|Ears of common wheat|
Common wheat (Triticum aestivum), also known as bread wheat, is a cultivated wheat species. About 95% of the wheat produced is common wheat, which is the most widely grown of all crops and the cereal with the highest monetary yield.
Numerous forms of wheat have evolved under human selection. This diversity has led to confusion in the naming of wheats, with names based on both genetic and morphological characteristics. For more information, see the taxonomy of wheat.
Bread wheat is an allohexaploid (an allopolyploid with six sets of chromosomes: two sets from each of three different species). Of the six sets of chromosomes, two come from Triticum urartu (einkorn wheat) and two from Aegilops speltoides. This hybridisation created the species Triticum turgidum (durum wheat) 580,000-820,000 years ago. The last two sets of chromosomes came from wild goat-grass Aegilops tauschii 230,000-430,000 years ago.
Free-threshing wheat is closely related to spelt. As with spelt, genes contributed from Aegilops tauschii give bread wheat greater cold hardiness than most wheats, and it is cultivated throughout the world's temperate regions.
Wheat first reached North America with Spanish missions in the 16th century, but North America's role as a major exporter of grain dates from the colonization of the prairies in the 1870s. As grain exports from Russia ceased in the First World War, grain production in Kansas doubled.
Worldwide, bread wheat has proved well adapted to modern industrial baking, and has displaced many of the other wheat, barley, and rye species that were once commonly used for bread making, particularly in Europe.
Modern wheat varieties have short stems, the result of RHt dwarfing genes  that reduce the plant's sensitivity to gibberellic acid, a plant hormone that lengthens cells. RHt genes were introduced to modern wheat varieties in the 1960s by Norman Borlaug from Norin 10 cultivars of wheat grown in Japan. Short stems are important because the application of high levels of chemical fertilizers would otherwise cause the stems to grow too high, resulting in lodging (collapse of the stems). Stem heights are also even, which is important for modern harvesting techniques.
Compact wheats (e.g., club wheat Triticum compactum, but in India T. sphaerococcum) are closely related to common wheat, but have a much more compact ear. Their shorter rachis segments lead to spikelets packed closer together. Compact wheats are often regarded as subspecies rather than species in their own right (thus T. aestivum subsp. compactum).