The Beak of the Finch by Jonathan Weiner
In 2004, researchers identified gene BMP4, one of the key genes that sculpts and resculpts the finches’ beaks under the pressure of natural selection. BMP4 is also the same gene that helps to shape the human face.
“If I have seen further it is by standing upon the shoulders of Giants.” - Isaac Newton
The volcanoes of the Galápagos are Darwin’s shoulders. “Origin of all my views,” he called them. Origin of Origin of the Species.
The Grants do what Darwin could not: they return to the Galápagos year after year. They see what Darwin did not imagine could be seen: natural selection and evolution in action. Their work becomes one of the most valuable animal studies ever conducted. Zoologists and evolutionists regard it as a classic.
To study the evolution of life through many generations you need an isolated population. If you detect a change in the wingspan of a bird, the teeth of a bear, the fins of a fish, or the mandibles of an ant, you want to be able to explain why the change occurred. You want to know the action to which the change is a reaction. For this you need something in nature approximating the simplicity and isolation of a laboratory. Islands are ideal for this purpose.
Two species of Darwin’s finches use tools. They pick up a twig, a cactus spine, or a leafstalk, and they trim it into shape with their beaks. Then they poke it into the bark of dead branches and pry out grubs.
One finch eats green leaves, which birds are not supposed to do.
Another, the vampire finch, found chiefly on the rough, remote, cliff-walled islands of Wolf and Darwin, perches on the backs of boobies, pecks at their wings and tails, draws their blood, and drinks it. Vampires also smash boobies’ eggs against rocks and drink the yolk. They even drink the blood of their own dead.
There is a vegetarian species that knows how to strip the bark off twigs into long curling ribbons like Geppetto’s shavings, to get at the cambium and phloem.
There are also species that perch on the backs of iguanas and rid them of ticks. The iguana invites a finch to perch by assuming a posture that makes it look like a cat that wants to be petted.
“The birds are Strangers to Man and think him as innocent as their countrymen the huge Tortoises,” Darwin writes in his diary.
Darwin was not yet an evolutionist: When the vice-governor of the islands told Darwin that the tortoises varied from island to island, Darwin ignored him. “I never dreamed that islands, about fifty or sixty miles apart, and most of them in sight of each other, formed of precisely the same rocks, placed under a quite similar climate, rising to a nearly equal height, would have been differently tenanted.…”
Darwin said what others were afraid to say. Ideas of evolution already existed. (French naturalist Lamarck. Darwin’s grandfather Erasmus. One of Darwin’s teachers, Robert E. Grant.)
“Did the Finches reach Galápagos from the coast of South America and then diverged from their ancestors, generation by generation? What if there were no limits to their divergence? What if they had diverged first into varieties, and then gone right on diverging into species, new species, each marooned on its own island? “—If there is the slightest foundation for these remarks,” Darwin wrote, “the zoology of Archipelagoes—will be well worth examining; for such facts undermine the stability of Species.” Then, in a scribble that foreshadowed two decades of agonized caution, Darwin inserted a word: “would undermine the stability of Species.”
That was the fabulous moment—not out on the islands but indoors in a cluttered office in London.
Breeders coined the term natural selection
Since breeders called the art of choosing “selection,” they called any changes in a breed that did not take place because of their conscious efforts—all of the casual, frustrating, and inexplicable changes in their flocks and herds behind their backs—“natural selection.”
Which specimens were the true species? Where should he draw the line?
“After describing a set of forms as distinct species, tearing up my MS., and making them one species, tearing that up and making them separate, and then making them one again (which has happened to me), I have gnashed my teeth, cursed species, and asked what sin I had committed to be so punished.” (Darwin’s friends knew just how he felt. The botanist Joseph Hooker wrote Darwin across the top of one letter, “I quite understand and sympathize with your Barnacles, they must be just like Ferns!”).
Each beak is a hand with a single permanent gesture. It is a general-purpose tool that can serve only a limited number of purposes. Woodpeckers have chisels. Egrets have spears. Darters have swords. Herons and bitterns have tongs. Hawks, falcons, and eagles have hooks. Curlews have pincers.
“To look at the bills of these birds in the hand, we would conjecture wholly different diets,” wrote the biologist and explorer William Beebe, who sailed out to Daphne Major through clouds of yellow butterflies in the wet season of 1923.
The Grant team, like Beebe, Lack, and all the rest, had visited the birds in the wet season. But the dry season might be the time to watch life squeeze Darwin’s finches.
Hybrid swarms and meaningless variation, like the coats of stray mutts and cats? (No. The dry season was when the Grants would watch life squeeze the finches, year after year. Not all finches would survive during periods of draught. Tougher beaks might be needed for tougher seeds, for example.)
There is a special providence in the fall of a sparrow. Even drought bears fruit. Even death is a seed.
To crack a grass seed, which is a speck about the size of a poppy seed, takes very little force, less than 10 newtons. A big cactus seed, the size of a peppercorn, takes more than 50 newtons. Cracking the toughest seeds in the Galápagos requires a force of 250 newtons, which is enough force to lift more than a thousand cactus finches into the air. (!)
There are only 24 Seed Types on Daphne Major.
In most parts of the world, one might find two hundred species of plants in a single spadeful of earth.
…on Daphne Major the finch watchers could get almost a God’s-eye view of their flocks, which never went anywhere and never migrated for the winter. And of course when the watchers unrolled their mist nets, adds Ratcliffe, “every bird you caught was a finch.”
Crossing a female horse and a male donkey will never change the gene pools of horses and donkeys by even a single gene, because all mules are sterile. Crossing a male horse and a female donkey will not change their lines much either, because hinnies are small, and less useful around the farm; so farmers seldom let them breed, though not all hinnies are sterile.
Darwin’s followers are always thrilled when they can confirm what Darwin saw, and they are even more thrilled when they can see what Darwin missed. Probably no other major branch of science today is so haunted, dominated, and driven by the thoughts of one man.
Why are there so many different kinds of animals? Or plants?
In the struggle for existence one variety or species must often squeeze another.
The lucky individual that finds a different seed, or nook, or niche, will fly up and out from beneath the Sisyphean rock of competition. Divergence.
Natural selection will have led in effect to another adaptation—the mutual adaptation of two neighbors to the pressures of each other’s existence. And the result of this sort of adaptation would be forks in the road, partings of the ways, new branches on the tree of life: the pattern now known as an adaptive radiation.
Competition among slightly divergent forms everywhere on the planet leads continually to new branches, radiating outward in all directions like a compass rose or the arms of a medieval sun. Darwin called this his principle of divergence.
If the island has only one niche for them, then either the line of the sharp beaks or the line of the small beaks is driven to extinction: one species outcompetes the other. The technical term for this outcome of a battle between two groups of living things is competitive exclusion. But if the island is tall enough to offer one species a new niche, a path out of the race, that species can evolve its way out of the competition. Then it changes in character. The beak bends, melts, morphs, changes shape, through evolution by natural selection, until that lineage of birds is freed from the dreadful war. The technical term for this outcome is character displacement.
Whenever the adaptive landscape heaves and flings about, like a sea under heavy winds, the hybrids among Darwin’s finches will be favored. They will intermingle their genes. But when the landscape returns to the pattern it held before the storm, the birds will settle back to their old peaks, and the sharing of genes will slow again.
The total number of bird species in the world is almost 10,000. Almost 1,000 of them, the Grants write, “are known to have bred in nature with another species and produced hybrid offspring…roughly one out of every ten species.”
The flowers we enjoy so much in the plant kingdom are really sperm throwers and sperm catchers. “As we delight in the strange and exotic beauty of orchid flowers,” writes a British biologist, “it is salutary to reflect that we are, in essence, looking at their genitalia.”
So as the plants’ pollen is swept from one plant to another by winds and insects, hybridization is not only inevitable but also desirable, because so many myriads of seeds will fall and sprout in adaptive landscapes that are different from those of their parents. Here natural selection favors great genetic variability, and hybridization is one way to generate it fast.
To the Grants, the whole tree of life now looks different from a year ago. The set of young twigs and shoots they study seems to be growing together in some seasons, apart in others. The same forces that created these lines are moving them toward fusion and then back toward fission. reticulate evolution
“Maybe we should all be grateful that Mother Nature is a bit slovenly when it comes to reproduction, for this may ultimately permit the unfolding of the bountiful diversity of life on Earth.” - Evolutionist, Robert Holt
“All is flux,” said the Greek philosopher Heraclitus; “everything flows.” The forms and instincts of living things, the invisible borders among them, and the very coasts and landscapes they inhabit are all more fluid and in more flux than even Heraclitus could have imagined..
In the Hawaiian Islands, the lineage of a single finch has radiated into more than forty species, with forty beaks, including seed crushers, bug catchers, nectar sippers.
A Galápagos finch has about one hundred thousand genes, roughly the same number as a human being. The genes are spelled out in a total of about one billion letters, an average of ten thousand letters to a gene. The story is big but the alphabet is small: there are only four letters, named for the four chemical compounds that, as Watson and Crick discovered, make the treads in every spiral staircase of DNA, rather like the leaden letters in a printing press. Their chemical names are guanine, adenine, thymine, and cytosine: G, A, T, and C.
Around the world almost one hundred species of birds are known to have gone extinct since the seventeenth century, together with more than eighty subspecies; more than nine out of ten of these lost species and subspecies lived on islands.
In 1967, a distinguished entomologist announced in Scientific American the discovery of a “resistance-proof” family of insecticides. The poisons were variants of some of the insects’ hormones. How could insects escape their own hormones? Yet within five years, flies had evolved one-hundred-fold resistance. “This seemed to surprise people,” says Taylor. “It would not have surprised an evolutionary biologist. But it surprised pesticide sprayers and the manufacturers of chemical compounds endlessly.”
“It always seems amazing to me that evolutionists pay so little attention to this kind of thing,” says Taylor. “And that cotton growers are having to deal with these pests in the very states whose legislatures are so hostile to the theory of evolution. Because it is evolution itself they are struggling against in their fields each season. These people are trying to ban the teaching of evolution while their own cotton crops are failing because of evolution. How can you be a Creationist farmer any more?”
A simple corollary to Darwinian law. Wherever we aim at a species point-blank, for whatever reason, we drive its evolution, often in the opposite direction from what we ourselves desire.
“Indeed,” note the ecologists Robert May and Andrew Dobson, “the fraction of all crops lost to pests in the United States today has changed little from that in medieval Europe, where it was said that of every three grains grown, one was lost to pests…leaving one for next year’s seed and one to eat.”
Global warming is of special interest here because in these islands the round of the seasons is driven by ocean currents. 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.
Our own tenure has been brief, and on average the term of a species is brief—a few million years. A species that can survive only by causing upheaval around it is in constant danger of extinction, like a tribe that lives for battle.
The rapid accumulation of change is not always progress, and forward motion is not always an advance.
The Book of Life is still being written. The end of the story is not predestined.