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Finch beaks after a drought
Topic: Natural selection
On Daphne Major in the Galápagos, the 1977 drought left medium ground finches with mostly large, hard seeds to eat, and most of the population died. Nearly every bird on the island was banded and measured, so researchers could compare the birds alive before the drought, the survivors that went on to breed, and those survivors' offspring once grown. Adult beak size does not change during a bird's life. The numbers below are rounded, illustrative values like those reported, not the published data set.
Data
| Group | Number of birds | Mean beak depth (mm) |
|---|---|---|
| Adults before the drought | about 1,200 | 9.4 |
| Adults that survived and bred | about 180 | 9.9 |
| Offspring of the survivors, as adults | about 300 | 9.8 |
What best explains why the offspring's mean beak depth (9.8 mm) is larger than the pre-drought mean (9.4 mm)?
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