Darwin discovered many different species of finches on the Galapagos Islands. Most folks think that he realized that they all evolved from one ancestor species; and that this is how he came to the idea of evolution by natural selection.
But in reality Darwin first developed his idea of evolution based on his studies of other animals and plants. It was only a few years later that, in collaboration with other scientists, that he discovered that the Galapagos finches also were good examples of evolution.
The following text has been excerpted and adapted from:
He had learned how to preserve bird specimens while at the University of Edinburgh and had been keen on shooting, but he had no expertise in ornithology and by this stage of the voyage concentrated mainly on geology.
On Chatham Island, he recorded that a mockingbird was similar to those he had seen in Chile, and after finding a different one on Charles Island he carefully noted where mockingbirds had been caught. In contrast, Darwin paid little attention to the finches. When examining his specimens on the way to Tahiti, Darwin noted that all of the mockingbirds on Charles Island were of one species, those from Albemarle of another, and those from James and Chatham Islands of a third. As they sailed home about nine months later, this, together with other facts, including what he had heard about Galápagos tortoises, made him wonder about the stability of species.
Following his return from the voyage, Darwin presented the finches to the Geological Society of London in 1837, along with other mammal and bird specimens.
The bird specimens, including the finches, were given to John Gould, the famous English ornithologist, for identification. Gould soon reported that the birds from the Galápagos Islands – that Darwin had thought were blackbirds, “gross-beaks” and finches – were actually “a series of ground Finches which are so peculiar [as to form] an entirely new group, containing 12 species”.
This story made the newspapers. … Gould found more species than Darwin had expected and concluded that 25 of the 26 land birds were new and distinct forms, found nowhere else in the world…. Darwin now saw that, if the finch species were confined to individual islands, like the mockingbirds, this would help to account for the number of species on the islands…The conclusions supported his idea of the transmutation of species.
Here we see that the diversity of finches is an adaptation to the type of food they eat.