The question
Name: ______________________ Date: ____________ Beak depth before and after a drought (9 points)
Data table
Finch beak depth across a drought
A population of seed-eating finches lives on a small island with no other finches. Before a severe drought, researchers banded and measured the beak depth of 120 adult birds. During the drought, small, soft seeds ran out first, and the seeds left were mostly large and hard. Only 28 of the banded adults survived; the researchers measured them again after the drought, and their beaks had not changed in size. Earlier breeding records showed that beak depth is heritable. When rain returned, the survivors bred, and the researchers measured 60 of their offspring once those offspring were fully grown.
| Group | Number of birds (n) | Mean beak depth (mm) | Standard error of the mean, SE (mm) |
|---|---|---|---|
| Adults before the drought | 120 | 9.30 | 0.05 |
| Adults that survived the drought | 28 | 9.82 | 0.10 |
| Offspring of the survivors, fully grown | 60 | 9.71 | 0.07 |
(A) Describe how the drought acted as a selective pressure on beak depth in this population. [1 point]
(B) (i) State the null hypothesis for comparing the adults before the drought with the fully grown offspring of the survivors. (ii) Justify measuring the offspring of the survivors, rather than only the survivors, to test whether the population evolved. [2 points]
(C) Construct an appropriately labeled graph that shows the mean beak depth of each of the three groups, with error bars showing the 95% confidence interval (mean ± 2 SE). [4 points]
(D) (i) Based on your graph, determine whether the drought changed the mean beak depth of the next generation, and justify your answer. (ii) After the drought, several years of heavy rain produce mostly small, soft seeds, which smaller-beaked birds handle faster. Predict the change in mean beak depth of the population over the following generations, and justify your prediction. [2 points]