In the 1980’s biologists Peter and Rosemary Grant and colleagues searched for evidence in support
of Darwin’s theory of natural selection. Each of 30 consecutive Spring seasons, they caught and
measured all of the birds from more than 20 generations of finches on the Galapagos island of
Daphne Major. In one of those years (1977), a severe drought caused vegetation to wither, and the
only remaining food source was a large, tough seed the finches ordinarily ignore. Were the birds with
larger and stronger beaks for opening these tough seeds more likely to survive that year and did they
tend to pass this characteristic to their offspring? It makes sense to compare 1976 finches (pre-
drought) to 1978 finches (post-drought) in terms of beak depth.
In 1976 they had caught and measured all 751 finches on the island. In 1978 they caught all 89
finches on the island. Here we compare the 89 finches of 1978 (post-drought, a population) to 89
randomly selected finches from the 1976 (pre-drought) group. Beak depth is measured in millimeters.
Here is a side-by-side stem and leaf plot for each group.
1976 1978
2 6 –
8 6 +
4 1 1 7 – 1
9 8 7 + 9
4 4 4 2 0 8 – 0 4 4
9 9 9 9 9 7 7 7 6 5 5 5 5 8 + 7 7 8
4 4 4 3 2 1 1 1 1 1 0 0 0 0 0 9 – 0 0 1 1 1 2 3 3 4 4
9 9 9 9 9 8 8 8 8 8 7 7 7 7 6 5 5 9 + 5 6 6 6 6 6 6 7 7 7 7 8 9 9 9 9
4 4 4 4 3 3 3 2 2 1 1 1 1 1 1 0 0 0 10 – 0 0 0 2 2 2 2 2 3 3 3 3 3 3 4 4 4 4
5 5 5 5 5 6 6 6 7 8 10 + 5 5 5 5 5 5 6 6 6 6 6 7 7 7 7 7 8 9 9 9
4 4 0 11 – 0 0 0 0 1 1 1 1 3 4 4 4 4
7 11 + 5 6 6 7
Here’s how to read this plot: For 1976 finches, the smallest beak depth was 62 mm, and the largest
was 117 mm. For 1978 finches, the smallest beak depth was 71 mm, and the largest was 117 mm.
a.) Compute the median beak length for each group. Then, describe the shape, center and spread in
each group. When you look at the plots above, does it look as though the post-drought finches
have typically larger beak depths than the pre-drought finches?
b.) Construct a side-by-side boxplot of these data. You need to put both boxplots on the same scale,
next to each other. Be sure to use the version of boxplots described in class – the one that
identifies outliers.
