55 pages • 1 hour read
The Beak of the Finch: A Story of Evolution in Our Time
A modern alternative to SparkNotes and CliffsNotes, SuperSummary offers high-quality Study Guides with detailed chapter summaries and analysis of major themes, characters, and more.
Part 1, Chapters 4-7Chapter Summaries & Analyses
Part 1: “Evolution in the Flesh”
Chapter 4 Summary: “Darwin’s Beaks”
Darwin’s theory of the importance of variations emerged in part from his immersion in natural theology and the work of Reverend William Paley. Paley argued that the beauty and intricacy of the natural world implied the presence of a magnificent Creator, just as “a watch [...] implies a watchmaker” (49). Paley’s influence motivated Darwin to learn what forces shaped the seemingly miraculous forms found in nature. Darwin theorized that variations provided the raw material upon which these forces worked, selecting only the most beneficial attributes in a species. However, Darwin offered no experimental evidence towards this point, and advocates of the theory clashed with its detractors well into the 20th century.
In 1938, the ornithologist David Lack traveled to the Galápagos to study the finches directly. He found the finches bred exclusively with their own kind, confirming their status as distinct species, not “hybrid swarms.” Further, he saw that species with nearly identical beaks did not live together on any island, and when species with similar beaks shared an island, they diverged from the average more significantly than when they were not neighbors. He believed that the differences in the finches’ beaks had secured each species’ place on its island; where competition between similar-beaked birds may have existed, it seemed that natural selection had driven one or another of the rivals extinct. Lack published his findings in a monograph called Darwin’s Finches, arguing forcefully in favor of Darwin’s theory.
Despite Lack’s compelling arguments, visitors to the islands had only seen the birds feeding in mixed flocks, with no selective advantage on display, until the Grants traveled to Daphne in a dry season. Food was scarce, and for the first time, they saw the birds specializing, each beak fit for a certain seed, such that “birds with the biggest beaks eat the biggest seeds, the birds with medium-sized beaks eat medium-sized seeds, and the birds with the smallest beaks eat the smallest seeds” (60). In some cases, differences of only half a millimeter determined whether certain birds ate particularly challenging and hard-to-crack seeds, such as those of the Tribulus plant, while other members of their species refused.
Even small variations in the birds’ beaks impact their survival. The next step of Darwin’s argument asserted that offspring could inherit favorable, adaptive variations from their parents, causing those variations to persist. Peter Boag, a graduate student of the Grants’, believed that beak size, among other traits, was heritable, or could be passed down; he sought to confirm this hypothesis through an experiment with the next year’s eggs. The Grants extended their study so that they might observe Darwin’s process in action across generations.
Chapter 5 Summary: “A Special Providence”
In 1977, the fifth year of the study, Peter Boag found himself in a severe drought, which stymied his planned experiment and culled the island’s wildlife population. Boag and his partner watched the finches struggle, waited for rain, and made measurements of the birds that lived and died. Processing the data later, they saw that the finches’ survival in the drought hinged upon beak size. Larger-beaked species and larger variations within smaller-beaked species fared best overall, and male birds, which tend to be larger in each species, greatly outnumbered females by drought’s end. The event provided clear, staggeringly convincing evidence of natural selection in the wild and further confirmed the significance of small variations, since “what made the difference between life and death was often ‘the slightest variation,’ an imperceptible difference in the size of the beak” (78).
This selection event on its own gave no evidence of evolution, but the following year, rain fell, and the finch watchers recorded a highly imbalanced breeding season. With many males to choose from, females consistently picked the largest of the large, creating generally larger offspring and driving the next generation towards a larger average size than had been seen before. Skeptics had claimed that it was an unfounded assumption to say natural selection leads to evolution; with Boag’s data, the finch team now had direct, scientific observation of evolution in action. They hoped to gain more evidence and more understanding with a continued watch.
Peter Boag appears throughout as a faithful guardian of the birds: “[he] lay awake and wondered how his flocks were doing, just as biblical shepherds once did in a far-distant desert” (75). The drought and the deaths that followed frustrate Boag and spoil his plans, but rather than plunging the study into chaos, or ending it, the event reveals the truth of evolution more clearly than he could have anticipated. The chapter ends with a meditation on the way that, through evolution, the seeming chaos of hardship and death holds meaning for future generations: “Even drought bears fruit. Even death is a seed” (82).
Chapter 6 Summary: “Darwin’s Forces”
In a sort of changing of the guard, Trevor Price, another of Grant’s graduate students, arrived on the island in 1979. Inheriting the work of his predecessors, Trevor stood “on their shoulders” (83) and came to know the finch population better than anyone before him, “so comprehensively and microscopically that the whole island seemed as small—for a brief interlude, at least—as a Petri dish” (84). Price observed the pressures of sexual selection at work among the birds in the aftermath of the drought. He witnessed the birds’ mating patterns and preferences, noting the dynamics of aggressive competition among males as they court potential mates. He noted that the females preferred the same features that allowed the birds to survive the drought: larger body size and larger beaks. Interweaving Darwin’s theoretical work on sexual selection with the narrative of Price’s watch, the chapter emphasizes that sexual selection acts in tandem with natural selection in the evolution of species.
John Endler, a biologist and contemporary of the Grants, observed a similar interplay of selective pressures in guppy populations in mountain streams in Trinidad. The guppies sport colorful spots that resemble the bright pebbles of the streambeds, giving them camouflage from predators. In the streams, there is a higher concentration of predators downstream, while there are fewer predators in the headwaters. Endler found that the size, color, and brightness of the guppies’ spots correlates with the concentration of predators in their portion of the stream. In areas with many predators, natural selection pushes the guppies towards smaller, more muted spots: better camouflage. In the absence of predation, the guppies’ spots grow gaudier as a result of sexual selection, by which females favor the gaudiest males. In Endler’s study, sexual selective pressure works against natural selection, morphing the pattern of spots depending on the environmental conditions in any one part of the stream. This creates wide variation within the population as a whole: “[...] so in this respect natural selection and sexual selection can oppose each other and push in the same direction: toward almost infinite diversity” (96).
On Daphne, both natural selection and sexual selection pushed the birds in the direction of larger size. Price suspected that this evolutionary trend would change or reverse itself in the next wet season. Price stayed on the island for years, waiting for precipitation, but, as with Boag, consistently dry weather foiled his plans. In 1981 a new graduate student, Lisle Gibbs, took up the watch, and in December 1982 it began to rain.
Chapter 7 Summary: “Twenty-five Thousand Darwins”
Gibbs witnessed a dramatic rainfall on Daphne Major, a flood caused by the erratic, recurring weather pattern known as El Nino. Following this intense wet season, the finches “bred like hell” (101), as Gibbs tells the author, and Gibbs recorded an unprecedented boom in the flora and fauna across the island. The population surplus proved unsustainable as the food supply dwindled during the subsequent dry seasons and many younger finches died in 1984. When analyzed in the lab, the flood had the reverse effect of the drought, with big birds dying out and smaller birds surviving. These results elucidated a less-understood fact of evolution: Looking closely at short timespans, one can witness evolutionary leaps in a certain direction, but other events can reverse those leaps just as largely. At a distance, it might look as if very little has happened—that change is gradual, when in fact it has been vast and dramatic. According to Dolph Schluter, another finch watcher, from these changes one may “start to view species not as constant entities but as fluctuating things” (108).
From his study of fossils, Darwin believed evolution was a slow, gradual process, and after the publication of his theory, many evolutionists operated under a similar assumption. In 1949 the evolutionist J.B.S. Haldane suggested that this presumably gradual rate of evolution be described in terms of a universal unit, which he named the darwin, defined as a change of 1% per million years. Looking at the fossil record, Haldane observed that over long stretches of time, the rate of change was 1 darwin, suggesting that evolution must be imperceptibly slow, and thus observing it in nature would be virtually impossible.
According to the finch watchers’ measurements, after the drought, the rate of change was 25,000 darwins. After the flood, the rate was 6,000 darwins. The Grants’ work reveals a more nuanced truth about Darwin’s process: that “the closer you look at life, the more rapid and intense the rate of evolutionary change. The farther back in time you stand, the less you see” (111). Part 1 closes with the assertion that the truest symbol for evolution is not a fossil in stone but an alert, perching bird, ready for flight.
Chapters 4-7 Analysis
Chapters 4 through 7 tell the story of the Grants’ study in the Galápagos while teasing out the nuances of Darwin’s theory and the decades of debate that followed. As in the first three chapters, Weiner interweaves past and present narratives. Pacing the finch watchers’ story across four chapters, Weiner employs a device of a “changing of the guard,” centering each chapter on a different student of Peter’s, each of whom expanded the scope of the study before their time on the island came to a close. Chapters 4, 5, and 6 all close with the tone of a cliffhanger, as a new question lingers, and a new student arrives to “take the next watch” (98).
This relay structure helps make the concepts in each chapter more memorable, since each concept is paired with a character and moment in the narrative. The motif of inheritance appears in these chapters as well: not only among the finches, but also among their human observers. The story of young David Lack in the Galápagos echoes the characterization of Darwin, casting Lack as a spiritual descendant of sorts. Within the Grants’ narrative, each of their students takes the research further thanks to the work of their predecessors, again “standing on their shoulders” (83).
References to religious imagery and art, as well as Biblical allusions, become more prominent in these chapters as well. The extreme weather events on Daphne Major in 1978 and 1982 are referred to as the great drought and the great flood, evoking stories of divine punishment. Weiner quotes the Book of Genesis in reference to each. During the drought, he says, “Boag lay awake and wondered how his flocks were doing, just as biblical shepherds once did in a far-distant desert: ‘In the day the drought consumed me, and the frost by night’” (75). Quoting Hamlet in Chapter 5, as the drought ravages the finch population, Weiner muses that “there is a special providence in the fall of the sparrow” (82), expanding the notion of providence, typically a reference to divine guidance, to refer to the process of natural selection. These choices continue to resist any easy dichotomies between evolution and creationism. Instead, Weiner implies, evolutionary science does not inherently exclude mystical thinking; in fact, it is defined by a similar sense of uncertainty and awe, as well as a deep curiosity about our origins.
The finches’ beaks appear repeatedly as the answer to the scientists’ questions. Beyond functioning as a well-known emblem of evolutionary science, they seem, with study, to be a sort of skeleton key that unlocks any mystery within Darwin’s theory. They play a role in surviving natural selection and succeeding in sexual selection. They seem to morph in response to the demands and challenges of their changing environment. Weiner juxtaposes the supposedly slow pace of evolution with the astonishing speed of change the Grants witness in the Galápagos. In reference to the birds and their evolution, Weiner uses images of fluidity and metamorphosis to highlight the many ways in which nature is not as stable or fixed as we think.
Unlock all 55 pages of this Study GuidePlus, gain access to 8,800+ more expert-written Study Guides.
Including features: