Final answer:
The significant variation that Darwin observed in the Galápagos finches was the beak shape, which adapted over time due to natural selection, enabling finches to exploit different food sources and demonstrating the process of adaptive radiation.
Step-by-step explanation:
The focus point of the finches that allowed Charles Darwin to observe significant variations was the beak shape. During his time on the Galápagos Islands, Darwin noticed that beak shapes varied among finch species and proposed that the beaks of an ancestral species had adapted over time to enable the finches to acquire different types of food sources. Each finch species displayed a beak shape that was well-matched to the seeds available on their respective island, suggesting that natural selection played a role in the evolution of finch populations on the islands.
For instance, finches with larger, stronger beaks could break open larger seeds, while those with smaller beaks were suited to eat smaller seeds. This adaptation allowed the finches to exploit various ecological niches. The differing beak shapes acted as key evidence for Darwin's theory of evolution by natural selection and demonstrated the concept of adaptive radiation, where a single ancestral species evolves into multiple species, each with distinct traits that fit different environments or food sources.
Moreover, long-term studies by the Grants further confirmed Darwin's observations, showing measurable evolutionary changes in beak shapes over generations in response to environmental pressures, such as changes in food availability due to climatic events like El Niño.