Cotton belongs to the genus Gossypium L., which comprises 50 species, 45 of which are diploid, the remaining five being tetraploid. Two A-genome diploid and two AD-genome allotetraploid species are grown for their lint. The centre of origin of the genus is unknown, but the primary centres of diversity rest in north-east Africa and Arabia, Australia, and Mexico. Tetraploid cotton would have arisen on the American continent from a unique hybridization event, which was then followed by diversification. Archaeological evidence point out to independent centres of domestication and diversification in the Old- and New World. Following early dispersion and diversification of tetraploid cottons, selection pressure has greatly reduced the genetic diversity of modern varieties.