Galapagos Islands

The original ancestor of the tortoises was probably of normal size and evolved into the present day giants after its arrival in Galapagos. This is due to a phenomenon seen in many island ecosystems where gigantism evolves because there is no longer any need to hide from predators and because there are no other similar animals to compete with for food. Once the tortoises spread around the archipelago, they evolved on their isolated islands into the different races we see today, some with domed carapaces (shells), and others with saddleback carapaces. The unusual saddle shape is believed to have evolved several times on different islands, showing that it must be a very successful design for life in Galapagos.

http://www.spallek.com/photo/galapagos/June7/Website/_MG_7507.jpg