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Did Galápagos Turtle Lineage Survive Ancient Blast? |
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John Roach for National Geographic News |
| October 2, 2003 |
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About 100,000 years ago the top of Volcano Alcedo in the Galápagos Islands exploded in a violent eruption that smothered the region in pumice and blew away all but one lucky lineage of the giant tortoises that lived there, according to a new study. "The only lineage that probably survived the eruption was the one that repopulated the region," said Luciano Beheregaray, a molecular ecologist at Macquarie University in Sydney, Australia. The Galápagos tortoises are perhaps the best known animals from the island archipelago off the Pacific coast of South America. They can grow as heavy as 550 pounds (250 kilograms) and live over 100 years. The tortoises are believed to belong to one species (Geochelone nigra) with 14 different subspecies spread throughout islands, including three species that have been hunted to extinction since humans arrived in the 1600s. Beheregaray is part of a research group that exploits advances in genetics to study the evolution and distribution of tortoises in the Galápagos. Their most recent paper appears in the October 3 issue of Science. Other members of the group include Claudio Ciofi, Adalgisa Caccone, and Jeffrey Powell at Yale University in New Haven, Connecticut; Dennis Geist at the University of Idaho in Moscow; and James Gibbs at State University of New York in Syracuse. According to the new research, the tortoises that live on Volcano Alcedo on Isabela Island today are descendants of the lone surviving lineage of the volcanic catastrophe, and they carry a record of the eruption in their genes. "Our study shows that we can infer specific historical events in a population using DNA markers from extant individuals," said Beheregaray. Christopher Schneider, a biologist at Boston University in Massachusetts, said that the use of genetic markers to infer population history has been around for more than a decade, but that estimates for when events took place are often quite broad. "However, we don't want to throw the baby out with the bathwater because sometimes broad estimates are useful, so I hope that theoreticians keep hammering away on this problem," he said. Genetic Analysis Today the 3,000 to 5,000 tortoises that make up the Alcedo tortoise population (Geochelone nigra vandenburghi) are the largest group in the Galápagos, but they have very low genetic diversity compared to four other populations on Isabela. Each population there occupies a different volcano. The low genetic diversity seems to suggest that the Alcedo population is relatively young, but all the volcanoes on Isabela are about the same age and thus presumably the tortoise populations should be about the same age, the researchers report. The one unique thing about Alcedo among Galápagos volcanoes is that it experienced an explosive eruption in its past, according to the researchers. All other Galápagos volcanoes are constructed on basaltic lavas that slowly seeped out from the ground. According to potassium-argon dating of the several-meters-thick layer of rhyolitic thephra—particulate matter from an eruption—on Alcedo, the researchers concluded that the catastrophic eruption took place about 100,000 years ago and that it probably wiped out most of the giant tortoise population there. To test their theory, the scientists analyzed genetic data from tortoises that live near the volcano today. They found a clear signature of a dramatic population decline. The signature, a reduction in genetic variety, is known as a genetic bottleneck. The bottleneck signature is absent from the other tortoise populations on Isabela, but none of these populations were impacted by the eruption, say the researchers. Using another genetic technique, the researchers determined that four new lineages of tortoises descended from the survivors of the Alcedo eruption appeared within the last 88,000 years. "In other words, these lineages appeared, by mutation, after the volcano eruption," said Beheregaray. Population History The scenario that emerges, conclude the researchers, is of all the Alcedo tortoises getting blown off the volcano at the time of eruption except for the lone, lucky lineage, which could have been as small as a single pregnant tortoise, but probably a group of related individuals. "The bottleneck test…has been used extensively to test for population contractions, especially for endangered species that experienced recent contractions," said Beheregaray. "In our study we were able to show the signature of a relatively old population contraction and estimate the age since contraction and relate it to a well dated catastrophe." Robert Rothman, a biologist at Rochester Institute of Technology in New York who is familiar with the group's work but not a member, said the research is interesting and the conclusions reasonable. "There doesn't seem to be any reason why Alcedo would have been colonized later, so to link it to the major rhyolite eruption is a very reasonable conclusion," he said. More About Turtles News Stories Satellite-tracked Turtles Survive Hurricane Isabel Photo Gallery of Endangered Turtles Rare Two-Headed Tortoise Found in South Africa Ichthyosaur's Turtle Supper Causes Extinction Debate Saving Turtles by Taking Them off the Menu (with photos of some of the world's most endangered turtles) Saving Sea Turtles With a Lights-Out Policy in Florida Girl Scouts Help Scientist Conserve Turtles in U.S. Leatherback Turtles Near Extinction, Experts Say Can Network of Colonies Save Asia's Turtles? China's Taste for Turtle Fuels Asian Crisis, Groups Say Turtles Smuggled to China as Food Find Haven in U.S. National Geographic Magazine Photos: 1930 image of bather "riding" on the back of a turtle in Australia David Doubilet image of a green turtle National Geographic Guide to Animals and Nature: Go>> |
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