In 1831, Charles Darwin visited the Galapagos Islands. While observing the giant land tortoises that lived on these islands, Darwin noted that the shape of the tortoise shell varied depending on which island the tortoise lived. Tortoises on one island had round shells, for example, whereas tortoises on a neighboring island had more flattened, saddle-shaped shells. Which statement BEST summarizes Darwin's explanation for these differences?
