good reference: Ernst and Barbour, 1989, Turtles of the World, Smithsonian Institution Press.

usually treated as a single species (Geochelone elephantopsis) with 12 subspecies

all 12 populations are allopatric (one possible exception) -- all morphologically distinct (often clearly so) -- in some cases produce hybrids with lower viability in captivity -- but blood enzymes show high degree of genetic similarity among the populations (92-99%) (Marlow & Patton, 1981, Journal of Zoology 195: 413-422)

genetic analysis also shows close relationship between Galapagos tortoises and much smaller tortoises in same genus on South American mainland -- Galapagos populations are no more similar to any one continental species than are mainland species among themselves (<= 87%)

two different carapace (dorsal shell) shapes -- saddle-backed and humped -- do not correlate with genetic differences -- saddle-backed forms have evidently evolved several times independently as adaptations for extending the neck upward for browsing on higher vegetation

most populations, even those with large numbers of adults, are in danger of extinction -- they have little or no recruitment of young because mammals (rats, pigs, dogs, goats) eat all hatchlings -- these mammals all introduced (intentionally or unintentionally) by humans in past two centuries

some notes on the 12 populations . . .

abingdonii (saddle-backed) -- Pinta -- one captive survives

chathamensis -- northeastern S. Cristobal -- >500 -- once occupied entire island -- dogs eat all hatchlings -- genetically most distinct

darwini -- S. Salvador -- >500 -- pigs eat all hatchlings

ephippium (saddle-backed) -- Pinzon -- >150 -- rats eat hatchlings -- restocking with captive-bred juveniles 4-5 years old

hoodensis (saddle-backed) -- Espanola -- >20 -- goats destroy nests -- adults rarely if ever meet for mating -- restockling with catpive-bred juveniles

nigrita -- S. Cruz -- >2000 -- pigs, rats, goats, cats

phantastica (saddle-backed) -- Fernandina -- last seen 1906 but possibly still exists in wild -- no human interference or introduced mammals

becki (saddle-backed) -- Isabela (northern end) -- >1000

microphyes -- Isabela (Volcan Darwin in north-central part of island) -- >500

vandenburghi -- Isabela (upper slopes of V. Alcedo in center of island) -- >3000 -- little disturbed as result of inaccessibility of habitat

vicina -- Isabela (Cerro Azul in southern part of island) -- >400 -- hatchlings eaten by rats, pigs, dogs, cats

guntheri -- Isabela (Sierra Negra in south-eastern part of island) -- >300 -- exploited by settlers -- possibly overlaps very similar vicina in a corridor of habitat along the coast

summary . . .

12 distinct populations (subspecies? -- species? -- what is your opinion?) on 8 islands -- 5 on different mountains on Isabela -- all 12 populations in danger of slow extinction