The Galápagos mockingbirds were the animal group Darwin noted as having distinct forms on different islands. During the voyage of HMS Beagle, Darwin collected mockingbirds and labeled specimens by island. Their island-by-island differences helped him recognize that geographically separated populations could diverge.
When ornithologist John Gould examined Darwin’s bird collections in London in 1837, he concluded that the mockingbirds Darwin had treated as varieties were separate species. The discovery was especially significant because the birds were closely related to one another and also had relatives on the South American mainland.
Darwin later became famous for his observations of Galápagos finches, so finches are a frequent answer to this question. However, the early observation that distinct species occurred on separate Galápagos islands specifically concerned the mockingbirds. The islands’ tortoises and other animals also show geographic variation, but they are not the canonical answer to this historical detail.