Galapagos Tortoises and Evolution

Lonesome George lived in the Galapagos, a chain of volcanic islands off the coast of Ecuador, in South America—islands that forever changed our understanding of the natural world. While visiting the Galapagos in 1835, British naturalist Charles Darwin observed local plants and animals. He became fascinated by species that seemed related to ones found on the mainland—but that also had many physical variations unique to different islands.

Over time, Darwin began to wonder if species from South America had reached the Galapagos and then changed as they adapted to new environments. This idea—that species could change over time—eventually led to Darwin’s theory of evolution by natural selection.

The Galapagos islands are a chain of volcanic islands off the coast of Ecuador. 
NASA

Evolution of Tortoise Shapes

Galapagos tortoises have two very different shapes, each adapted for different feeding habits needed on low, arid islands versus high, lusher islands.

Saddle-back Tortoises

Some tortoises, including Lonesome George, have shells that rise in front, like a saddle. This adaptation makes it easier for them to lift their heads high to eat tree cactus and to resolve disputes over limited food resources, which they do through raising their heads as high as possible during social interactions.

Pinta Island tortoises have shells that rise in the front (right in the overhead view above), like a saddle. 

Dome-shaped Tortoises

Tortoises with dome-shaped shells live on islands where there is an abundance of vegetation close to the ground, making it less necessary for the animals to raise their heads to feed.

The majority of tortoises on Isabela Island have dome-shaped shells.