They have evolved slight differences in the shell shape, so Charles Darwin could see that those living on the wetter Galápagos Islands had the normal round shape, while those living on drier islands had a high saddle-shaped peak on the front of the shell, so their necks could reach higher and obtain food during droughts.

The differences are so striking that many people could tell which island a particular tortoise came from. The only explanation that made sense to Darwin, and to scientists today, is that they must have evolved from a common ancestor about 5 million years ago, then began to diverge in genes and in shape as they adapted to the conditions of their local environment. Today, they are considered one of the first great examples of evolution in action.