The Beak of the Finch
By Jonathan Weiner
Knopf, 321 pages, $25
"Darwin's finches," the 13 species of small, brown-black birds that live in the Galapagos Islands, "
The Beak of the Finch
By Jonathan Weiner
Knopf, 321 pages, $25
"Darwin's finches," the 13 species of small, brown-black birds that live in the Galapagos Islands, "look so much alike," Jonathan Weiner tells us in "The Beak of the Finch," "that during the mating season they find it hard to tell themselves apart. Yet they are also spectacularly and peculiarly diverse." And as we have all learned-from a story that has passed from science to myth-it was this paradox that inspired Charles Darwin's earliest thinking on the theory of evolution.
Weiner draws the title of his book from the salient feature that distinguishes one species of Galapagos finch from another. The variably-shaped beaks of the 13 different finches have been compared to a tool set that contains everything from needlenose pliers to heavy-duty lineman's pliers. For closely related species that must share an island and its available food sources, this diversity appears to make an ideal case for evolutionary specialization: one species of finch eats cactus flowers, one green leaves, one picks ticks off the backs of iguanas and one, the vampire finch, actually drinks the blood of other birds.
In the century and a half since Darwin first visited the Galapagos, the finch's beak has come to embody evolution. Yet, as creationists are quick to note, evolution remains a theory-one that is rejected by nearly half of the people in the U.S., according to a 1990 survey.
When Peter and Rosemary Grant, evolutionary biologists at Princeton University, arrived in the Galapagos in 1973, they faced head-on the prevailing belief that evolution cannot be observed firsthand-thus, cannot be proved-because the process moves too slowly, occurring over tens of thousands of years. Darwin himself said so. The Grants' intensive, meticulous research-from counting seeds to crunching numbers on their computers-has turned this basic assumption about evolution upside down.
The Grants and a succession of dedicated research associates have focused their study on the three species of ground finches that live on Daphne Major, one of the least accessible and least disturbed volcanic islands in the archipelago. Having trapped, banded, measured, photographed and drawn blood samples from virtually every ground finch on Daphne Major over the last 20 years, the Grants have accumulated an extraordinarily detailed family history of 12 generations of ground finches-nearly 20,000 birds.
The three closely related species of ground finches are all primarily seed-eaters. One species has a large beak, one a medium beak and one a small beak, which suggests that each feeds on different kinds of seeds. After their first season of finch-watching, during which they measured the hardness of every type of seed on the island, counted thousands of seeds in study sites and watched the medium ground finches eat 42,582 meals, the Grants found that most of the finches were eating the same seeds-an unpleasant contradiction to what evolutionary biology told them should be the case.
That was during the wet season, however, when seeds were relatively plentiful. When the Grants returned to Daphne Major during the dry season, they watched the seed supply dwindle until the finches were often competing for one particular difficult-to-crack seed. The species with the largest beak had the least trouble with it. The species with the smallest beak found it impossible.
Most interesting were the finches with the medium-sized beak, Geospiza fortis. Within the species, the size of the beak varied somewhat from one individual to another. Those whose beaks were 11 millimeters long could crack the seed; those with beaks of 10.5 millimeters could not. That tiny difference-half a millimeter-could determine which birds would live and which would die. Yet nine out of 10 of the fortis finches survived the dry season to reproduce. There were just enough seeds to go around.
Then, in 1977, the Grants suddenly were witness to evolution as it occurred. A terrible drought dramatically reduced the seeds available to the finches; 85 percent of the fortis finches died. Those that survived were the individuals with the largest beaks. By the following season, that trait was passed on to the next generation of fortis finches, pushing the species one quarter of the way toward the 