Scientists have been studying various species of brown frogs that live on the Greek islands. The frogs are very similar in appearance. One species of brown frog spread to the islands over five million years ago when the basin between them was dry. The basin re-flooded and the frogs became separated by salt water. Today different species of these frogs live on different islands. These species of frog are very similar but have unique characteristics such as differences in body size, size of the pads on their feet, and coloring. Which of the following best explains these differences