Insular dwarfism

s – when their gene pool is limited to a very small environment, primarily islands.

This effect has made itself manifest many times throughout natural history, including dinosaurs and modern animals such as elephants and human beings.

There are several proposed explanations for the mechanism which produces such dwarfism, which are often considered likely to be co-contributing factors, including an evolved gene-encoded response to environmental stress, as well as a selective process where only the smaller of the animals trapped on the island survive, as food declines to a borderline level. The smaller animals need fewer resources, and so are more likely to get past the break-point where population decline allows food sources to replenish enough for the survivors to flourish.

Examples

Among the most famous examples of insular dwarfism are:

* The Channel Islands Mammoth which lived on the prehistoric island of Santa Rosae in the California Channel Islands, and the small woolly mammoths of Saint Paul Island off Alaska, and Wrangel Island north of Siberia.
* Dwarf elephants in the recent natural history of Malta, Crete, Cyprus, and Sicily.
* Dwarf stegodons (elephant relatives) from the recent natural history of Flores, Sulawesi, Sumba and Timor.
* A (disputed) species of hominin called "Homo floresiensis", from fossils found on Flores Island, Indonesia.
* Small-Bodied Humans from Palau, Micronesia, similar in size to the Flores hominins. [ [http://www.independent.co.uk/news/science/skull-may-identify-tiny-islanders-who-shrank-to-survive-794487.html Skull may identify tiny islanders who shrank to survive] , The Independent]
* Dinosaurs, including the recently validated "Europasaurus", on Mesozoic islands such as Haţeg Island.

There are also proposed instances of this process occurring among plant life, the appearance of dwarf sequoia / redwood trees being one such proposal.Fact|date=June 2007

This process, and other "island genetics" artifacts, can occur not only on traditional islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, and isolated valleys. An example of this is the "pygmy" people of Africa, such as the Mbuti, who evolved a small size while in genetic and ecological isolation in a dense jungle region.Fact|date=May 2008

There is an inverse form of this process, island gigantism, wherein small animals, lacking the predators of their normal homes, may become "gigantic" when breeding in isolation. An excellent example is the dodo, the ancestors of which were normal-sized pigeons.

Additional examples

Carnivora
*Channel Island fox in California, United States, and the enigmatic, nearly extinct Cozumel fox
*Honshū Wolf in Japan
*Bali Tiger
*Cozumel Island Raccoon
*Sardinian Dhole
*Sardinian Lynx
Ungulates
*The strange, tiny Balearic Islands Cave Goat ("Myotragus balearicus") in Majorca and Minorca and its close relative "Nesogoral", from Sardinia which became extinct after human settlement
*Philippine water buffalo, Tamaraw, Anoa, the extinct Bubalus cebuensis and other dwarf bovids from South East Asia
*Several prehistoric species of hippopotamus from the Pleistocene of Mediterranean islands (such as "Phanourios minutus") and Madagascar
*Dwarf deer species on the Philippines (Philippine Sambar, extant), and Crete ("Candiacervus ropalophorus"), and Ryukyu Islands of Japan ("Cervus astylodon") (all extinct)
*Key Deer

Other
* The tiny flightless Inaccessible Island and Laysan Rails
*The dwarf nodosaurid "Struthiosaurus" from Europe, and the dwarf allosaur from South Australia

ee also

*Island gigantism
*Foster's rule
*Deep-sea gigantism
*Baron Franz Nopcsa

References

External links

* [http://www.guardian.co.uk/life/science/story/0,12996,1340300,00.html Strange world of island species October 31, 2004 The Observer ]