Australasian Science: Australia's authority on science since 1938

Darwinian Relic Survives

By Stephen Luntz

DNA analysis reveals that a species with a significant place in scientific history may not be extinct after all.

You have to respect anyone who weighs 400 kg and might have personally inspired Charles Darwin. Dr Ryan Garrick is on a quest to revive a species that was once considered extinct. He is studying the DNA of the Galapagos tortoises that inspired the concept of natural selection. Given the remarkable age to which the giants live, some of his subjects may have been around when the Beagle visited in 1835.

Tortoises are believed to have first arrived at the Galapagos Islands from South America. While poor swimmers, they can float for many months without food. Famously, the tortoises evolved two differently shaped shells, with protective rounded carapaces on wetter islands and a saddle shape for dry locations where longer necks are required to reach vegetation. Among these two dominant shapes, however, are many subtle variations.

In the 16th century an estimated quarter of a million tortoises inhabited the islands, but hunting reduced their numbers to just 3000 by the 1970s. Chelonoidis elephantopus, native to Floreana Island, was believed to have been wiped out entirely.

Disastrous as the arrival of humans on the islands proved, ships accelerated the movement of tortoises between islands. The tortoises’ capacity to survive long periods without food saw them treated like a canned meal to be eaten late in a voyage. However, when a ship was chased by pirates the tortoises were thrown overboard to reduce weight, and they sometimes washed up on a different island.

Before Garrick arrived at Yale University, researchers in his lab had made an unexpected discovery. “They had been able to get hold of bones and shells from museum collections and got DNA out and created a reference database,” he says. “When collecting blood samples from live tortoises, they came up with a few who had the same ancestry as those from Floreana.”

It seems some C. elephantopus had arrived on Isabela, the largest Galapagos island, which is already home to five tortoise species.

Researchers went back to Isabela and sampled more than 1600 tortoises. The results came back to Yale, where Garrick was part of a more remarkable discovery – some of the tortoises have a C. elephantopus parent. Moreover, about 30 of these are juveniles less than 20 years old.

Full-blooded C. elephantopus were roaming Isabela in the past two decades, and quite likely are doing so today. Although it is not known if these were the original transportees from Floreana, or if some immigrants managed to find each other among the far larger number of C. beki that form the majority of tortoises sampled, but the offspring do not all come from a single very fertile individual – Garrick estimates around 38 full-blooded individuals are responsible for the hybrids they have found.

“To our knowledge, this is the first report of the rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring,” says Garrick. He acknowledges a debate as to whether the different tortoise forms are better classified as subspecies but says: “We look at it from a conservation perspective. The question is what we are trying to conserve. These lineages are on different evolutionary paths.”

The initial discovery of C. elephantopus DNA inspired an ambition for a captive breeding program, selectively breeding for elephantine characteristics with a view to returning them to Floreana, but if full-blooded individuals can be found the path to their recovery may be much shorter. Captive breeding of other tortoise species has seen the total number recover to approximately 20,000 over the past 40 years.

“I originally wanted to be a park ranger,” Garrick says. However, he became interested in the genetic techniques that rangers used and decided to switch to science. After an undergraduate degree at La Trobe University, Garrick did his doctorate at the same university.

“I worked on a project on the Great Dividing Range in NSW, looking at invertebrates on the forest floor,” says Garrick. “I was looking at population genetics to try to understand how the different genetic groups interact to see if there were any hotspots for conservation. The work is still ongoing and I am still involved.”

Nevertheless Garrick moved to Yale, where his first project was on the Baja Peninsula, looking at the genetics of plant/species pairings to see how diversity in one affects that in an interdependent species.

Having followed up with the tortoise work, Garrick is now at the University of Mississippi where he will be studying the insects of the Appalachian forests, combining his original ecological work with the genetic techniques he applied to tortoises.

The Galapagos work continues, however. Having arrived at Yale a few weeks too late to take part in the last collecting trip to the famous islands, Garrick hopes to get another chance to meet the species he is helping to save face to face.