Final answer:
Darwin thought the Galápagos finches might be related to the finches of South America because they all exhibited different traits yet shared similarities that suggest a common ancestor, exemplified by various beak shapes adapted to different food sources on the islands.
Step-by-step explanation:
Darwin may have come to think that the finches of the Galápagos Islands might be related to the finches of South America despite their different appearance because of the concept of shared ancestors. When Darwin visited the Galápagos Islands, he observed that even though the finches on different islands had distinctly different beak shapes and sizes, they bore similarities to a finch species found on the mainland. This suggested to Darwin that all of these finches shared a common ancestor and had diversified on the islands through the process of natural selection, adapting to different food resources across the islands. For instance, seed-eating finches developed stronger, thicker beaks for breaking seeds, while insect-eating finches had more pointed, spear-like beaks for catching insects. This trend of adaptive changes based on food sources and habitats is an example of the evolution of species as proposed in Darwin's theory.
The correct answer to why Darwin thought the different species of Galápagos finches might be related to the South American finches would thus be B) Shared ancestors.