The Beak of the Finch: Chapter 11 Summary & Analysis

Darwin himself believed that “the possibility of making distinct races by crossing [or hybridization] has been greatly exaggerated.” But the Grants now suspect that selection and crossing work together—that they are part of the same process of species creation. To explain this phenomenon, Weiner turns to the two kinds of shorelines in the Galápagos—visible and invisible. The visible shores are the rocks where volcanoes rise up out of the Pacific—the borders of air, sea, and land. But the invisible shores are the borders that exist between the birds themselves. These boundaries are not so easily seen. The boundaries between the species hold each one apart from the rest—each species is, with the exception of the hybrids, an island unto itself.

Darwin was confused by these invisible coasts between species. He did not fully survey the barriers that exist between them—and what makes those barriers easier or harder to cross. Modern ecologists and evolutionists now know that the isolation of species isn’t just in the visible barriers between them, but the invisible ones. Instinct and behavior hold them apart—that is, the innate instinct to keep a line alive rather than blend with the lines around it, and thus disappear. The process of selection helps animals to abide by these invisible coasts, because the individuals that trespass between coasts will suffer a disadvantage: their offspring will have a harder time surviving.

The finches on Daphne Major have a talent for telling one another apart—even with the many shades of variation within each species. Even though their territories, mating habits, and plumage colors overlap, there are marked distinctions between these behaviors in each species. These variations are passed down, sometimes for good or for ill. For instance, finches that somehow learn the wrong mating calls (by, say, picking up a neighbor’s call) rather than internalizing the mating songs of their progenitors find themselves in trouble, unable to attract mates.

While the number of species of finches in the Galápagos is not eternally fixed, they are, right now, true species—they are distinct. So even while some hybrids breed, the species mostly remain apart. To test why birds rule one another out—and what they’re looking for when they’re seeking mates—Peter Grant and one of his associates, Laurene Ratcliffe, conducted some “ghoulish” experiments using finch corpses. By propping the decoys up and displaying them throughout different territories, they could watch the living finches’ responses. The experiment showed that male finches chose the decoy of their own species—even when the heads and bodies of different species were switched, they gravitated to decoys that had the heads of their same species.

Mating patterns, though, can change and evolve just as quantifiably as the beak of the finch. Experiments with fruit flies bred in total darkness for 14 generations essentially erase the intricate mating patterns that flies usually engage in. All instincts but the instinct to keep the line going evaporate.

Sexual selection is a powerful force in the initiation of the speciation process—and researchers like Ken Kaneshiro, who works with Hawaiian flies (of which there are more than 700 species), are studying how sexual selection drives divergences. The pressures of sexual selection are acting fast on these insects—just as the pressures of sexual selection are changing the finch populations in the Galápagos. A preference for even the smallest details during mating can, over the course of a few generations, cause a major split.

In 1978—the year after the great drought—the Grants began camping near a lagoon on the island of Genovesa. They found that among the cactus finches that surrounded them, there were two groups of singers—A singers and B singers. The As’ beaks were narrow, shallower, and longer than the Bs’—even though the difference was just about a millimeter. The As drilled holes in the cacti to get at the fruit while the Bs ripped and tore fallen cactus pads. The Grants believed the drought had caused a split in the ground finches on Daphne Major—and that it could have caused a split among these cactus species on Genovesa, too.

After observing the cactus finches, the Grants began to believe—based on mating pattern and beak size—that the drought had caused a disruption during which selection widened the difference between the beak size. The two groups had now become separate enough, such that they were not mating with each other and were showing signs of the potential for distinct speciation.

However, by 1981, the link between beak shape and songs was completely gone—the birds stopped dividing. In 1985, another drought came, and this time the drought did not partition the birds. The island was a different place by then and many of the cactus plants were gone—and the pressures of selection had begun to favor the short, deep beaks for ripping up fallen cactus pads. There is no new niche—yet—for the finches to split and begin to occupy. Yet the Grants suspect that the finches are in a constant process of being forced slightly apart, then drifting back together. Fission and fusion, Weiner writes, “fight forever among the birds.”