The centerpiece of evolutionary biology in the Galapagos are the 13 species of Darwin's finches. The finches are small, drab birds that differ from one another primarily by their beak shape and, therefore, the ecological niche in which they live. They represent a group of very closely species at the point at which they are still in the process of diverging away from each other. The finches were far too complex for Darwin to understand - indeed, their history is still in the process of being unravelled, but he observed that with this birds, it was as if a single pair reached the island and then they evolved into the various types which he saw. The birds in the slides below are the small, medium and large ground finches, the cactus finch, the large cactus finch, and the warbler finch. The differences in plumage are sex and age related: mature males are black (except for the warbler finch).