Evolution in Action: Graphing and Statistics
11. Based on the graphs you have drawn, how does wing length compare between survivors and non-survivors?
What about body mass?
The surviving medium ground finches had slightly longer wings and slightly larger body masses than medium
ground finches that did not survive the drought of 1977.
12. What do the results illustrated by your graphs indicate about the effects of the drought on birds with
particular wing lengths and body masses?
The results suggest that it may have been an advantage during the drought to have a larger body mass and
longer wings. Students may also point out that larger birds probably also have larger wings and larger beaks,
and are thus more likely to survive.
13. The Grants say in the film that a key trait that made the difference in survival for the birds during the drought
was beak depth. Is that conclusion consistent with the data presented in this activity (including Part B)?
Explain your answer.
Beak depth was larger for the surviving birds compared to the birds that did not survive. However, body mass
and wing length were also larger among survivors. It could be that larger beaked birds simply have larger body
masses and longer wings. All three traits could be important in survival.
14. Explain why the Grants concluded that beak depth may have played a more important role in survival during
the drought than wing length or body mass. Correctly use the terms natural selection, adaptation, and fitness
in your answer.
The major environmental change caused by the drought was a change in food source. The larger beaks of some
medium ground finches became advantageous when the small, soft seeds disappeared and only large, hard
seeds remained. Birds with larger beaks were able to use large, hard seeds as a food source and were therefore
more likely to survive the drought and reproduce than were birds with smaller beaks. Therefore, large-beaked
birds were more fit than small-beaked birds. Because the survival challenge posed by the 1977 drought had to
do with a change in the food supply, natural selection probably acted primarily on beak depth, not wing length
or body mass. An explanation for why wing length and body mass were also greater for surviving birds may be
that birds with larger beaks were also larger overall—they had longer wings and were heavier than birds with
smaller beaks.
Students may also indicate that having a larger body mass may have helped birds withstand lack of food better
than birds with smaller body mass.
15. Explain the role of variation in important traits (like beak depth) in a population for the survival of a species.
Students should indicate in their own words that variation among individuals in important traits like beak depth
makes it more likely that at least one form of the trait will be good enough for individuals to successfully survive
a change in their environment.
AUTHOR
Paul Strode, PhD, Fairview High School, Boulder, Colorado
Edited by Laura Bonetta, PhD, HHMI, and Ann Brokaw, Rocky River High School, Ohio
Reviewed by Brad Williamson, University of Kansas; Peter Grant, PhD, and Rosemary Grant, PhD, Princeton University
Origin of Species Revised December 2017
www.BioInteractive.org Page 7 of 7