The Beak of the Finch

Having studied natural theology at Christ’s College, Cambridge, Darwin was inspired and influenced by religious thinkers. So when he and his fellow crew members on the Beagle saw the finches of the Galápagos, they viewed the finches’ adaptive traits to life on the harsh islands as “admirable provisions of Infinite Wisdom”—that is, that God had made them well-suited for their environment. But Darwin also had the idea that if living things were well made, then even the slightest variations must make a difference, even perhaps helping an organism to find a new spot in the larger organization of the natural world. 

Because a bird’s beak is the most important part of its anatomy—it is the tool a bird uses to eat, dig, and defend itself—there are many different kinds of beaks. Egrets have spear-like beaks, herons’ beaks are like tongs, and so on. Neither Darwin nor his contemporaries doubted the “adaptive value[s]” of these tools. But many of Darwin’s colleagues did doubt that individual variations in these different sorts of beaks meant that much at all. 

Darwin saw variations as essential to the longevity of a species. For instance, in a long dark winter with few deer around for prey, the slimmest wolves in the pack would be expected to do best. The same idea, then, applied to all animals: the fox with the best sense of smell or the rabbit with the longest legs would fare the best in tough conditions and pass its traits along. Yet Darwin himself never tried to produce an experiment that would confirm this point—though it was a “logical” hypothesis, it was a difficult one to prove.

A British ornithologist named David Lack arrived in the Galápagos in the late 1930s to research Darwin’s finches—and at first, his research seemed to suggest that the birds’ beaks offered “no scope for natural selection.” But upon returning home to England and looking over data, he noticed stunning anomalies which suggested the opposite: the birds, as they bred, seemed to be “consciously trying to get out of each other’s niches.” They were making themselves as different as possible from one another in order to survive. In 1947, Lack published a monograph called Darwin’s Finches, proving that there was enough difference between the beaks of individual finches on the islands to confer competitive advantages that would in turn power natural selection.

To illustrate the finches’ “bitter struggles for existence,” author Jonathan Weiner describes one of their food sources: a weed called caltrop, Latin name Tribulus. The weed defends its fruits, or mericarps, with sharp spines. When the mericarps fall to the ground, the finches eat them—but the mericarps are awkward in a finch’s beak, and some species don’t even try to open them. The finch species Magnirostris crushes the mericarp in its powerful beak, but fortis, whose beak is weaker, must slowly pry the mericarp open. Finches’ lives depend on how efficiently they can forage—and their specialized beaks can help them save precious energy as they do so.

The tiny variations between the species, then, are critical, and finches with stronger beaks survive better. And, at the same time, the caltrop is evolving in response to the finches: spinier plants with less fruit survive longer and proliferate more. The finches and the caltrops are driving one another’s development. Darwin himself commented on the “trifling difference[s]” that can determine whether a species survives, or whether it perishes. Peter and Rosemary Grant can see how these “trifling differences” pan out on the islands of Daphne Major and Genovesa—how those differences decide which individual birds live, and which die.

Darwin argued that favorable variations would be more likely to be passed down, spread throughout the population, and benefit the survival of the entire species. But not all of his contemporaries—or even evolutionists working in the early 20th century—believed him. Peter Boag, a contemporary of the Grants, decided to undertake a study measuring relationships between parent beak size and offspring beak size in Darwin’s finches to see how accurately favorable variations are passed down, and he found that they did. However, Boag didn’t get to perform an experiment in which he switched eggs from different nests to see whether the inheritance of beak size was a function of genetics or upbringing. But another scientist, Jamie Smith, did, and he found that foster birds took after their biological parents rather than their adoptive ones. Smith showed that the smallest details are heritable, passed down through the generations.

While the Grants first traveled to the Galápagos, they planned to stay for just a few months and take a “snapshot” of the environment. But soon they began to understand that it was worth watching the finches over a long period of time so that they could see in real time how the variations between finches impacted their survival, and offspring, and how those variations were passed on from one generation to the next.