The two forms are the most visible difference among Galápagos tortoise populations. Domed shells occur on larger, higher islands where rainfall supports grass and low herbs within easy reach. Saddleback shells, with a raised front margin and longer neck and legs, occur on dry, low islands where much of the available food is cactus pad and shrub browse above ground level.
The pattern was noticed early. The governor of the penal colony on Floreana told Charles Darwin in 1835 that he could identify a tortoise's island from its shell, a remark Darwin recorded and later found significant.
The forms are not two species. Shell shape varies along a gradient, intermediates exist, and genetic work groups the populations by island history rather than by shell profile. Reading shape as a direct guide to relatedness gets the tree wrong.