Final answer:
Charles Darwin observed variation in the Galapagos finch population that can be attributed to differences in climate among the islands.
Step-by-step explanation:
Charles Darwin observed a unique beak size and shape in the finch population of each of the Galapagos Islands he visited. The most likely cause of the observed variation is differences in the climate among the islands. The islands have different climates, which result in varying food availability and environmental conditions. This leads to natural selection favoring finches with beak sizes and shapes that are best suited to their specific island's resources.
Learn more about evolutionary adaptation in finches