tracking the tortoise

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Genetic bottlenecks

This variety of Galapagos giant tortoise lives only on the Alcedo volcano in the Galapagos. Its genes carry a record of a volcanic eruption 100,000 years ago. Photo courtesy Michel C. Milinkovitch

On the Galapagos island of Isabela, tortoises on the volcano Alcedo are isolated from others on nearby volcanoes. Original photo: NASA.

Asteroids changed biology forever. What about volcanoes?

up close front view of turtleScientists are fairly sure that a global catastrophe wiped out the dinosaurs and a gazillion other species 66 million years ago. The cause? A huge asteroid impact or a gigantic volcanic eruption. Can smaller upheavals also affect the course of evolution?

Apparently so, according to research published this week. Molecular ecologist Luciano Beheregaray and colleagues looked at a distinct type of giant tortoise that lives on one volcano in the Galapagos islands. Here, you may recall, Charles Darwin had insights that guided him toward his theory of evolution by natural selection. Darwin famously ogled many unique species of finches, but he also gawked at giant tortoises lumbering across the volcanic landscape.

Just as those finches told a story of adaptation to the environment, those tortoises have now told a story of response to geologic change.

Using genetic techniques, the researchers examined different DNA markers, in a subspecies of Galapagos giant tortoise that lives on the upper slopes of Alcedo, a volcano on the island of Isabela.
map of Galapagos, showing alcedo volcano on Isabela Island. World map shows islands situated west of South America

Because DNA changes slowly but steadily, counting the genetic differences between two animals creates a "genetic calendar" of when the animals last shared a common ancestor. Although it's a mite controversial, a similar technique has traced all modern humans to an "African Eve" -- a small group of Homo sapiens who left Africa roughly 100,000 years ago.

Similarity breeds...
Because Alcedo has the largest population of giant tortoises in the Galapagos, you might expect them to have the greatest genetic diversity. But in fact Beheregaray found a surprisingly low level of genetic diversity. After studying the DNA and doing the math, his group concluded that the genetic similarity could only be explained by a drastic reduction in the tortoise population about 1,000 centuries ago.

As even a fruit fly will tell you, such a "genetic bottleneck" slashes genetic diversity.

The timing of the bottleneck correlated nicely with a giant eruption of Alcedo about 100,000 years ago, which layered Alcedo's flanks with several meters of deadly, red-hot ash. Only a few giant tortoises - perhaps just one egg-carrying female - survived the cataclysm to found today's population.

What's new here is the nice correlation between genetics and geology, says Beheregaray, who is now an assistant professor of biology at Macquarie University, in Sydney, Australia. "This is the first study to provide a conclusive example of past demographic changes associated with prehistoric volcanism. ... By using sophisticated methods that analyze DNA markers, we are showing how a specific event, a volcano eruption, affected the history of a population -- an event that happened 100,000 years ago!"

Playing the bottleneck
Islands, especially volcanic ones like the Galapagos and Hawaii, are headquarters of evolution, due to evolutionary processes that accelerate the formation of new species. Here's our gross oversimplification of island biodiversity:

The few organisms that manage to cross the ocean bring a random selection of genes in what's called the founder effect. "Diversification may happen much faster because by chance the few individuals that colonize the population will be a small sample of the original population," Beheregaray explains.

As the founder's offspring fill vacant ecological niches, they change in response to natural selection. This "adaptive radiation" explains, for example, why the Galapagos has so many finch species.

As new volcanoes arise from the ocean floor, both the founder effect and adaptive radiation are repeated, forming more species and sub-species.

Volcanic eruptions, like the giant one on Alcedo 100,000 years ago, reduce the gene pool in a random fashion. These "population bottlenecks" tend to leave a genetically distinct surviving population.

turtle sticks head in waterPhoto courtesy Michel C. Milinkovitch

The Galapagos study is, in one sense, simply a comforting confirmation that the genetic record matches the geological record. But it could also be useful for conservation, says Beheregaray, who notes that even as Darwin's scientific offspring were grooving on the evolutionary marvels in the Galapagos, whalers were slaughtering tortoises for food and oil.

Today, the major threats to giant tortoises and other unique species on the Galapagos are the goats, cattle, pigs and other animals introduced by whalers and human colonists.

The Conservation Genetics of Giant Tortoises: Galapagos project, based at Yale University, intends to use what it calls "conservation genetics" to exploit DNA markers while probing biological diversity on the islands. The goal is to develop management techniques that will protect the giant tortoises, says Beheregaray.

Although Alcedo is isolated by lava fields, goats have finally reached it. Thousands of goats are born each year, and they are quickly destroying habitat, Beheregaray says. "The number of tortoises is declining, not because an eruption, but from the feral goats."

David Tenenbaum

The Why Files

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