See also: paper in PLoS: Morphological Evolution Is Accelerated Among Island Mammals
And: New Scientist: Island creatures evolve to be great and small
Islands spark accelerated evolution
“Giant tortoises, dwarf elephants and little people. All are examples of species that became very large or very small when they colonised islands.”
Dwarf elephant and hippo of Malta next to a modern Indian Elephant.
(From The First Eden: The Mediterrranean World and Man by David Attenborough)
Among insular mammals, there is a general tendency for small mammals to evolve toward larger size and larger species to evolve toward smaller size. This tendency has come to be known as the island rule.
For the first time, biologists have found empirical evidence that morphological evolution accelerates on islands – resulting in dramatic evolutionary changes.
“The size, isolation and boundaries of islands combine to create selective pressures that are quite distinct from those that act on the mainland. Also, these species often evolve from very small populations that are subject to genetic bottlenecks, which spur change.”
If island species can evolve quickly, Millien [McGill University] argues, it stands to reason that mainland species retain a similar capacity. As habitat destruction continues to pose the number one threat to biodiversity, many mainland habitats are beginning to resemble islands, with isolated pockets of wildlife separated by degraded or developed lands. Thus, island species may serve as a model for understanding how mainland species will adapt to the rapidly changing environmental conditions brought on by habitat destruction and global warming.