Genes, Hybrids, and Giant Tortoises

Charles Darwin visited the Galápagos Islands in 1835. As he moved from island to island, he saw the subtle differences between finches, tortoises, and other animals. These observations led to the discovery of the theory of evolution as an expanding “tree of life,” first sketched by Darwin in his notebook entry dated just two years later in 1837.

The great tortoises of the Galápagos could not fail to impress. The greatest of all, the tortoise Chelonoidis elephantopus, can live to be a hundred years old and grow to six feet and almost 900 pounds.

Until now, it was believed that whalers hunted the great C. elephantopus to extinction shortly after Darwin’s visit. Now, however, new research suggests that a few of the great tortoises may still be alive.

Caption: G. Becky tortoises are native to Isabela Island in the Galapagos chain and have more domed shape shell. Credit: Courtesy Yale University.

Researchers have found what they believe are direct offspring of purebred C. elephantopus tortoises. By testing the genes of living tortoises, researchers concluded that they were studying hybrids. One parent was from a related species, C. becki. But the other parent was clearly C. elephantopus. And since the living tortoises were still quite young, researchers were drawn to the obvious conclusion that the C. elephantopus parent lived until a few decades ago and may still be roaming the slopes of Isabela Island.

So now it’s a race against time to find surviving purebred C. elephantopus tortoises in hopes that enough of them still exist so the species—truly one of the great animal species—can be brought back from what seemed like extinction. According to the report, “purebred tortoises of the recently ‘extinct’ C. elephantopus from Floreana Island are very likely still alive today.”

Caption: This tortoise is a hybrid of G. Becky and C. elephantopus, a species native to Floreana Island some 200 miles away and thought to be extinct. Genetic analysis of tortoise population on Isabela Island suggests purebred individuals of C. elephantopus must still be alive on Isabela. Credit: Courtesy of Yale University

One interesting parallel. Using a similar approach, researchers have recently concluded that human beings are also hybrids. For example, many of us contain genes from our Neandertal ancestors. The big difference, of course, is that our interbreeding occurred tens of thousands of years ago. In either case, hybridization or interbreeding occurs when the twigs at the end of Darwin's tree of life come together. As evolutionary biologists are discovering, speciation (or branching) is critical to evolution, but so is interbreeding or hybridization.

According to the report, “To our knowledge, this is the first rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring.” The report, "Genetic rediscovery of an ‘extinct’ Galápagos giant tortoise species," appears in the January 9, 2012 issue of Current Biology.