Tortoises chronicle eruption in their genes.

An ancient volcanic eruption in the Galapagos Islands bequeathed diminished genetic diversity to one group of the archipelago's famed giant tortoises, a new analysis suggests.

Five subspecies of the Galapagos tortoise live on the island of Isabela, which is also home to five major volcanoes. Two of the subspecies live along Isabela's southern coast and are now represented by only a few hundred tortoises each, says Luciano B. Beheregaray, a molecular ecologist at Macquarie University in Sydney, Australia. Those groups are the remnants of populations that were decimated by whalers and other seafarers who killed the creatures for food. Members of another subspecies, which escaped the stew pot by inhabiting the relatively inaccessible slopes of the island's Alcedo volcano, number in the thousands.

When Beheregaray and his colleagues conducted detailed DNA analyses of tortoises from these three subspecies, they unexpectedly found that the more-populous Alcedo tortoises had one-third to one-fifth the genetic diversity of the two subspecies inhabiting the island's southern shore. Analyses of several genetic mutations unique to the Alcedo tortoises suggest that those mutations appeared about 88,000 years ago.

That timeframe roughly matches the span since a major eruption of Alcedo that dumped more than 3.4 cubic kilometers of pumice and ash on the volcano's slopes, says Dennis Geist, a volcanologist at the University of Idaho in Moscow. The lava flow from that event appears to have occurred sometime between 120,000 and 74,000 years ago. Beheregaray, Geist, and their colleagues report their findings in the Oct. 3 Science.

The scientists propose that the Alcedo subspecies exhibits less genetic diversity than the other subspecies do because its members are the descendants of just a few progenitors that repopulated the area after the ancient eruption. It's impossible to tell whether the ancestors of the current Alcedo tortoises lived in the area and survived the eruption or emigrated there afterwards, says Beheregaray.

The team's genetic analyses suggest that 82 of the 90 Alcedo tortoises examined descended from a single ancient female.

If the analyses are correct, then it's good news, says Howard L. Snell, a conservation biologist at the University of New Mexico in Albuquerque. The results suggest that a population of a species or subspecies that has been reduced to just a few individuals isn't necessarily doomed to extinction, he notes. The Alcedo tortoises apparently skirted extinction and rebounded to their present population, which may include as many as 10,000 individuals.