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A Volcanic Discovery on Tortoises

On the Galapagos Islands, the creatures carry a remarkably similar DNA. Scientists credit an eruption 100,000 years ago.

October 04, 2003|Rosie Mestel | Times Staff Writer

A volcanic eruption about 100,000 years ago in the Galapagos Islands left its mark on giant tortoises that are still plodding the planet today, according to a report in the current issue of Science.

The tortoises dwell on the island of Isabela in the Galapagos and live out their days in the shadow of Alcedo Volcano. Other distinct, giant tortoises live on Isabela's other four volcanoes.

But something about the Alcedo tortoises is different, reported the authors, after analyzing tortoise DNA. The creatures are three to five times more genetically similar as a group than the others -- the exact opposite of what you'd expect, since the Alcedo population is the largest.

Sometime between 78,000 and 118,000 years ago, the population crashed and then reestablished itself, according to the DNA evidence.

The likely cause: Alcedo Volcano.

"We propose that this eruption, which covered most of the prime tortoise habitat with meters of hot pumice, caused a dramatic population reduction," the authors wrote. Only a few tortoises would have survived the inferno to found a new colony with their DNA and theirs alone.

The other groups of tortoises experienced nothing like this because their volcanoes only dribbled out slow creeps of lava, enabling even a tortoise to flee, said lead author Luciano Beheregaray of Macquarie University in Sydney, Australia.

Beheregaray said the study suggests that such eruptions can contribute to genetic changes in populations and may help fuel the process of evolution.

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