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.
Once the students have made their predictions, they can be
shown the histogram in Figure 6. Working in small groups using
Figure 6. Used with permission from the Howard Hughes Medical Institute, © 2017. All rights reserved. https://www.hhmi.org/
biointeractive.