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Evolution of Beak Size

A group of researchers spent 8 years in the Galapagos Islands measuring beaks, observing feeding behavior, and measuring rainfall in a population of a particular species of Darwin's finch. They found that this species eats only small seeds and that its beak enables the bird to crack and eat such seeds quickly and efficiently. The beak-size data are in the table below.

Develop a hypothesis that explains what caused beak size to increase in dry years. Your hypothesis should specify how the fates of individuals determine changes in the population.

Year 1 2 3 4 5 6 7 8
Rainfall normal normal normal very low normal normal normal normal
Avg. beak size 10.1 10.3 10.2 12.1 12.5 10.1 10.0 10.1


Solution Preview

Based on the table, beak sizes are 10.0 - 10.3 in normal years, but over 12 in the year when there less rain and the subsequent year. Therefore, we can assume that low rainfall somehow causes an increase in beak size in the population.

The amount of rainfall probably doesn't affect the beak size directly. Rather, the rainfall probably affects the seeds that the finches eat, which, in turn, ...

Solution Summary

The solution suggests and explains the reasoning behind a hypothesis aimed at explaining the data on beak sizes of finches in the Galapagos Islands.