One-hundred-thousand years ago, the Volcano Alcedo on the Galapagos island of Isabela erupted, leaving a lasting mark on the genetic make-up of the local tortoise population. The Alcedo tortoise population is thought to have been severely reduced after the eruption and consequently exhibits far less genetic variation than neighboring tortoise populations.

This came as a surprise to scientists studying the genetics of the Galapagose tortoises. They had expected wide genetic variation in the Alcedo tortoises, since the population is the largest group of tortoises in the Galapagos archipelago.

Isabela is an island consisting of five young, active volcanos. These include the Volcano Wolf (1707 meters), Volcano Darwin (1330 meters), Volcano Alcedo (1097 meters), Volcano Sierra Negra (1370 meters), and Volcano Cerro Azul (1689 meters). There are also remnants of a sixth, partially destroyed volcano on the northwest arm of Isabela Island, Volcano Ecuador. During the past century, Cerro Axull, Sierra Negra, and Wolf have erupted a combined total of 13 times.

Volcano Alcedo is beautiful location, with a 7 km-wide caldera and steaming fumarole where giant tortoises are often sighted. These Alcedo tortoises have recently attracted scientists' attention. The tortoise population on Alcedo has been found to have very low genetic variation when compared to populations of tortoises on Isabela's other volcanos. Scientists studying the genetic make-up of the Alcedo tortoise population found evidence in the tortoises' DNA that something occurred about 100,000 years ago to cause a drastic reduction, or bottleneck, in the tortoise population.

The timing of the genetic bottleneck coincides with a large eruption of Volcano Alcedo. The eruption is thought to have reduced the population down to a group of only a few individuals, possibly including as few as just one female.

Find out more: Genes Record a Prehistoric Volcano Eruption in the Galápagos (Science)

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