Final answer:
Darwin's observations that led to his theory of evolution occurred in the Galápagos Islands, where he studied the diverse beak shapes of ground finches, leading to his concept of natural selection.
Step-by-step explanation:
Charles Darwin made key observations that led to his understanding of the mechanism of evolution during his expedition to the Galápagos Islands off the western coast of South America. These observations were made from 1831 to 1836 while on a voyage around the world aboard the H.M.S. Beagle. Among his significant findings were the variations between species of ground finches, each exhibiting unique beak shapes and sizes adapted to different food sources on the islands. These variations mirrored the diversity of similar finches found on the mainland, leading Darwin to propose that the island species had evolved from a common ancestor, illustrating the process of natural selection.