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From Darwin to DNA: evolution, genomics, and conservation of the Galapagos giant tortoises

When Charles Darwin arrived on the Galapagos Islands in 1835, the remarkable morphology of the native breeds of giant tortoise, unique to each island struck him as significant. Later on, these thoughts contributed to his ideas about the theory of evolution through natural selection.

In the 18th and 19th centuries, Galapagos giant tortoises were not just interesting scientific specimens, they were also a critical source of fresh meat for sailors during long sea voyages into the Pacific Ocean. With the Galapagos acting as a larder, more than 200,000 giant tortoises were taken from the islands for human consumption. Hundreds more were sacrificed to fill natural history museums throughout the world with specimens of these extraordinary beasts.

Image credit: Pinzón Galapagos giant tortoise. Photograph by Evelyn Jensen.

By the 1960’s, the giant tortoise populations were in rough shape; some were extinct, some were reduced to a handful of individuals, while all were facing severe habitat destruction and competition from invasive species, such as goats and rats. Concerted conservation actions began, focusing on recovering species by addressing the challenges specific to each case.

Image credit: Adult Pinzón Galapagos giant tortoise, underneath an Opuntia cactus. Photograph by Evelyn Jensen.

Key to these efforts was determining the scale at which to act. And for tortoises, this meant following up on Darwin’s observations and confirming that each population was a distinct evolutionary unit. To this end, phylogenetic and population genetic techniques have been applied to study the Galapagos giant tortoises since their development in the 1990’s. This body of work showed that the tortoises alive today could be divided into 12 unique lineages, and conservation actions have been made accordingly.

Most recently, we have used advanced sequencing techniques to conduct a genome-wide assessment of genetic diversity. These methods produce orders of magnitude more data than had previously been used, and can detect subtle signals inaccessible to traditional methods. However, the results from this study in The Journal of Heredity confirmed the previously found genetic lineages, and showed that measures of diversity were significantly correlated with those derived from the previous generation of genetic markers. These are welcome findings that confirmed the genetic uniqueness of each species and management plans for their conservation.

In another study in the same issue, we focused on just one species, the Pinzón Island giant tortoise, and questioned if genetic diversity before and after the population crash on the island was being accurately measured by the smaller datasets common to previous genetic sequencing studies. We found that if we had sequenced less than the whole mitochondrial genome (a feat which has only been possible on a large scale recently), we would have incorrectly estimated the differences in diversity between the population in 1906 and 2014, depending on the specific genes selected for the study. A loss of diversity between temporal samples is one of the genetic signatures of population decline, so a biased estimate may have implications for the conservation actions taken to protect a species.

For the Pinzón giant tortoise species, our studies using nuclear genomic and mitochondrial sequence data concluded that genetic diversity has persisted despite a dramatic population decline. This is thanks to a 50-year long head-start program that led the population to recover in size and an eradication effort that successfully eliminated invasive rats from the island that had been preying upon hatching tortoises.

The close genetic study of the Galapagos giant tortoises has yielded many important insights for conservation, but none so exciting as the discovery of living hybrids of two extinct species. The last Pinta Island tortoise was the famous Lonesome George, who died in 2012. A few years before his death, a genetic survey of the tortoise population on Volcano Wolf discovered individuals that are the offspring of the species native to the area and Pinta Island tortoises, as well as a second extinct species from Floreana Island. In both cases, the presence of the extinct tortoises was attributed to sailors dropping the tortoises overboard near Volcano Wolf.

Image credit: Adult Pinzón Galapagos giant tortoise, showing the “saddle backed” shell morphology. Photograph by Evelyn Jensen.

The happy accident of the existence and discovery of these hybrid individuals has opened up new possibilities for repopulating Pinta and Floreana Islands with giant tortoises that are partly related to the extinct native species. We recently reported on the success of the first three breeding seasons of the pilot Floreana giant tortoise breeding program. Once bred together, the majority of the progeny of the hybrid individuals showed ancestry from the Floreana species. The breeding program will be closely genetically monitored into the future to ensure that inbreeding is held at bay, and genetic diversity is maintained. Once they are large enough, the offspring from the program will be reintroduced to their ancestral home, Floreana Island, and help to ecologically restore it to a more natural state.

The continuous application of genetic, and now genomic, methods to study Galapagos giant tortoises has consistently provided information that is both scientifically interesting and a valuable aid in conservation. Galapagos giant tortoise species have been brought back from the brink of extinction, and perhaps even back from extinction to a certain extent. They are undoubtedly a conservation success story. One we can hopefully learn from to better protect other species.

Featured image credit: Maud Quinzin

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