Final answer:
The decrease in average beak depths after 5 years of plentiful rainfall can be explained by evolution through natural selection in response to changes in food availability. The drought on the Galápagos island caused a decline in small seeds, leading to the survival and reproduction of finches with larger beaks. This resulted in an increase in the average beak size of the finch population.
Step-by-step explanation:
The decrease in average beak depths after 5 years of plentiful rainfall can be explained by evolution through natural selection in response to changes in food availability. In the case of the Galápagos finches studied by Peter and Rosemary Grant, a drought caused a reduction in small seeds, which resulted in many small-beaked finches dying as they were unable to crack open larger seeds. The finches with larger beaks, on the other hand, were able to survive and reproduce, passing on their genes for larger beak size to the next generation. This led to an increase in the average beak size of the finch population.