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Galápagos tortoise evolution
To educate young children about evolution, you can do no better than the giant tortoises that live on the islands of the Galápagos, off the coast of Ecuador. They are freakishly heavy (up to 700 lbs, the size of a young Jersey cow), live incredibly long (up to 188 years), and can be ridden like a horse (they also taste great, but don't tell the kids). But the greatest thing about the Galápagos tortoises is that they provide an amazingly clear example of evolutionary change -- evidence of descent with modification. This page gives a brief overview of their evolution and provides links to resources that might be useful if you wanted to construct a lesson plan for your elementary school students. 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 (at all), 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 (and yes, they probably were cute, just like baby Galápagos tortoises today). Shown below is a map of the region, showing how far the journey was (and still is):
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.
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 (photos 1, 2, 3, 4).
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.
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:
Students might recognize that Lonesome George, the most famous tortoise alive today, is of the "saddle" type. The previously most famous tortoise, Harriet (who was at the Australia Zoo until her death in 2006), was a "domed" tortoise. If your local zoo has Galapagos tortoises, you might find that it has representatives of all three types; only recently have zoos attempted to breed only "within" type. Galápagos tortoises are thus a great example of how populations adapt to local conditions through the process of natural selection. Kids will enjoy it, and you can hit a lot of the science standards with it. I'll be updating this page, so if you have ideas on exactly how to integrate tortoises into the classroom, please get in touch with me -- I'd love to hear from you. I'm especially keen on finding instructions for making tortoises out of clay, so that students could make a population of the little beasties. All I can find is a YouTube link for making turtles out of polymer (really slick, but perhaps too advanced for elementary school kids). Another fun project (shown below) is to make paper maché shells and glue socks in them to act as necks. Then you can demonstrate the advantage of long necks and "saddleback" shape when a tortoise is trying to eat something tall.
Links: San Diego Zoo Special thanks to my talented sister, Teal Purrington, for the line drawings. Ways to flaunt your love of the Galapagos Islands and Galapagos tortoises: http://www.cafepress.com/galapislands
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