Grant Handout

Grant & Grant 2002 “Unpredictable evolution in a 30-year study of Darwin’s finches.”
1. What are the main questions/goals of the paper? There are more than one.
2. Why is this study unique? Do you know of other studies that might have different
conclusions if they were conducted over longer periods of time?
3. Set up the field situation for each other:
 Who are the birds?
Where are the birds?
What was measured on the birds?
Natural selection and evolution:
4. What is the null hypothesis?
5. Figure 1 tests the null hypothesis. Go through Figure 1 like this:
What do the axes mean? (what is a PC on the Y axes?)
What do the 95% confidence intervals mean?
What happened to beak size and body size for both species over the study period?
What happened to beak shape of G. scandens over the study period?
6. Figure 2 shows selection on the traits that were shown in Figure 1. Go through the figure
like this.
What is a selection differential?
What do each of the bars mean?
What do the dots on top of the bars mean?
Which traits underwent selection? Which trait underwent the LEAST selection?
In relation to what happened to G. scandens beak shape from Figure 1, what can you
conclude about this pattern after looking at Figure 2? Try comparing each panel of
Figure 1 to Figure 2. When are they similar? When different?
7. Figure 3 uses heritability and selection differentials to evaluate the effect of selection on the
evolution of traits. How does it do this? What is heritability? What would the graph look like if
traits did NOT respond to natural selection? What would it look like if natural selection were the
ONLY force?
Introgressive hybridization:
8. The authors suggest that change in G. scandens beak shape is not explained by natural
selection. This may be because of “introgressive hybridization.” What is this?
9. Why are only G. scandens seemingly affected by the hybridization? Use Figure 4 to support
your statement.
What can you conclude is happening to G. scandens over the study period from this
figure (also use Figure 5 to support your statement)?
10. Figure 5 shows the effects of hybridization on beak shape measurements.
What would you EXPECT the figures to look like if hybridization does not affect
beak shape? What would you expect if hybridization DOES affect beak shape?
What is the difference between the white and grey bars?
11. What are the main agents of SELECTION in this study? What are the main sources of
RANDOMNESS (i.e. what is unpredictable)? When is evolution predictable?
12. This paper is a cool example of the intersection between ecology and evolution. Why so?
What is an example from the paper where we need to understand the ecology in order to
understand the evolution of these organisms?