Final answer:
The Grants' 40-year study of Galápagos finches is significant because it provides direct evidence of natural selection by documenting the change in beak sizes in response to environmental changes, validating Darwin's theory of natural selection.
Step-by-step explanation:
The importance of the Grants' study of finches for understanding natural selection is that it allows for the direct observation of evolutionary changes. Over the span of 40 years, Peter and Rosemary Grant have documented the changes in beak sizes of the Galápagos finches, particularly the medium ground finch on the island of Daphne Major. Their research has shown that during periods where environmental conditions change, such as during an El Niño event where rainfall was higher than normal, the availability of different seed types changed. This led to small-billed birds being more successful at survival and reproduction due to an abundance of small soft seeds. As a result, in subsequent years, the Grants observed a decrease in the average bill size within the population, demonstrating natural selection and evolution of an inherited trait in response to environmental pressures.
Such long-term studies are critical because they provide concrete evidence of how environmental factors can direct the course of evolution in a population. The Grants' work validates Charles Darwin's theory of natural selection by showing that traits that enhance survival and reproduction can become more common in a population over time. While option (a) reflects the direct observation of evolutionary change, option (b) affirms the important role the study plays in validating the theory of natural selection proposed by Darwin. Options (c) and (d) are incorrect as the study does not suggest that finches are the only species affected by natural selection or that natural selection has no influence on speciation.