 Giant Tortoise Of The Galapagos
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Oct. 3, 2003 — Giant tortoises that live on the slopes of Alcedo, a volcano on one of the Galapagos Islands, are far more inbred than those on nearby islands as a result of passing through a genetic bottleneck after an eruption there 100,000 years ago, a new study said.
Alcedo's ancient explosive past may have reduced the tortoise population at the time to just a few related individuals, perhaps even a single pregnant female, according to Luciano Beheregaray, of Macquarie University in Sydney, Australia, who led a team that reports its findings today in Science.
Alcedo is located on Isabela, an island with five giant tortoise groups each occupying the slopes of its five major volcanoes. The group living in vegetated areas on Alcedo is the most numerous in the archipelago, with between 3,000 and 5,000 individuals.
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"Their lack of genetic diversity was a real surprise," Beheregaray told ABC Science Online. "We expected Alcedo to have the largest genetic diversity because it has the largest tortoise population."
But just as human genetic studies have previously suggested that our own origins boast a similar bottleneck from relatively few founding females — the a so-called "African Eve" — it turned out that Alcedo seems to have had its own tortoise equivalent, a "Galapagos Eve."
Beheregaray was studying the tortoises to gain insights into the process of island colonization and population history. A large-scale survey of their mitochondrial DNA — which is inherited along maternal lines — surprisingly revealed that those on Alcedo had three to five times less matrilineal diversity than the other Isabela tortoises, the report said.
That finding could indicate a relatively young population, despite the fact that Alcedo emerged above sea level at about the same time as the other Isabela volcanoes, about 500,000 years ago. As well, Alcedo was probably never exploited by whalers, who hunted tortoise populations on more accessible islands.
What does set Alcedo apart, however, is that it is unique among Galapagos volcanoes in having experienced a major explosive eruption in prehistoric times, 100,000 years ago (the rest are made up of basalt lavas that did not erupt explosively).
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