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

Adaptive radiation describes the way evolution unfolds after natural selection drives similar, competing species to diverge. After the divergence pushes the rivals into exclusive niches, each competitor begins adapting expansively within that niche, forging new “forks in the road […] new branches on the tree of life” (142).

Character displacement is the phenomenon by which a species evolves its way out of competition with a rival by changing its characteristics. David Lack suspected character displacement among Darwin’s finches. Dolph Schluter confirmed it via his adaptive landscape tool, which correctly and precisely predicted the adaptive peaks the finches had evolved based on competition and seed supply; thus, “[the finches] are the best case of character displacement ever found” (154).

Creationism is the belief that God created Earth and its inhabitants, including humankind. Though he founded evolutionary science, Charles Darwin studied in a creationist tradition shaped by the thoughts of Karl Linnaeus, William Paley, and others. Modern creationism is often juxtaposed with evolution and sometimes presented as its ideological opposite. The Beak of the Finch works to undermine this simple binary.

“Darwin’s finches” is a term used frequently among biologists to refer to the finch species of the Galápagos Islands. David Lack coined this moniker as the title of his 1938 monograph on the birds. There are 13 species in all, but only a subset appear substantially in the book: the small, medium, and large ground finches (Geospiza fuliginosa, fortis, and magnirostris, respectively); the cactus finch (Geospiza scandens); and the sharp-beaked finch (Geospiza difficilis).

A darwin is a unit that measures the rate of evolution by percentage change over time. One darwin represents a change of 1% per million years. It was conceived by the early-20th-century evolutionist J.B.S. Haldane.

DNA, or deoxyribonucleic acid, is a type of molecule containing long sequences of genes arranged in a double helix. The cells of all living things contain DNA. The genes in DNA form a code indicating what form and function a cell can take. Darwin suspected that something like DNA was responsible for the variations in all living organisms, believing variety was written by “invisible characters” in the blood. Contemporary evolutionists use DNA to determine shared ancestry and moments of divergence in species. While DNA often stands as a symbol of unchanging essence, Weiner notes that DNA constantly changes due to damage, constant repair, and mutation, all of which affect variation and evolution.

Heritability describes the ability of a given trait to pass from generation to generation. When a trait (such as beak size) is heritable, it can contribute to a species’ evolution via natural selection. Whatever traits survive selective pressure, when they are heritable, will persist across generations. In the text, the idea is introduced by the Grants’ graduate student Peter Boag and later explored and confirmed in a parallel experiment with sparrows by evolutionist Jamie Smith in British Columbia.

Introgressive hybridization is the act of “backcrossing” by hybrids into the purebred lines of either of their parents. Although this behavior does not expand the hybrid population, it supplies new genetic material to the pool of the purebred line. Infusions of new genes increase a species’ potential for variation. The offspring of a hybrid parent and a purebred parent will be fit for unusual or hybridized habitats. Under conditions of chaotic environmental change, the products of introgressive hybridization may take advantage of newly formed niches, adapt, and form new species. The botanist Edgar Anderson coined the term in the mid-20th century. The concept has informed the Grants’ work on hybridization as a possible cause of speciation in Darwin’s finches.

Natural selection is the process by which certain features of a species are preserved and refined. All characteristics vary slightly among individuals within a species; nature selects for variations that are advantageous and against variations that are less beneficial. Natural selection is not the same as evolution, but by scrutinizing, culling, and preserving certain variations, it is a primary factor that can lead to evolution.

Darwin’s principle of divergence explains the mechanism by which varieties branch so far apart that new species form. In an environment where similar varieties of a species share space, they compete for food and territory to survive. Certain exceptional individuals who vary further from the average have exceptional advantage here; over generations, such advantages persist and amplify, pushing each variety towards further divergence until the competition relaxes. Inevitably, one species either drives the other extinct or drives its rival into a new niche.

Sexual selection is the process by which traits in a species are preserved as a result of mating rituals and preferences. Just as natural selection favors certain traits, members of a species select certain desirable traits in each other. These traits pass to the next generation. Working in tandem with natural selection, sexual selection has a strong impact on evolution.