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

Darwin believed that the process of evolution was so gradual that to see a species at conception would be impossible. Today, scientists are inching closer to witnessing the so-called point of divergence as it occurs. In one case, scientists unintentionally generated a new species of Drosophila in their own lab when they weren’t looking: A strain they held in captivity adapted so rapidly in isolation that, in a few years, it no longer produced fertile offspring with strains to which it had been closely related.

Critics of evolution (and Darwin himself) struggled to articulate how adaptation leads to evolution. Many insisted, echoing Paley, that nature’s most intricate and useful creations, like the human eye, could not possibly come from random variations. Modern thinkers such as British evolutionist Richard Dawkins responded with counterarguments that illustrate how a rudimentary cluster of cells—for example, one for sensing light and dark—might be gradually refined into a sophisticated visual apparatus. Scientists Craig Benkman and Anna Lindholm tested this argument in an experiment on the beaks of crossbill birds, whose overlapping upper and lower mandibles allow them to twist open pinecones and retrieve the seeds. Benkman and Lindholm trimmed the beaks of a set of crossbills to remove this overlap so that the birds could no longer open the cones. They watched as the birds’ beaks regenerated in a rough, short-term mimicry of how a variant beak with a slight overlap may have slowly evolved into the crossbill beak. As the overlap returned, the birds grew progressively better at opening cones, once again affirming that a variation of a few millimeters, preserved and amplified by natural selection, could result in the crossbill’s unique and powerful tool.

While mimicking evolution gives convincing support to the theory, observing actual evolution across generations offers proof. Dolph Schluter has initiated a long-term study of a set of fish species, sticklebacks, that live relatively isolated in freshwater lakes in Western Canada. Schluter observed that in each lake pairs of stickleback species live side by side in “mutual repulsion,” with one species feeding at the bottom of the lake and the other feeding up and down the water column but not at the bottom, suggesting that each caused “character displacement” in the other. Schluter intends to place the species into laboratory ponds, remove competition, and witness the resultant change across generations. Schluter expects to see the fish expand into each other’s niches but admits that it may take years to observe an evolutionary response. He says, “We’ve got our work cut out for us” (189), acknowledging the vast scope of his task. The chapter closes inconclusively, with the “point of divergence” still an unknown.

During their sabbatical, the Grants notice new trends in the yearly seed census data, changes in the food supply, and other new ecological conditions caused by the drought and flood. They suspect the changes relate to the increased fitness of the hybrid finches. Peter Grant recalls an impactful paper on interspecies breeding that illustrated the ways that hybridization diversifies the gene pools of purebred lines. The paper asserts that a hybrid finch breeding with a purebred finch will create offspring closer in characteristics to the purebred line than its hybrid parent, but diverse genetic material will make its way into the gene pool of the extant species, enabling increased variation in future generations. In the face of drastic environmental change, hybrid individuals may hold greater potential to adapt and survive. Taken in full, Peter and Rosemary’s data indicates an oscillation between “hybrid superiority” and “hybrid inferiority” alongside changes in the landscape:

Whenever the adaptive landscape heaves and flings about, like a sea under heavy winds, the hybrids among Darwin’s finches will be favored. […] But when the landscape returns to the pattern it held before the storm, the birds will settle back to their old peaks, and the sharing of genes will slow again (198).

Following the Grants’ discoveries, the tree of life metaphor is less useful in describing the way the process works than perhaps a thicket or a net—a model known as reticulate evolution, in which branches diverge and come together again and again: “This [model] doesn’t bind lineages together forever; eventually they part ways or fuse. But it may be a general and hitherto neglected feature of the origin of species” (202). The author goes on to emphasize that neither evolutionary forces nor the conception of species are fixed; all are fluid. This passage echoes the concluding passage from Part 1, urging the reader to embrace more complex, dynamic images as truer encapsulations of life on earth.

Weiner ruminates on what the Grants’ work with the finches has revealed and what mysteries remain. He describes an 1879 painting called Science is Measurement by Henry Stacy Marks, depicting a befuddled Victorian gentleman standing with calipers before the skeleton of a stork. The painting appears in a historical science book alongside a quote emphasizing that unless something can be measured and quantified, it cannot be known—together, the painting and the quote implicitly critique ecology as unmeasurable and unscientific. The Grants keep a copy of this book in their lab, where they continue to sift through a massive collection of finch measurements, a testament to the progress of evolutionary science.

After they fed their data on annual seed supply into a computer, the Grants’ evolutionary formula accurately predicted the mean beak size in each species on Daphne Major for every year of their study, all within a fraction of a millimeter. The precision and consistency of the calculations confirm again that Darwin’s process is at work among the finches. The measurements of the finches’ variability suggest also that they are “new beings on this earth” (206), young species in the process of a swift evolution. Undergoing a period of hybrid superiority, “one in ten finches born […] on Daphne Major are hybrids” (206); the separate species have the potential to fuse, but forces of competition and selection are still at work pushing the species apart. This is an active, dynamic, not wholly predictable process.

Weiner concludes Part 2 with the Grants enjoying a more imaginative moment, discussing the origin of species as it relates to hybrid forms. They contemplate how such creatures might participate or themselves cause the formation of a new species. Sketching a hypothetical graph, they theorize that a new species could occur when there is exceptionally strong and relatively narrow selective pressure for a hybrid body; through that small window of opportunity, a new species might evolve. The Grants step back from any hard assertions here, acknowledging that this is a flight of fancy, currently unmeasurable. They resolve to turn back to their data, pushing the narrative forward with the mystery lingering.