The Beak of the Finch: Chapter 12 Summary & Analysis

Nowhere in his writings does Darwin contemplate the moment of divergence itself—the moment when one line of a species splits into two. Weiner calls this “the cosmic parting.” But today, as more and more naturalists study evolution in action, they keep an eye out for these “cosmic” splits. Researchers have been able to observe these splits in species like fruit flies and conclude the definitive fact that “each species first existed as a variety,” as Darwin wrote so long ago. By observing many generations of fruit flies in lab conditions, evolutionists have watched “incipient species” arise—these new species are unable to produce fertile offspring with specimens that were, just a few generations ago, their kin. 

The question of how new adaptations arise from gradual beginnings is one of the central questions of Darwinism. Darwin himself used the example of the evolution of the human eye to suggest that all the sophistications that exist within it grew gradually, in stages, through natural selection for advantageous traits, also called adaptive traits. Adaptive traits are more likely to be preserved—even though the process of creation is, essentially, “blind”—with the result that the advantageous traits build on themselves over time.

In 1991, two researchers at the University of British Columbia studied a genus of finches native to North America, Europe, and Asia called crossbills—their beaks, as their name suggests, are crossed. Their twisted bills are adaptive, because these finches eat pinecones and need twisted beaks that allow them to pry the cones open. The researchers clipped the bills of the finches—a harmless process akin to clipping fingernails—so that they were uncrossed. The birds struggled with their food sources, but as their bills grew back, they did better and better.

Dolph Schluter has begun a new study near Mandarte Island, in the Strait of Georgia—a body of water which is full of three-spined sticklebacks, small salt-water fish who sometimes migrate into fresh water. For 12,500 years, the fish have been evolving in freshwater lakes—just like the finches on Daphne Major. Most lakes in the region hold just one species, but a few lakes hold two. One species (benthics) feeds along the bottom, and one (limnetics) above it. By carefully measuring the fish, Schluter and his team have mapped how selection is acting on the fish.

In every lake that contains two species of sticklebacks, the team has found, the two species are divergent in the same way: one always feeds on the bottom, and one always feeds higher up. Benthics are bigger and fatter, while limnetics are small and thinner. The pattern has repeated itself in five different lakes. Even in one lake where all of the fish are technically the same species, the smallest difference in their sizes dictates where they’ll feed. Speciation, in other words, is in progress. The populations are ready to respond, at all times, should selection pressure arise. And what’s more: these species have altered the course of one another’s evolution, filling niches that the other cannot.

Schluter predicts that the species will continue evolving toward their respective peaks. Because the species are rare and protected, Schluter can’t do the kinds of experiments he’d like to—but the ones he has done have made it clear that competition affects populations today, and that natural selection pressurized by competition can be observed. Schluter plans to create a hybrid of benthics and limnetics—then place those hybrids in two false ponds, one containing only benthics and one containing only limnetics, and see how the hybrids change in each pond to fill the niche exposed by their counterparts.