Sunday, July 10, 2011

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The Galápagos tortoise or Galápagos giant tortoise (Chelonoidis nigra) is the largest living species of tortoise, reaching weights of over 400 kg (880 lb) and lengths of over 1.8 meters (5.9 ft). With life spans in the wild of over 100 years, it is one of the longest-lived vertebrates. A captive individual lived at least 170 years.

The tortoise is native to seven of the Galápagos Islands, a volcanic archipelago about 1,000 km (620 mi) west of the Ecuadorian mainland. Spanish explorers who discovered the islands in the 16th century named them after the Spanish galápago, meaning tortoise.
Shell size and shape vary between populations. On islands with humid highlands, the tortoises are larger, with domed shells and short necks. On islands with dry lowlands, the tortoises are smaller, with "saddleback" shells and long necks. These island-to-island differences played a role in the inception of Charles Darwin's theory of evolution.

Tortoise numbers declined from over 250,000 in the 16th century to a low of around 3,000 in the 1970s. The decline was caused by hunting for tortoise meat and oil, habitat clearance for agriculture, and introduction of non-native animals such as rats, goats, and pigs. Seven subspecies of the original ten survive in the wild. An eighth subspecies (C. n. abingdoni) has only a single living individual, in captivity, nicknamed Lonesome George. Conservation efforts beginning in the 20th century have resulted in thousands of captive-bred juveniles being released onto their home islands, and it is estimated that numbers exceeded 19,000 at the start of the 21st century. Despite this rebound, the species as a whole is classified as "Vulnerable" by the International Union for Conservation of Nature (IUCN).

galapagos tortoiseRecognition of subpopulations
Early classification
The Galápagos Islands were discovered in 1535, but first appeared on maps, of Gerardus Mercator and Abraham Ortelius, in about 1570.The islands were named "Insulae de los Galopegos" (Islands of the Tortoises) in reference to the giant tortoises found there.[nb 1]

Initially, the giant tortoises of the Indian Ocean and those from the Galápagos were considered to be the same species. Naturalists thought that sailors had transported the tortoises. In 1676, the pre-Linnaean authority Claude Perrault referred to both species as Tortue des Indes. In 1783, Johann Gottlob Schneider classified all giant tortoises as Testudo indica ("Indian tortoise"). In 1812, August Friedrich Schweigger named them Testudo gigantea ("gigantic tortoise"). In 1834, André Marie Constant Duméril and Gabriel Bibron classified the Galápagos tortoises as a separate species, which they named Testudo nigrita ("black tortoise").

The first systematic survey of giant tortoises was by Albert Günther of the British Museum, in 1875. Günther identified at least five distinct populations from the Galápagos, and three from the Indian Ocean islands. He expanded the list in 1877 to six from the Galápagos, four from the Seychelles, and four from the Mascarenes. Günther theorised that all the giant tortoises descended from a single ancestral population which spread by sunken land bridges. This theory was later disproven by the understanding that the Galápagos, Seychelles and Mascarene islands are all of recent volcanic origin and therefore could not have been linked by land bridges. It is now thought that the Galápagos tortoises descended from an ancestor from South America. The Indian Ocean tortoises derived from Madagascar

At the end of the 19th century, Georg Baur[ and Walter Rothschild recognised five more populations of Galápagos tortoise. In 1906, the Academy of Sciences collected specimens and gave them to John Van Denburgh for study. He identified four additional populations, and proposed the existence of 15 species.[32] His list still guides the taxonomy of the Galápagos tortoise, though now ten populations are thought to have existed.

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Floreana Island
Phylogenetic analysis may help to "resurrect" the extinct subspecies of Floreana (nigra). The subspecies was only known from subfossil remains. Some tortoises from Isabela were found to be a partial match for the genetic profile of Floreana specimens from museum collections, possibly indicating the presence of hybrids from a population transposed by humans from Floreana to Isabela speculated to be caused either by deliberate moving between the islands or from individuals thrown overboard ships to lighten loads.Nine Floreana descendants have been identified in the captive population of the Fausto Llerena Breeding Center on Santa Cruz. This permits the possibility of re-establishing a reconstructed subspecies from selective breeding of the hybrid animals.
Pinta Island
The Pinta Island subspecies (abingdoni, now extinct in the wild) is most closely related to the subspecies on the islands of San Cristóbal (chathamensis) and Española (hoodensis) which lie over 300 kilometers (190 mi) away,[29] rather than neighbouring Isabela as previously assumed. This relationship is attributable to dispersal by the strong local current from San Cristóbal towards Pinta. The discovery informed further attempts for the preservation of the abingdoni lineage and the search for an appropriate mate for Lonesome George, who had been penned with females from Isabela. This hope was bolstered by the discovery of an abingdoni hybrid male in the Volcán Wolf population on northern Isabela, raising the possibility that there are more living undiscovered Pinta descendants.

Santa Cruz Island
Mitochondrial DNA studies of tortoises on Santa Cruz show up to three genetically distinct lineages found in non-overlapping population distributions around the regions of Cerro Monturra, Cerro Fatal and La Caseta. Although currently united in a single subspecies (porteri), the lineages are all more closely related to tortoises on other islands than to each other; Cerro Monturra tortoises are most closely related to duncanensis from Pinzón, Cerro Fatal to chathamensis from San Cristóbal, and La Caseta to the four southern races of Isabela.
galapagos tortoiseEvolutionary history
All subspecies of Galápagos tortoise evolved from common ancestors that arrived from mainland South America by overwater dispersal. The minimal founding population was a pregnant female or a breeding pair.[29] Survival on the 1000-km oceanic journey is accounted for by the fact that the tortoises are buoyant, can breathe by extending their necks above the water, and are able to survive months without food or fresh water. As they are poor swimmers, the journey was probably a passive one facilitated by the Humboldt Current, which diverts westwards towards the Galápagos Islands from the mainland.

The closest living relative (though not a direct ancestor) of the Galápagos giant tortoise is the Argentine tortoise (Chelonoidis chilensis), a much smaller species from South America. The divergence between C. chilensis and C. nigra probably occurred 6–12 million years ago, an evolutionary event preceding the volcanic formation of the oldest modern Galápagos Islands 5 million years ago.[52] Mitochondrial DNA analysis indicates that the oldest existing islands (Española and San Cristóbal) were colonised first, and that these populations seeded the younger islands via dispersal in a "stepping stone" fashion via local currents. Restricted gene flow between isolated islands then resulted in the independent evolution of the populations into the divergent forms observed in the modern subspecies. The evolutionary relationships between the subspecies thus echo the volcanic history of the islands.

Subspecies genetics
Modern DNA methods have revealed new information on the relationships between the subspecies:

Isabela Island
A distinct population was once thought to inhabit each of the five main volcanoes of the largest island Isabela: (Wolf, Darwin, Alcedo, Sierra Negra, and Cerro Azul). The four southern populations on Isabela, though separated from each other by barren stretches of lava between volcanoes, are in fact a single genetic unit derived from colonists from Santa Cruz. The genetically distinct Volcán Wolf subspecies in northern Isabela (becki) is probably the result of a separate colonisation event from Santiago. Tortoises from Sierra Negra in southern Isabela (formerly guentheri) are possibly the ancestral source of dispersal to the volcanoes Darwin (formely microphyes), Alcedo (formerly vandenburghi) and Cerro Azul (vicina). On this basis, the southern populations on Isabela may be considered as a single subspecies vicina, with morphological differences attributable to age, sex or local environment
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