Emily Grace: Galápagos tortoise evolution

There is so much in these small islands that is interesting, I will do several blogs to cover it all. The free internet in the harbor (and therefore on my boat) is pretty good when someone remembers to turn it on. It seems we see some new cool thing every day we go exploring and I take another 150 photos. Today's topic, Dear Reader is tortoises and evolution.

Galápagos tortoises provide the Lawler Academy of the High Seas an amazingly clear example of evolutionary change -- evidence of descent with modification.

The Galapagos islands are, geologically, brand new -- they were formed by volcanic activity approximately 3-5 million years ago. Therefore, all plants, insects, birds, etc. on the island are species that must have arrived on the islands via rafting in the water (perhaps after a storm blew them off the coast of South America), or via wind currents or self-powered flight. Because tortoises cannot fly, it is most likely that several million years ago, some tortoises on the beach in South America were somehow blown out to sea during a storm, and then, after an undoubtedly unpleasant and long journey ("are we there yet?") bobbing around in shark-infested waters, deposited on the beach on one of Galápagos islands. Given the really, really small size of the islands, it is perhaps most likely that only a single individual arrived -- so envisioning a sole female that was carrying eggs is a likely scenario. Once she laid these eggs, the island was soon populated with little baby tortoises.

Over many, many generations, the tortoises dispersed (perhaps during storms) to other islands in the archipelago. Because the conditions on each island are slightly different, the types of tortoise that survived the best (and that had the greatest success rearing young) were also different.

Map of Galapagos Islands. Some of these islands are very, very dry, and there is not much vegetation to eat. On those islands, cactus "trees" are a good source of dinner for tortoises.

On relatively dry islands, tortoises with long necks had a slight edge over the tortoises with slightly shorter necks -- long necks, and the notch in their shell that allows them to fully stretch it, allow an individual to reach the foliage on the cactus plants that grow on the island.

Tortoise stretching to reach foliage.

Shorter-necked individuals without the saddle shells might find enough foliage to survive, but they just would not grow as fast, and thus wouldn't produce as many progeny as would those tortoises with longer necks. (Longer necks might also be useful when competing for food on the ground with a neighbor.) This is, essentially, natural selection in action -- if there were 1000 tortoises on an island, and a couple had necks that were 1 mm longer than the rest, then these longer-necked individuals would leave a few more offspring than their competitors. And if (as is likely!) these longer-necked individuals happen to have babies with equally longer necks (i.e., due to DNA variations passed from parent to offspring), there would be a bit more long-necked tortoises in each generation, as a percentage of the total population.

Photograph showing neck extension of a saddleback Galapagos tortoise.

Over millions of years, the process has resulted in rather dramatic differences in neck and shell dimensions. Or at least that's what scientists hypothesize has happened, given the current shapes of tortoises on the different islands. Shown below is a sampling:

The three main types of Galapagos tortoises (there are lots of "in-between" types, too). The "saddle" types tend to occur on islands (or parts of islands) that are relatively dry, and the long neck is probably an adaptation for foraging on high stems of cactus.