Final answer:
Charles Darwin's key observations that led to his theory of natural selection were made on the Galápagos Islands, where he noted significant differences among different finch species, particularly in beak shapes adapted to varied diets.
Step-by-step explanation:
Charles Darwin observed biological diversity that led to the development of his theory of natural selection on the Galápagos Islands during his five-year voyage on the HMS Beagle.
Darwin noted distinct variations among species that were similar yet had slight differences, especially evident in the varied beaks of Galápagos finches. These birds, despite belonging to the same group, were adapted to different ecological niches within the islands. The finches had different beak shapes suited to their specific diets. This observation among others led Darwin to conceive the foundations of natural selection, positing that species evolve over time with traits that better enable them to survive and reproduce in their environment becoming more common.
This idea was a cornerstone for his later work, "On the Origin of Species," which changed how we understand the process of evolution and the development of biodiversity on Earth.