The American Biology Teacher

Changes in Beak Depth of Darwin’s Finches

The story of the adaptation of Darwin’s finches during a 1977 drought provides our second example of evolution in action. This example involves a quantitative trait, beak depth. Quantitative traits produce a continuous distribution of different phenotypes rather than a dichotomous split between two contrasting phenotypes. Variants of the trait can be displayed on a graph called a histogram that may approximate the shape of a normal distribution, and can be characterized with two numbers, the average and the standard deviation. By examining histograms from successive generations, students can see that evolution involves cumulative changes in the proportions of different variations at the population level, not changes in individual organisms.

An HHMI BioInteractive resource (HHMI BioInteractive, 2015b) presents students with the histogram in Figure 6, which displays data collected by Peter and Rosemary Grant. The histogram shows the distribution of beak depths of Darwin’s finches in 1976 (white bars), with the finches that survived a drought overlain in black. In the early 1970s, the Grants selected the small island of Daphne Major in the Galápagos archipelago as a laboratory to study evolution. Since the island was small, they could manage the task of measuring all the medium ground finches on the island. During the first four years of their study, they caught, banded, and measured birds but observed little change in their traits. However, in 1977, a severe drought lasting 18 months hit the island. In May 1976, before the drought, the Grants measured an average beak depth of 9.42 mm for the population of medium ground finches. In 1978, after the drought, the average beak depth of the finches had increased to 9.96 mm, 6% larger than in 1976 (Boag & Grant, 1981).

The drought altered the food supply, creating conditions in which many of the birds from the 1976 population were ill-equipped to survive. Before the drought there was a range of seeds varying in size and hardness, but as the drought persisted the only seeds remaining were large, hard seeds from the cactus bushes that were able to weather the drought. Smaller finches were unable to crack these large, hard seeds and died at a higher rate than birds with larger beaks. The survivors that reproduced had, on average, larger beaks and produced offspring that also had, on average, larger beaks. Parents tend to produce offspring that look like themselves. The shift in the population distribution toward larger average beak depth was a result of differences in survival and reproductive success for individual birds with different beak depths. This is natural selection in action.

To begin the study of this case, students must be provided with the backstory about the Grants and their study of the finches on Daphne Major without telling them the final outcome. They need to know that the Grants were collecting data on finch beak depth, body weight, wing length, and leg length for a number of years, with little change in the finches until a drought occurred in 1977. The drought caused a change in the vegetation and a resulting change in the types of seeds available for the finches to eat. Before the drought there was a range of seeds varying in size and hardness, but following the drought there were only large, hard seeds. Ask students to predict what happened to the finches as a result of the changing conditions caused by the drought. It is likely that many will suggest that individual finches had to grow larger beaks, or produce offspring with larger beaks, in order to survive on the supply of large seeds.