Selection

Selection is the choice of individuals from a mixed population that carry one or more desirable traits, and it is the oldest and most basic method of plant breeding. Natural selection operates on its own in nature, following the Darwinian principle of survival of the fittest, but it works slowly. Artificial selection is the human-directed alternative, and it takes three main forms. In mass selection, a large number of plants with a similar phenotype are chosen from a mixed population and their seed is bulked together to form a new, more uniform variety without individually testing each plant's progeny; after about five to six years of repeated selection, the resulting seed is multiplied and distributed to farmers, though it remains hard to separate hereditary variation from purely environmental variation by this method (Figure 9.12). Pureline selection, a term coined by Johannsen in 1903, isolates plants through repeated self-pollination of a single homozygous individual, producing a highly homozygous variety; its drawback is that it creates no new genotypes, so the resulting line tends to be less adaptable to changing environmental conditions. Clonal selection is used in asexually propagated crops, where progeny derived from a single plant are genetically identical to it because they arise through mitotic division; superior clones are identified by their phenot …