Final answer:
Charles Darwin developed his theory of evolution by natural selection based on his studies of organisms on the Galápagos Islands, specifically the different sizes and shapes of the finches' beaks.
Step-by-step explanation:
Charles Darwin developed his theory of evolution by natural selection based on his studies of organisms on the Galápagos Islands. These islands are known for their unique species that have adapted to their specific environments. Darwin observed that the finches on the islands had different sizes and shapes of beaks, which helped them acquire different types of food. These observations led him to propose that species can change over time through natural selection.