Final answer:
Charles Darwin's key discoveries contributing to the theory of evolution come from his observations on the Galápagos Islands.
Step-by-step explanation:
Some of Charles Darwin's most crucial discoveries, which aided in the development of his theory of evolution, were based on studies of birds and tortoises he observed on the Galápagos Islands. Located 966 kilometers off the coast of Ecuador, the Galápagos are composed of 16 small volcanic islands.
Darwin noticed both that each island hosted distinct species and that species differed even between islands, such as the giant tortoises with varied shell shapes and the renowned Galápagos finches with different beak structures.
One of Darwin's key observations was that tortoises on different islands had developed different shell shapes that correlated with the environment of their particular island.
For instance, on islands with tall vegetation, the tortoises had longer necks and upward-bent shells, which helped them to reach for food. Likewise, Darwin noticed that the finches on different islands had unique beaks suited to the types of food available on their island.
These observations led him to wonder how species originate, thus forming the basis of his theory that organisms develop adaptations over time due to environmental pressures, a process now known as natural selection.