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Darwin and the Giant Tortoises

The world is rich in animals and plants, some of which still remain to be discovered. A small area of the Tropical Forests of South America will still yield insects that have never been described, the difficulty is finding a specialist whose is able to classify them. The understanding of such biodiversity would have been almost impossible, if it had not been for Charles Darwin and his trip around the world. For example Darwin described the adaptations of the Giant Tortoises (Geochelone nigra) that occur on the Galapagos Islands in the South Pacific.

Tortoises occurring on the well-watered islands, with short, cropped vegetation had gently curved front edges to their shell.

An example of dome-shell Galapagos Tortoise that occurs on the well-watered parts of the islands.

Tortoises occurring on more arid islands had to stretch their necks to reach branches of cactus and other vegetation. Consequently, these later individuals had longer necks and a high peak to the front edges of their shells, which enabled them to stretch their heads almost vertically.

A "saddle-back" Galapagos Tortoise that inhabits drier areas of the islands and has a longer neck and a high peak to the front edge of its shell, this enables it to stretch it neck further out and obtain food higher up off the ground.

Observations such as these were the foundations for the theory of evolution, which suggests that species were not fixed for ever but changed with time and thereby contribute to the immense diversity of life.

Darwin's argument for the evolution of different necks in these tortoises was as follows:- all individuals of the same species are not identical. In a single clutch of eggs there will be some hatchlings, which, because of their genetic constitution, will develop longer necks than others. In times of drought such individuals will be able to reach leaves higher off the ground than their siblings and therefore will survive. The brothers and sisters in the clutch who possessed shorter necks would be unable to stretch and reach food and therefore would starve to death. Since this time natural selection has been debated and tested, refined, quantified and elaborated. Later discoveries about genetics, molecular biology, population dynamics and animal behaviour have developed the theory of natural selection still further. It remains the key to our understanding of the natural world and it enables us to recognize that life has a long and continuous history during which organisms, both plants and animals, have changed, generation by generation, as they colonized all parts of the world.