When Evelyn Jensen visits a museum to scrape bone from a long-dead Galápagos tortoise, she has two hopes in mind.
First, that the specimen’s genetic material will be well-preserved. Second, that she will find that it is a Floreana tortoise — a species that has been extinct for 180 years.
A lecturer in molecular ecology at Newcastle University, Jensen has, over the last four years, studied 78 Galápagos tortoises at museums in Britain and the United States. But she has found only five from Floreana. Only one yielded high-quality DNA.
“It just kills me that after all of this — just one,” she says.
Nevertheless, that single sample is helping to guide the restoration of giant tortoises that are remarkably similar to the original Floreana tortoise to that Galápagos island, a project that is critical to restoring its depleted ecosystem.
Historical DNA is helping conservationists repopulate the island of Floreana with a tortoise that’s well adapted to the ecosystem.
When 19th century hunters, explorers, and naturalists killed fauna across continents, some of their trophies and specimens went to museums and private collections, forming a record of wildlife before many of their populations drastically declined. As the power of genetic sequencing technology has advanced, and become both cheaper and faster, researchers have begun to compare the genomes of ancient and museum specimens with those of their living descendants. Scientists are now using historical DNA to establish baselines for assessing how much genetic diversity has been lost over time — an indicator of a population’s health and its ability to adapt to a changing world. They’re using it to identify the genealogical continuity of populations and to make decisions about whether remnant populations should be combined, connected with others, or kept separate.
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In Africa, for example, scientists are using historical DNA to help guide critical conservation decisions for black rhinos and lions. In Europe, similar work is informing a breeding program for Spain’s bearded vulture, and it is being used to assess the effectiveness of current conservation strategies for the Iberian lynx and the Iberian imperial eagle. In Australia and New Zealand, scientists are using historical DNA to assess the current genetic health of remnant and translocated populations of the burrowing bettong, a marsupial, and of the takahē, a flightless swamphen. In the Galápagos, similar work is helping conservationists restore the most ecologically devastated island, Floreana, by repopulating it with a species that’s relatively well adapted to that particular island’s ecosystem.
Starting in the 1800s, demand for tortoise oil and meat, plus the introduction of invasive species, drove three of the 15 known Galápagos tortoise species and lineages, including those on Floreana Island, to extinction. But 20 years ago, conservationists spotted tortoises with shells that had an unusual shape living on the north of Isabela Island, about 125 miles from Floreana. Scientists wondered if they were closely related to the extinct tortoises of Floreana.
A team led by Adalgisa Caccone, director of the Center for Genetic Analyses of Biodiversity at Yale University, turned to museums for an answer. The American Museum of Natural History and Harvard’s Museum of Comparative Zoology kept boxes of bones and shell fragments gathered from caves in Floreana, where they had lain possibly for thousands of years. Despite the age and condition of the fragments, the team managed to extract some maternal DNA, large quantities of which float in structures known as mitochondria in every cell. They compared segments of this DNA with those of the mystery tortoises and found a match: Floreanas had somehow reached Isabela and hybridized with its local species.
The discovery of the genetic signature of a long-extinct species was “a unique conservation situation,” says Jensen.
On average, wild populations have lost 6 percent of their genetic diversity over the last few hundred years, says a geneticist.
The scientists used this genetic reference material to choose the most Floreana-like of the Isabela hybrids and are now selectively breeding them in captivity. The goal is to push the genome more toward Floreana and away from Isabela. This is important because the Floreana tortoise is a keystone species — it shapes its ecosystem — and is therefore critical to the larger project of restoring the island, says Jensen. Early this year, some 300 offspring will be released into Floreana’s interior.
Since they had only maternal DNA, the scientists could identify only hybrids whose mothers had Floreana ancestry. That is why Jensen continues to look for more recent and better-preserved specimens, like the one she found in London’s Natural History Museum, in the hope of accessing full genomes, tucked away in the cell’s nucleus. It’s not ideal to have only a single historical genome, she says, but it is helping to hone the selection of hybrids for the next breeding round.
Guiding breeding programs is just one way that historical DNA may help to conserve species. Every member of a species has a slightly different genetic code. This diversity is critical if a population is to adapt over generations to environmental change. But as population sizes decline, they lose their genetic diversity. On average, wild populations have lost 6 percent of their genetic diversity over the last few hundred years, estimates Deborah Leigh, an ecological geneticist at the Swiss Federal Institute for Forest, Snow and Landscape Research. Comparing historical genomes with present-day genomes can help quantify this erosion in a way that simply making head counts of a species’ population cannot.
