structural:expansion
Recognition of subpopulations

The first systematic survey of giant tortoises was by the zoologist Albert Günther of the British Museum, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther hypothesized that all the giant tortoises descended from a single ancestral population which had spread by sunken land bridges. This hypothesis was later disproven by the understanding that the Galápagos, the atolls of Seychelles, and the Mascarene islands are all of recent volcanic origin and have never been linked to a continent by land bridges. Galápagos tortoises are now thought to have descended from a South American ancestor, while the Indian Ocean tortoises derived from ancestral populations on Madagascar.

At the end of the 19th century, Georg Baur and Walter Rothschild recognised five more populations of Galápagos tortoise. In 1905–06, an expedition by the California Academy of Sciences, with Joseph R. Slevin in charge of reptiles, collected specimens which were studied by Academy herpetologist John Van Denburgh. He identified four additional populations, and proposed the existence of 15 subspecies. Van Denburgh's list still guides the taxonomy of the Galápagos tortoise, though now 10 populations are thought to have existed.