Final answer:
The changes in the finch population after the period of abundant rainfall were caused by evolution through natural selection. During a drought, fewer seeds were available for finches to eat, resulting in the death of many small-beaked birds. Birds with bigger beaks were able to crack and eat seeds of all sizes and therefore survived and reproduced.
Step-by-step explanation:
The changes in the finch population after the period of abundant rainfall were caused by evolution through natural selection. During a drought, fewer seeds were available for finches to eat, resulting in the death of many small-beaked birds. Birds with bigger beaks were able to crack and eat seeds of all sizes and therefore survived and reproduced. This led to an increase in the average beak size in the finch population.