It’s all about the shell shape
It would be interesting to find out which domed tortoise Darwin tried to mount and couldn’t. There are two options from which to choose: the dome-shelled giant tortoise and the saddle-backed tortoises. It is precisely the shells that separate the different species and subspecies of giant tortoises.
Resembling half of a soccer ball the dome-shelled tortoise has a neatly round, shiny shell and is generally larger than its cousin. This tortoise is also fond of more humid climates that offer a wealth of low-hanging vegetation which is easily reached by the crawling tortoises.
On the other hand, the saddle-backed tortoise is a true freak of nature, or simply a true evolutionary champion. Endemic to hotter, drier, islands where vegetation is more sparse these giants developed throughout the years a saddled shell that allows the tortoise to stretch its neck upward and reach higher growing plants
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These adaptations in between different species, can in fact be considered the birth of a new breed. The type of vegetation on each island, the survival conditions and the challenges towards finding food is perhaps the main catalyst in these changes in shape and size.
Saddle backed tortoises species are found on smaller arid islands with sparse vegetation, hence the Espanola tortoise as well as those found on Pinzon Island and of course lonesome george who was a species from Pinta island all have something in common: saddleback shells.
In contrast, a larger Galapagos island that has developed cloud forests in it's highlands and has a more humid vegetation that is closer to the ground is the ideal environment for the domed tortoise. This can be observed in the endemic species found on Isabela Island, San Cristobal and Santa Cruz island.











