Final answer:
Darwin's observation of varying beak shapes among Galápagos finches supports his hypothesis of species evolution through natural selection, as further evidenced by research demonstrating beak size changes in response to environmental factors.
Step-by-step explanation:
The observation by Darwin that most supports his hypothesis of gradual species evolution through small changes in ancestral species is the variation in beak shape among finch species. Darwin noticed that each finch species had a beak shape uniquely adapted to its diet and the particular environment it inhabited. This phenomenon of multiple species evolving from a single ancestral species to fill various ecological niches is known as adaptive radiation. This was evident in the Galápagos finches, with beak shapes perfectly suited for different food sources, indicating that natural selection based on availability and competition for food could lead to the development of new species.
Extensive research by Peter and Rosemary Grant provided empirical evidence supporting Darwin's hypothesis. Natural selection was observed in action when the Grants documented beak size changes in finch populations in direct response to environmental changes caused by an El Niño event. Small-beaked finches had better survival and reproductive rates due to a shift in the available food sources, leading to a measurable evolution in beak size over generations.