In natural populations, the distinction between species is fuzzy, so that anatomists have varieties of species. Darwin saw this as evolution in progress. The islands are volcanic and were never connected to the mainland. In the beginning, there would have been no life. Then the finches would arrive.
Finches are small seed-eating birds.
On the islands, the multiplied and diversified. There were three seed-eating finches, of three different sizes, large, medium and small. None of them ate the same food and none of them interbred. They were in separate niches. Two others fed on cacti and the rest on insects.