galapagos
CNH Tours - Cultural and Natural Heritage Tours Galapagos
Thursday September 12, 2024
Shaped by Survival: The Evolution of Galápagos Giant Tortoises
Galapagos is a great place to see, first hand, how biological evolution happens - you just have to know where and how to look (hence our belief that a good naturalist guide is critical to getting the most out of your visit to the islands).
Darwin spent 5 weeks there in Sept/Oct 1835, collecting specimens, but also looking around and trying to understand how things were the way they were. He observed that each island was "inhabited by a different set of beings."
During his time in the islands, he had the chance to have a chat with the local bigwig, the Ecuadorian vice-governor, Manuel Julian Cobos (the human population at the time likely didn't surpass 200). Ecuador had claimed Galapagos 3 years earlier and was intent on colonizing the archipelago. The vice-governor told Darwin that he "could at once tell from which island any one [tortoise] was brought."
These kinds of statements were catnip to Darwin. He had to dig deeper and try to understand. It turned out that indeed, there were two basic giant tortoises morphologies in the islands, defined by shell shape i) The dome on the left in the pic below, and ii) the saddle on the right.
If you can visit different islands, and if you're a keen observer, you'll find that the dome shaped tortoises live on high altitude islands (Isabela), where there is plenty of rain in the highlands. They have a lot of food growing on the ground.
In contrast, the saddle-shaped tortoises live on low altitude islands - where conditions are a lot dryer, and there is a lot less vegetation on the ground. There, tortoises struggle to reach up for succulent cactus pads.
Over the two to three million years since the first tortoises arrived in Galapagos, any young tortoise born on low-lying islands with a slight recess on its shell, above the neck, would have found it a bit easier to reach for food than others without it. In extremely dry years, such individuals were more prone to survive and reproduce, and pass that characteristic down to its offspring. Repeat this process over millions of years and... presto! You have the saddle-backed tortoise populating low-lying, dry islands in Galapagos.
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