The Beak of the Finch: Chapter 6 Summary & Analysis

The next “watch” over Daphne Major fell to the researcher Trevor Price, a student of Peter Grant’s who landed on the island early in 1979. Because of the research of those who came before him—Boag, the Grants, and their teams—Price could follow Darwin’s finches more closely than anyone else. He banded finches quickly and came to know them by sight, as a shepherd knows members of his flock. He quickly learned that the smallest of the young fortis were surviving—they were the most likely to succeed. Because juveniles must hunt and peck for small seeds—yet need the most food of all—big soft beaks don’t help them get it. Instead, smaller, beaks are favored amongst younger birds. Price set to work measuring these “conflicting waves of natural selection” as they moved through the finch population.

One of the many “waves” of selection these birds must contend with is sexual selection. Put another way: in times of environmental stress, nature scrutinizes the birds, but in times of plenty, the birds scrutinize one another. Trevor Price watched as the island became a “comedy of sexual selection” rather than the “tragedy of natural selection” it had been just two years earlier during the drought.

Darwin assumed that in mostly monogamous species like his finches, the pressures of sexual selection would be less intense than the pressures of other kinds of selection. But after the drought, the skewed sex ratio between the finches meant that mating became a “winner-take-all” game. Many male finches were unable to mate—the biggest males with the biggest beaks were taking all the mates, and some females were mating with multiple males who had favorable traits.

Plumage color, territory size, and more play a role in the finches’ mating process—but because males with the black plumage of sexually mature adults are often attacked by other mature males, some males will strategically hold off changing their plumage as they set up their territory under the radar.

The forces of sexual selection often wrestle with the forces of natural selection. The naturalist John Endler is one of the foremost experts on the intersection of these forces. Endler is to guppies what the Grants are to finches—he studies guppies that live in the streams of northeastern South America. The male guppies he observes are covered in spots of varying size, shape, color, iridescence, and location—the palette of colors and the size and brightness of a guppy’s spots are all heritable traits. While the spots might seem like an insignificant trait, they are, like the finches’ beaks, often a matter of life and death.

In the 1970s, Endler began watching the guppies and noticing that their spots helped them to camouflage themselves in the gravel at the bottoms of their riverbeds. The guppies are vulnerable to predators: both other fish and prawns. Guppies higher upstream face a lower risk, but those downstream are in a constant and intense battle with such predators, and therefore with natural selection. Endler developed a method of measuring guppy spots—and found that the spots are directly related to the number of enemies the guppies have. Those with fewer enemies have brighter spots, for attracting mates; those with more enemies have duller spots, so as to evade the notice of predators, because natural selection is a greater pressure than sexual selection. The gaudier the male, the more mates he attracts—but the more risk he attracts, as well.

Endler saw that the forces of selection were at work—but as he noticed more and more specificity in the coloring and patterning of the guppies’ spots, he knew he needed to experiment on the guppies and see these selection processes in action. He built test ponds at Princeton and filled them with colorful gravel, guppies, and predators such as cichlid fish. Guppies can bear young at just five or six weeks of age, so it didn’t take long to have enough guppies to stock each of his 10 ponds with 200 fish. By creating a huge assortment of guppies, he could see how they evolved.

After many months (and many generations of guppies) had passed, Endler could see that the guppies placed into ponds with fewer enemies got gaudier—they had more spots that were bigger and in brighter colors, like blue, to which guppies’ eyes are very sensitive. But the guppies with more predators to contend with stayed relatively drab. Even though Endler’s experiment was taking place in a controlled setting, the data was plain: evolution was happening, daily and hourly, based on different pressurized selection mechanisms. Natural selection with the guppies, as with the finches, was “swift and sure.” 

As Trevor Price’s time on the island drew to a close, the members of Princeton’s Finch Unit had spent nearly a decade on Daphne Major—and that decade had taught them that natural selection was making the finches bigger. But Price wondered why there were still, in spite of that fact, small birds on the island—he felt the trend toward selection for bigger birds had to come to an end soon. He believed the next heavy wet season would bring a new development.

But the rains didn’t come—and Price traveled to the other islands of the Galápagos to kill time as he grew increasingly frustrated. When the rains finally came, in March of 1981, it was “too little too late”—not many finches bred. Price left, and Lisle Gibbs, another of Peter Grant’s graduate students, took up the watch. A year passed with no rain. But in late 1982, as Lisle had left the island for Christmas, he got a postcard from a contact in the populated Galápagos village of Puerto Ayora—it was raining at last, and it was raining hard.