The Beak of the Finch: A Story of Evolution in Our Time

Opening with an anecdote of Darwin walking around a “Thinking Path” behind his country home, the chapter emphasizes that Darwin’s thoughts continue to dominate evolutionary science. The Grants not only work where Darwin first traveled, but they measure change the same way he did: in individual variations, taking dimensions in millimeters. Teaching students at Princeton, Peter Grant frames evolution as simply “change in variation” (128), explaining that trends in variation over time amount to evolution. A biologist’s work, Grant says, is to understand the invisible forces that drive those changing trends.

Initially many scientists struggled to accept this leap in Darwin’s argument; that natural selection refines adaptations was acceptable, but that it created new species was not. The vocal skepticism on this point prevented a widespread acceptance of Darwinism for much of the 20th century. Even Darwin struggled at first to believe that natural selection could produce some of the more intricate features of living organisms (for example, the human eye). Writing decades after Darwin, evolutionist George Williams noted that such opposition doesn’t refute the theory so much as it highlights the limits of the human imagination.

While formulating his theory, Darwin began to suspect that the profusion of varieties he saw in the Galápagos was the result of a few immigrants—plant and animal specimens that blew in, settled there, and evolved. He drew the origin of species “as a few rough diverging lines, calling the sketch at first the Coral of Life. One species splits into two, two split into four, growing and radiating into branches that will fork again” (134). David Lack reiterated this version of the story, visualizing a family tree for the finches, with the 13 species descended from an original ancestor.

The Grants observed frequent instances of migration and exploration during their study. They have theorized, based on the behavior of birds, that much of the flora and fauna on the island could have traveled from the mainland by air, by sea, or on the feet and bodies of passing travelers. They observe finches and other birds flying in and out: “endless streams of wanderers between the islands” (138). Prior to the flood, visiting birds had little observable impact upon the established finch population, but afterward, immigration to Daphne increased alongside hybridization. The Grants suspect that the new tendency of visiting birds to mate with resident species and settle in the islands may relate to the development of new species, though this has not yet been confirmed.

During a carriage ride in England 10 years after his voyage, Darwin realized how natural selection might lead to speciation, which he called his “principle of divergence” (144). He imagined that when varieties within a species share a single environment, they must compete to survive, going after the same food and territory. An exceptional individual, one who varies more drastically from the group, might have an upper hand in these situations, with more opportunity to pass on its qualities. Over time, this would drive the variations further apart until the competition relaxed, creating “the mutual adaptation of two neighbors to the pressures of each other’s existence [...] forks in the road [...] new branches on the tree of life: the pattern now known as an adaptive radiation” (142).

This aspect of evolution, “competition theory,” was controversial. David Lack placed the finches at the center of the debate when he argued forcefully that their specialized beaks must be a result of such competitive pressures, which caused their divergence. Many ecologists resisted this circular argument, in which the lack of visible competition somehow proves that it occurred. The debate inspired Peter Grant to study the finches, as Lack’s work was persuasive, but Grant felt a need to verify Darwin’s theory through direct observation rather than dogmatic argument.

Dolph Schluter, who was one of Peter Grant’s PhD students, undertook a closer study of the adaptive radiation at work among the finches. Like Lack, Schluter found no evidence of competition while observing the birds in the field. In the lab, Schluter pioneered a visualization technique to examine the birds’ varying beaks within an adaptive landscape including peaks of optimal fitness and valleys of unfitness. Schluter’s adaptive peak model synthesized the data on beak size and seed supply to predict an optimal beak size for the conditions on Daphne. Instead of a single peak, the model produced three adaptive peaks, which corresponded to the small-, medium-, and large-beaked finches on Daphne. This result indicated that their beaks evolved as an optimal response to their environment. Further study in the Galápagos and elsewhere has shown that, while Darwin claimed competition would be most intense between the “closest neighbors,” there is evidence of character displacement even between organisms “far apart on the evolutionary tree,” such as between nectar-sipping finches and bees (155).

Peter Grant believes that character divergence is impactful but challenging to observe: It is “likely to be quite common and important […] but […] of rather small magnitude, in terms of the quantity of the evolutionary shift” (156), possibly only accounting for a small percentage of difference between characters. Regardless of its magnitude, though, divergence is another factor—another clue to understanding the origin of species.