Crop Improvement

Crop improvement refers to the genetic alteration of plants to satisfy human needs. In prehistory, human forebears in various parts of the world brought into cultivation a few hundred species from the hundreds of thousands available. In the process they transformed elements of these species into crops though genetic alterations that involved conscious and unconscious selection, the differential reproduction of variants. Through a long history of trial and error, a relatively few plant species have become the mainstay of agriculture and thus the world's food supply. This process of domestication involved the identification of certain useful wild species combined with a process of selection that brought about changes in appearance, quality, and productivity. The exact details of the process that altered the major crops is not fully understood, but it is clear that the genetic changes were enormous in many cases. In fact some crop plants have been so changed that for many of them, maize, for example, their origins are obscure, with no extant close wild relatives.
The selection process was unconscious in many cases. For example, in wild wheats, the grains scatter by disarticulation, separation of the seed from the seed head. When these grains were harvested by cutting the heads with a sickle, an unconscious selection occurred for "nonshattering" types that would then be continually replanted. For some crops a clear conscious selection occurred, especially when the variant was obvious and would be maintained by vegetative propagation. Something so clearly useful as a seedless banana must have been immediately seized upon and maintained ("fixed") by planting offshoots of the plant. The changes wrought in domestication included alteration in organ size and shape; loss of many survival characters, such as bitter or toxic substances; disarticulation of seeds in grains; protective structures, such as spines and thorns; seed dormancy; and change in life span—increased in...