East and in the West,” Walsh wrote in 1867, “it attacks the cultivated apples only in a certain limited region, even in the East, for … this new and formidable enemy of the apple is found in the Hudson River Valley, but has not yet reached New Jersey.” He predicted that the flies would continue to spread, and that they might eventually begin to diverge. The haw flies and the apple flies might go on living side by side, yet turn into two separate species, as Darwin suggests in the Origin.
Unfortunately, Walsh died soon after publishing his paper on the apple fly; but the flies did spread as he predicted. They were reported north of the Hudson River Valley, in Vermont and New Hampshire, in 1872, in Maine in 1876, in Canada in 1907. By 1894 they were marching through Georgia, and by 1902 they were in Michigan. They are now eating apples up and down the East Coast of North America and in the Midwest; in the last decade they arrived on the West Coast. The fly also eats rose hips, and sometimes pears and plums, and in the Door Peninsula in Wisconsin it has just begun eating sour cherries. Meanwhile haw flies still eat haws.
Currently most evolutionists regard the possibility of speciation among neighbors as unorthodox, even though Darwin himself proposed it. The standard model of speciation requires geographic isolation. That has been the canonical pattern for half a century, and many evolutionists believe it is the universal pattern. But evolutionists are forever dividing and subdividing into schismatic sects, kingdoms of Either and Or. Do new species arise in archipelagoes, like Darwin’s finches, or do they arise among neighbors? Is the origin of species fast or slow? Is the mechanism natural selection or sexual selection? And so on. None of these questions really have to be framed either-or. It is almost a law of science: the more indirect the evidence, the more polarized the debate. Evolutionists sometimes catch themselves sounding like the Little-Endians and Big-Endians in Gulliver’s Travels, fighting tooth and nail over the proper way to crack an egg. Meanwhile, the more direct the evidence, the less the answers look either-or.
Flies that have switched from haws to apples lay their eggs in an apple while it is still on the tree. The eggs hatch within two days, and the larva grows and grows, hollowing and honeycombing the fruit as it eats. When the apple ripens and falls to the ground, the larvae dig down into the soil. They spend the winter dormant in the ground at the foot of the tree they infested the summer before. By the time the next summer comes, they are ready for another crop of apples.
The flies mate only on the trees where they feed. Early in the season, according to one apple-fly expert, the male flies toward the female and stares at her, face to face, within one or two centimeters; “and then, if the gestalt of visual characteristics suggests that the insect is indeed of the same species, jumps onto [her] abdomen and attempts to copulate.” Often, on top of an apple.
Later in the season, the courtship pattern changes. The males establish territories and defend their turf from other males while they wait for females to alight nearby. Two apple flies sometimes battle each other for possession of an apple. So these flies’ lives seem to be growing more and more entwined with the apple trees. They may be in the act of cutting themselves off from the haw flies and forming a new species, just as Walsh predicted a century ago.
The apple on the left has been attacked by a codling moth, the apple on the right by an apple fly. From Benjamin D. Walsh, “The Apple-Worm and the Apple-Maggot.”
Library, the Academy of Natural Sciences of Philadelphia
A young evolutionist, Jeffrey Feder, is now investigating this possibility at the molecular level. In one study he looked at haw and apple flies in an old, abandoned orchard near the town of Grant, Michigan. He and his coworkers examined six enzymes in the flies. For all six enzymes, Feder found slight differences between the flies on the haws and the flies on the apples. The differences really are extremely slight. In fact, Feder found precisely the same six enzymes in both the haw flies and the apple flies. He also found the same variant forms of the six enzymes. The haw and apple flies differed only in the proportions, the relative numbers, of the variant forms. It is as if human beings on two continents each carried the same set of genes for eye color—blue, brown, black, green—but on one continent blue was more common than black, and on the other black was more common than blue. The gene pools of the two races of flies in the orchard seem to be partly joined and partly isolated. We may be watching the very earliest stages in the divergence of two species.
In a more extensive study, Feder looked at two dozen enzymes in flies from a dozen orchards across the United States and Canada. He saw that all over the continent the gene pools of the apple race and the haw race have diverged in this way.
Apples ripen and fall about one month earlier than haws. When the flies tie themselves to the apples, their feeding and breeding schedules have to shift about a month ahead in the calendar. One month is a lifetime for these flies: it is nearly the entire life-span of an adult fruit fly. So the switch to apples is pushing the two gene pools apart. Haw flies and apple flies are marooned on islands in time rather than islands in space. They seem to be taking the first steps toward speciation, just as Darwin and Walsh imagined.
The same thing must be happening unseen in thousands of other species around the world, because farms offer new islands and niches for insects everywhere, as every farmer knows and regrets. Codling moths too have adapted to North American apple orchards since the moths arrived in the Hudson River Valley two hundred years ago. Other populations of codling moths specialize in Persian walnuts, Swiss apricots, South African pears, California plums. Treehoppers have diverged and branched in six divergent directions, to specialize in six different genera of trees and bushes, including bittersweet, butternut, Viburnum, and Cercis. Treehoppers in each genus of tree do not mate with treehoppers in another tree, even if the two trees grow side by side. The reproductive isolation of the treehoppers is complete.
Feder has also studied a line of flies that looks identical to haw flies and apple flies but infests blueberries and huckleberries. It is a menace in blueberry patches from Nova Scotia to Florida, and as far west as Michigan. If you take some apple flies and some blueberry flies and put them in a jar together, they will mate and produce perfectly normal-looking, healthy, hybrid flies. They appear to be absolutely interfertile in the jar. But Feder has inspected the genes of adult flies collected from neighboring blueberry bushes and apple trees. He finds that their genes are clearly distinct and “species-specific,” peculiar to the apple fly or the blueberry fly. So they hybridize rarely, if at all, in the wild. They are not passing genes back and forth between them. Even where the highest branches of a blueberry bush and the lowest branches of an apple tree are interdigitated, mingling like the fingers of two hands, the two species of flies are still not mixing. Each fly is feeding on the fruit of its kind, and mating true to its kind, as isolated as if the blueberry and the apple flies lived on far-flung islands.
Chapter 17
The Stranger’s Power
We may infer from these facts, 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.
—CHARLES DARWIN,
Journal of Researches
In the Galápagos, large tracts of the highlands of Floreana, Santiago, San Cristóbal, and Santa Cruz have been cleared for cattle pasture. Farmers have planted tomatoes, avocados, guavas, papaya, oranges, lemons, bananas, potatoes, cabbages, coffee, and a fruit known locally as Norwegian pear. On Santa Cruz, which is in sight of Daphne Major, the dirt road up through the highlands and over the summit of the island is full of wide views of these young orchards and pastures, and dotted with small white crosses, like wayside shrines, to commemorate fatal car accidents. Horses, donkeys, cows, and cattle egrets (all of them, like human beings, new to the islands) stand in the fields, through which a few giant Galápagos tortoises still wander or browse in green pond scum w
ith Darwin’s finches riding on their domes. The tortoises help the farmers by eating grapefruit, oranges, and lemons whenever they can, and dispersing the seeds throughout the uplands.
The biggest settlement in the archipelago is the village of Puerto Ayora, on the southern shore of Santa Cruz. Darwin’s finches are the sparrows of the village. In the cafés, while men call greetings to one another, mock-distinguished epithets—“Capitán!” “Profesor!”—the finches hop beneath their feet, looking for crumbs and spills. They hunt and peck in every dooryard garden and bathe in puddles in the middle of the dirt roads, braving the bicycles and pick-up trucks.
The local people call the finches chiques, for the sound. They know the birds are famous, but they don’t know why. They simply call them chiques—all of the finches, from the small-beaked to the big-beaked; the warblers are marias or canarios.
Darwin knew that the finches could adjust their habits quickly to take advantage of the banquets spread by human beings. In the highlands of Floreana he watched the birds stealing seeds from the fields of the Ecuadoran political prisoners. “The Gross-beaks are very injurious to the cultivated land,” Darwin writes in his Ornithological Notes; “they stock up seeds & plants, buried six inches beneath the surface.”
Similarly, Alfred Russel Wallace in an essay on the variability of species tells the story of the kea, a mountain parrot of New Zealand. Before Europeans began raising sheep in New Zealand, the kea fed on the honey of flowers, and on the insects that buzzed around the flowers, rounding out its diet with fruits and berries. Soon after shepherds came to New Zealand, keas began picking at sheepskins hung out to dry and the mutton hung up to cure. Then shepherds began finding sheep with raw and bleeding wounds on their backs. About 1868, Wallace says, some of the shepherds actually saw the parrots attacking their sheep. “Since then,” Wallace reports, “it is stated that the bird actually burrows into the living sheep, eating its way down to the kidneys, which form its special delicacy.” The shepherds meanwhile declared war on the parrots, shooting them on sight. “The case,” Wallace concludes, “affords a remarkable instance of how the climbing feet and powerful hooked beak developed for one set of purposes can be applied to another altogether different purpose, and it also shows how little real stability there may be in what appear to us the most fixed habits of life.”
In the Galápagos the lives of the settlers and Darwin’s finches have almost instantly intertwined. “Finches eat the flowers of the crops,” says Fabio Peñafiel, a young Ecuadoran naturalist and Galápagos guide who lives in the village of Puerto Ayora. “The señora who lives next door to me has had to give up planting pineapple in her garden,” he says, “because of the finches picking in the middle. Cactus finches and tree finches. It’s nibble, nibble, nibble, and no fruit will come out.
“The señora has eaten finches. Yes! She says they have a very mild taste. She makes finch soup. And of course she tells me that! She says, ‘Your finches, your silly finches have eaten my plants—but I have eaten the finches!’
“Highly illegal! Finch soup! The people at the station would just fall on their backs! But she was honest enough to tell me. Imagine what the people up in the highlands are doing, when the finches are eating their crops. And after all, the finch population is probably increasing up there because of those crops. I don’t like sentimentality. Facts are facts.”
At the Charles Darwin Research Station, which is just outside the village, Darwin’s finches hop in the gravel between the low buildings, hunting for seeds among the pebbles and the pocked pale coral. They look down at visiting scientists from the edges of the corrugated roofs and the railings of the dormitory porches. They perch on the arrows along the tourist footpaths as if seeking evolutionary guidance. The flocks of finches are thickest outside the station’s library, where the librarian, Gayle Davies, throws rice to them, wearing a shirt tied at the waist and hair pulled back in a bun, tropical-librarian-fashion. She also feeds the finches on the veranda of her home, a mile down the dirt road from the station. There, she dumps the rice on a hanging tray, while dozens of fuliginosa, fortis, magnirostris, and scandens hop and cling to the wires, cheeping and snatching a few grains right out of her hands.
Darwin’s finches on a tourists’ footpath at the Charles Darwin Research Station, in the village of Puerto Ayora.
Drawing by Thalia Grant
“Since my husband is gone a lot—he goes out to sea very often—it’s nice to have living things around,” Gayle says. “They are there in the morning. They’re there at noon. And they are hopeful all day long. If someone comes to the house when I’m away, it’s like spreading a rumor: all of a sudden there’s a flock.”
She used to keep the rice in an open bowl in the kitchen, and some of the finches, the most confident, would fly right in and feed from the bowl. Most preferred to wait until they were served outside. “But some few who didn’t like to eat with the hoi polloi would come right through the window to the inside bowl and eat in peace, even if the outside bowl was full,” she says.
“This one day, I was inside the house, sitting on the bed reading. The house we lived in then was just one room—the bed doubled as a couch by day. So I’m sitting propped up on pillows, and a finch landed on a pillow right next to my head. And I could see that he had something wrong with his beak. It was pox. Pox shows normally on feet, but sometimes you get a nasty growth on the beak, inside.
“I managed to scrape it off. Then I painted the spot with gentian violet. I tried to help him out.
“I never had a finch do that before—fly right up and look into my face. They cock their heads and look at you when you are feeding them, but this was something very different. It felt—to get thoroughly anthropomorphic—like a tiny cry for help. Of course you’ll never know. It could have been that he could not eat, was basically starving, and I was the source—the one who put out the rice. Who knows? To me, it felt like a ‘help me.’ ”
THE ARRIVAL OF HUMAN BEINGS means a new phase in the evolution of Darwin’s finches, and its directions are still unclear. “On the inhabited islands,” says Peter Grant, “we haven’t done studies of the effects of cats, rats, mice, dogs, goats, donkeys, fire ants, pineapples, bananas, or guavas—and so on. Are they having an effect on the finches? Strictly speaking, we don’t know from our studies. We assume they are,” he says, “but we have observed none of these changes.”
Rosemary and Peter do think they see something odd about the finches of Santa Cruz. The birds around the research station, and in the village, seem to be blurring together. The Grants have never made a systematic study of this: but to their eyes the species almost look as though they are fusing. “They just sort of run into each other,” says Rosemary. There is no difference between the largest fortis and the smallest magnirostris.
“Really, we would need to compare the finches one hundred years ago in that part of the island with what they are now,” Peter says cautiously. But he and Rosemary wonder if the provision of so much water and food in Puerto Ayora may have enabled the birds’ breeding densities to rise around the village. The struggle for existence may have grown less intense. As the growth of the village relaxes the selection pressures on generation after generation of finches, the birds may be turning into a hybrid swarm.
This has not happened on Daphne or Genovesa because, as the Finch Unit has shown, the struggle for existence on those uninhabited islands is intermittently so intense. In hard times the varied finch beaks have such great adaptive value that they are preserved again and again by natural selection. On Genovesa, for instance, late in 1983, after the great Niño, the island was unusually lush and conditions were unusually soft for Darwin’s finches, as they are all year round on Gayle Davies’s veranda. At that time the depths of conirostris and magnirostris beaks on Genovesa were very variable, and the deepest conirostris beak and the shallowest magnirostris beak were close together.
If conditions had continued lush, those finches would likely have continued to fuse. But t
he years 1984 and 1985 were one long dry period. “All the intermediates went out,” says Rosemary. “Died, to be more precise: all the finches whose measurements lay between conirostris and magnirostris died.”
In magnirostris, length and depth went up. But in conirostris they went down. As a result, the two species were divided by the selection episode, says Rosemary, gesturing with her hands as if she were drawing out a lump of clay. “The net result was divergence. You got an actual gap.”
So the Grants speculate that in places like Santa Cruz, where there is little selection against hybrids, some of Darwin’s finches might yet merge. Human beings may have tipped the balance between fission and fusion. Around the villages and farms, many of the celebrated differences in their beaks may simply disappear.
SOME DECADES AGO, the botanist Edgar Anderson wrote a speculative essay, “The Hybridization of the Habitat.” Anderson is the evolutionist who coined the term introgressive hybridization, to emphasize the point that the backcrossing of hybrids with the lines of either of their parents provides a means for the mixing of genes between the two lines, and so may be an important evolutionary step.
In his essay, Anderson argues that the disturbances that human beings are visiting on the planet must be leading to increasing cases of hybridization everywhere, and that thousands of these hybrids and their habitats may prove to be the seedbeds of new evolutionary lines. In his view the crucial evolutionary step comes not with the first hybrid generation, but with the second. “The first hybrid will be uniform in its requirements and on the whole they will be for conditions intermediate between those required for the two parents,” Anderson writes. This is just what the Grants are seeing with Darwin’s finches: the first generation of hybrids is intermediate in beak and body. But from their offspring, surprises may come. “The second generation will be made up of individuals each of which will require its own peculiar habitat,” Anderson writes, and repeats the statement in block letters: “THE SECOND GENERATION WILL BE MADE UP OF INDIVIDUALS EACH OF WHICH WILL REQUIRE ITS OWN PECULIAR HABITAT FOR OPTIMUM DEVELOPMENT.”
The Beak of the Finch: A Story of Evolution in Our Time Page 28
