Finches in the Galapagos Islands
For example; The Galapagos Finches.
He studied the similarities between the island finches and the mainland finches.
He came up with the idea that he would swap the environment that each species lived in and see how well they adapted.
They each had different beak shapes and sizes suited to different foods in their different habitats, and the two species became dissimilar and a new species was created.