Hint: Crop improvement refers to the genetic alteration of plants to satisfy human needs. It is the traditional method. In prehistory, human forebears in various parts of the world brought into cultivation a few hundred species from the hundreds of thousands available.
Step by step answer:In the process they transformed elements of these species into crops through genetic alterations that involved conscious and unconscious selection, the differential reproduction of variants.
-To meet the ever-increasing demand for food, crop improvement can help to fulfill the demands. 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 are 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.
-In the twentieth-century, plant breeding developed a scientific basis, and crop improvement was understood to be brought about by achieving favorable accumulations and combinations of genes.
-Taking advantage of genetic diversity facilitates appropriate combinations that were achieved through recombinations brought by the sexual process (hybridization).
-Furthermore, it was possible to move useful genes through special breeding strategies. Thus a gene discovered in a wild plant could be transferred to a suitable adapted type by a technique known as the backcross method.
-A sexual hybrid was made, followed by a series of backcrosses to the desirable (recurrent) parent, while selecting for the new gene in each generation.
-A number of genetic techniques were developed and refined in twentieth-century breeding, such as improved techniques to search for and store increased genetic variability, different techniques to develop variable populations for selection, and improved methods of testing to separate genetics from environmental effects.
-The exact details of the process for crops necessarily differed among naturally cross-pollinated plants (such as maize) and naturally self-pollinated plants (such as soybean or tomato) as well as those plants in which vegetative propagation (usually cross-pollinated) permitted the fixing of improved types directly.
-Conventional plant breeding can be defined as systems for the selection of superior genotypes from genetically variable populations derived from sexual recombination.
-The system is powerful because it is evolutionary; progress can be cumulative, with improved individuals continually serving as parents for subsequent cycles of breeding. Genetic improvement by conventional breeding has made substantial changes when the efforts have been long-term.
Note: Recombinant DNA technology, called transgene technology or genetic engineering, is the most powerful and revolutionary of the new genetics developed in the last half of the twentieth century. It is possible to isolate stretches of DNA from one organism, store it in a bacterial host, select unique combinations, and then incorporate them into the DNA of another species, where it can be expressed.