The Beak of the Finch: Chapter 10 Summary & Analysis

Darwin himself wondered, through his private writings, if the different strains of life in the Galápagos diverged by adapting to their isolated new surroundings. But Darwin’s finches—at least in the modern era—aren’t marooned: there is constant traffic of visiting finches between islands. While the birds might have started their divergences in isolation, they’re not isolated now. Darwin’s writings hold an answer to the question of what transpires when divergent lines of life meet up again after a long period of time.

Darwin believed that as natural selection acted on species that were still in the process of formation, it would drive wedges between them and push them apart. In other words, two varieties living side by side are in constant competition with one another—like the contest that’s happening between ground finches for similar resources. In the struggle for existence, it’s hard for two variations that are so similar to both survive. Any escape from this oppressive competition—a variety that finds a different seed or a new niche of territory—will get out from under that competition and begin to thrive and flourish. Eventually, the competition will slack off once the varieties have diverged. This process, Weiner writes, is known as “adaptive radiation.”

Darwin himself never saw this process happen—but by studying pigeons, he came to believe that efficiency drove diversification. In other words, he discovered that natural selection organizes life by creating divergent species that fill separate niches.

This theory of adaptive radiation, though, remains one of the “most contested questions” in Darwin’s entire body of work—even the Grants, observing competition between the finches on the Galápagos, struggle to measure competition amongst species that are no longer actively competing with each other. But David Lack, who published Darwin’s Finches in the middle of the 20th century, recognized that certain islands in the Galápagos don’t hold two species of ground finch—he believed this was the “aftermath of a great war” across the archipelago. Lack’s theory is that the species realized, over time, that when they were on the same island they were thrown into competition—so they began to seek other niches on other islands.

An American ornithologist named Robert Bowman visited the Galápagos in 1952—and after observing the contents of dead birds’ stomachs, he found that many different species of finches were feasting at the same source. Bowman didn’t believe that “the absence of competition is a proof of its power”—in other words, he wasn’t certain that competition between species was enough to push them into such different niches.

Yet the Grants observe, each year as the rains on Daphne Major end, how the finches move apart and specialize based on the level of competition with one another. Each year, in times of drought, the finches are, through specialization, “making room for one another”—which allows them to coexist on the island continuously. Even though in such cases it their behavior that is changing and not their anatomy, that change is still evidence of selection at work.

The Grants, and researchers like them, are coming to understand that the hierarchy of nature—the “food pyramid” or food chain—isn’t necessarily fixed. Within each level there are specialists, called “guilds”—leaf eaters, stem borers, root chewers, nectar sippers, and so on—and these guilds are frequently in flux. Harder times create smaller and smaller guilds, and the many pressures (like competition for food and mates) that are acting on guilds in tough times push those guilds further apart with each passing year.

Dolph Schluter, one of the Grants’ former mentees, is one of the foremost researchers on Darwinian divergence in the modern era. While studying with Peter Grant, Schluter wanted to focus on the war between small-beaked and sharp-beaked finches on the island of Pinta. On Pinta, the small-beaks’ and sharp-beaks’ territories overlap—but their diets hardly do. Schluter found very little competition between the birds. He was disappointed in his inability to find competition between the finches to study, so he moved to several other islands, mapping how competition drove up beak size and influenced how the finches ate. He ultimately found that the small beaks and sharp beaks essentially divvied up the islands of the archipelago between themselves—they were staying out of one another’s way.

After a few years, Schluter had come up with a new way of visualizing Darwinian pressures on the finches. Adaptation and specialization, Schluter suggested, was a process similar to a long climb up to a mountain peak of “maximum fitness.” A bird blown to a new island will hardly ever land at the top of this metaphorical peak right away because it is unlikely to be perfectly fit for that new environment. But if it lands somewhere on the side of the mountain rather than down in the valley at its foot (as it will likely die if it is so unfit as to be at the bottom), it can begin to climb, and its descendants will continue the journey until they reach their “adaptive peak”.

Schluter used his data to run experiments through a computer back on the mainland. By entering ranges of beak sizes available to the ground finches on the islands, as well as the range of seed sizes and their numbers, he calculated how many finches a hypothetical island could support—then he made the calculation again and again. The computer drew three peaks, with many deep valleys between them. The computer had, in effect, predicted the existence of three species of finches, each with a beak precisely the right shape and size to crunch the seeds available to it. There could be three adaptive beaks for ground finches on the island—which means that a single species arriving in a new land could, and likely would, diverge into three.

Schluter has applied his research to different species of plants and animals throughout the Galápagos—he thinks that even distant branches in the trees of life are competing, each day, throughout the archipelago. Bees and birds are in competition for the nectar of certain flowers on certain islands; but on islands without bees, the finches have made themselves smaller to fill the bees’ niche. Peter Grant believes that divergence is a consequence of the struggle for existence.