many are stable polyploids , with four sets of chromosomes ( tetraploid ) or six ( hexaploid ). [ 24 ] Most tetraploid wheats (e.g. emmer and durum wheat ) are derived from wild emmer , T. dicoccoides . Wild emmer is itself the result of a hybridization between two diploid wild grasses, T. urartu and a wild goatgrass such as Aegilops searsii or Ae. speltoides . The unknown grass has never been identified among now surviving wild grasses, but the closest living relative is Aegilops speltoides . [ citation needed ] The hybridization that formed wild emmer (AABB) occurred in the wild, long before domestication, [ 24 ] and was driven by natural selection. Hexaploid wheats evolved in farmers' fields. Either domesticated emmer or durum wheat hybridized with yet another wild diploid grass ( Aegilops tauschii ) to make the hexaploid wheats, spelt wheat and bread wheat . [ 24 ] These have three sets of paired chromosomes, three times as many as in diploid wheat. The presence of certain versions of wheat genes has been important for crop yields. Apart from mutant versions of genes selected in antiquity during domestication, there has been more recent deliberate selection of alleles that affect growth characteristics. Genes for the 'dwarfing' trait, first used by Japanese wheat breeders to produce short-stalked wheat, have had a huge effect on wheat yields world-wide, and were major factors in the success of the Green Revolution in Mexico and Asia. Dwarfing genes enable the carbon that