The Beak of the Finch: Chapter 16 Summary & Analysis

From his very first moments in the Galápagos, Darwin knew that the birds of the archipelago had no experience with human beings—they didn’t recognize Darwin and his team as predators, and they’d perch on their shoulders or on their cups. But Darwin did know that invaders always bring change—one of Darwin’s contemporaries used the example of polar bears arriving in Iceland to illustrate this point. Polar bears, arriving in the country on an iceberg, began eating the animals like foxes, deer, and seals, who didn’t recognize them as predators. With fewer deer on the island, the plants and insects prospered—and with fewer foxes, the ducks multiplied, thus reducing the fish population. Darwin could only imagine “what havoc the introduction of any new beast of prey must cause in a country”.

Another example of this phenomenon Darwin referenced involved introducing cats into an English village. The cats would eat the mice, he predicted, and so a lesser population of mice (which ate honeycomb) would allow beehives around the village to thrive and expand. More bees would be able to pollinate more flowers—and so in time, Darwin predicted, the arrival of the cats would directly result in a flourishing of local flower populations.

Just as El Niño spurred an evolutionary event on Daphne Major, the introduction of a new species of plant or animal, too, could create conditions that would pressurize selection toward evolution. Darwinian pressures, Weiner suggests, are intensifying “everywhere we look.”

In 1898, the evolutionist Hermon Carey Bumpus was living in Providence, Rhode Island when a strong blizzard killed hundreds of sparrows. Bumpus collected their corpses, took them back to the lab, and studied them—he determined that the sparrows had been through what’s called a “stabilizing selection” event, because there were simply too many of them. The ones less equipped to survive—the ones that were largest and smallest—died quickly, which means that the rest of the species was pushed toward a more stable mean.

The chain of events spurred by the introduction of a new species, too, can be seen in everyday life. An evolutionist named Scott Carroll is studying beaked bugs called soapberry bugs, and just as finches’ beaks adapt based on the food they eat, so too do those of the bugs. The bugs are evolving rapidly in many different locations based on new plants that are introduced to their territories.

In the late 1860s, a British naturalist living in America, Benjamin Walsh, became inspired by Darwin’s writings and decided to study the haw fly, a native American fruit fly that consumes hawthorn. But in Walsh’s time, some of the flies had abandoned haws and begun to eat apples. This was only happening in certain parts of the country, but Walsh predicted the flies would multiply and expand their region. Sure enough, by 1902, the flies were not just in the Hudson River valley, but also in Vermont, New Hampshire, Maine, Michigan, and Georgia.

Today, biologists have observed that the flies’ lives continue to grow more and more entwined with the apple trees—the trees are where they feed, mate, and grow from larvae into insects. New technology allows modern-day researchers to study what’s behind these changes on the molecular level—and it’s clear that these flies are still in the early stages of divergence.

The same thing is happening, unseen, in thousands of other species around the world. For instance, as farms become more and more specialized, focusing on individual crops, the insects, too, are specializing, evolving to feed on specific types of fruit and becoming isolated based on that specialization.