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

Inspired by the finches’ variety and by Charles Lyell’s work on geologic change, Darwin wrote of the ways in which all living things exist in an ever evolving but interdependent relationship, a “web of complex relations” (225). Within that web, the introduction of one new species has far-reaching effects: “the invader itself evolves rapidly as it adapts to its new home; meanwhile, everything already living there either adapts to the invader or becomes extinct” (226). With the advent of human travel and human technology, these invasions occur more frequently than ever before.

Weiner describes multiple case studies that demonstrate this point. An American biologist and peer of Darwin’s, Hermon Carey Bumpus, conducted a study of English sparrows, newly introduced to America, that died during a severe blizzard in Rhode Island. By comparing the sizes of the birds that died and those that survived, Bumpus concluded that the blizzard had acted selectively on the birds, allowing survival of males more than females, and of small birds rather than large. More broadly, the study demonstrates the impact that immigration can have on the immigrant itself, as it is continually shaped by its new environment. A recent study on the length of the proboscis (the “beak”) of the soapberry bug, native to North America, indicates that the introduction of new plant species has led the species to diverge in beak length, depending on whether it feeds on its native host plant or on newer plants, immigrants from Europe in the past 100 years.

Even short-distance migrations, as from a wooded area to a neighboring orchard, have shown variation and divergence within a species: in the 19th century, a particular strain of fruit fly was observed moving from wild hawthorn into nearby cultivated apple trees, where it began mating and laying eggs in apples rather than haws. Recent studies of these flies’ DNA indicate that apple and haw flies are almost genetically identical, but proportions of certain genes in each have begun to vary, suggesting what may be the earliest beginnings of a divergence into two species, another clue to how speciation may begin.

On the inhabited Galápagos Islands of Floreana, Santiago, San Cristobal, and Santa Cruz, human settlement affects the course of the finches’ evolution. The Grants observe that the birds on these islands don’t display the same easily recognizable distinctions as species on islands with no humans, perhaps due to the abundant food and reduced selection pressure. In their studies on Genovesa after the flood, the Grants saw similar indications that a lush environment tends to dissolve distinctions. However, in those environments, periods of abundance alternate with periods of scarcity, which reestablish the distinctions. In the populated islands, humans regulate the food supply such that the culling process of natural selection is not at work quite so harshly.

Weiner notes that humans’ ecological dominance has global ramifications: increased migration and hybridization, which accelerate species’ evolution within a rapidly changing environment. Edgar Anderson, a botanist in the mid-20th century, speculated that increased hybridization over successive generations would produce organisms uniquely fit for new, unprecedented habitats. A chaotic, rapidly changing ecosystem could create such exclusive niches—newborn micro-habitats that “may prove to be the seedbeds of new evolutionary lines” (241). Anderson argues that humans create these radical changes in ecosystems and species across the globe, effectively “hybridizing the planet” (242). While humans may be unusually self-aware compared to previous dominant species, this pattern of drastic change in response to a single species’ heavy footsteps has been seen a handful of times in Earth’s history.

Island species are particularly vulnerable to sudden change, since they evolve in fragile, isolated environments. The Grants exercise remarkable vigilance not to bring even a fire ant along with them to Daphne Major, knowing the potential for disruption. Sometimes humans relocate a vulnerable species in a changing ecosystem for the sake of conservation, as in the case of an endangered Hawaiian finch moved from a populous island to a remote one. On their new island, scientists observed significant adaptive radiation in the species, such that the humans’ intent to preserve the species ultimately accelerated its evolution. Weiner emphasizes the point that species can’t exactly be preserved, since evolution drives them in new directions almost constantly. As the chapter closes, Weiner quotes Peter Boag saying that the change occurring in the Galápagos is probably happening faster than it ever has.