From Scientific American:
When the 26-year-old Charles Darwin sailed into the Galápagos Islands in 1835 onboard the HMS Beagle, he took little notice of a collection of birds that are now intimately associated with his name. The naturalist, in fact, misclassified as grosbeaks some of the birds that are now known as Darwin’s finches. After Darwin returned to England, ornithologist and artist John Gould began to make illustrations of a group of preserved bird specimens brought back in the Beagle’s hold, and the artist recognized them all to be different species of finches. From Gould’s work, Darwin, the self-taught naturalist, came to understand how the finches’ beak size must have changed over the generations to accommodate differences in the size of seeds or insects consumed on the various islands. “Seeing this gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends,” he noted in The Voyage of The Beagle, published after his return in 1839.
Twenty years later Darwin would translate his understanding of finch adaptation to conditions on different islands into a fully formed theory of evolution, one emphasizing the power of natural selection to ensure that more favorable traits endure in successive generations. Darwin’s theory, core features of which have withstood critical scrutiny from scientific and religious critics, constituted only the starting point for an endlessly rich set of research questions that continue to inspire present-day scientists. Biologists are still seeking experimental results that address how natural selection proceeds at the molecular level—and how it affects the development of new species.