Final answer:
The finches' beak shape variations, as observed in the Galapagos finches, are heritable. The Grants' research demonstrated that during environmental changes, finches with certain beak shapes had higher survival and reproduction rates. This led to an evolution in beak size over time.
Step-by-step explanation:
The Galapagos finches are a well-known example of natural selection and variation. Peter and Rosemary Grant's research on the finch populations demonstrated that some of the variations in the finches' beak shapes were heritable. They found that during periods of environmental change, such as a decrease in the availability of certain types of seeds, the survival and reproduction of finches with specific beak shapes were favored, leading to an evolution in beak size over time.