Plant Breeding Reviews, Volume 17

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There have been several important stages during the history of planting beginning with the domestication of wild species approximately 10, years ago. For purposes of discussion in the evolution of plant breeding methods, I have arbitrarily partitioned them based on the primary methods of selection and the information available to plant breeders; selection based on phenotypes, breeding values, and genotypes.

These are not distinct stages because there are different time periods for each of the three stages and each overlaps with others. Each stage however, has been very important although the time frames are very different. Phenotypes result from the combination of genetic and environmental effects of individuals. Phenotypes are the visual trait s observed and are the first features that are obvious whether expressed as a beautiful ornamental flower, maturity, plant stature, resistance to pests, or any other observable trait.

Phenotypes of individuals were obviously the unit of selection by the earlier plant breeders because selection based as phenotypes usually referred to as mass selection was the obvious method of selecting for the desired types. Initially, selection was for the more productive phenotypes among the wild, weedy plants species to provide adequate quantities of food for the primitive civilizations.

Objectives of Plant Breeding [Year-3]

Gradually, the range of traits considered in selection was expanded to include better adaptation, preferred plant and seed types, greater resistance to pests, and other traits considered for decorative and ceremonial purposes. Phenotypic selection is certainly the plant selection method with the longest, continuous use in plant improvement. It is a simple method that requires minimum resources and has been an effective method in many instances. The greatest contributions from phenotypic selection have to include making the transition from the wild, weedy plant species to cultivated crop species.

In most instances, it probably included making small, incremental changes that, with a few useful mutants, eventually led to crop species dependent on humans for their survival. Progress was not a smooth positive trend, but included hundreds of generations of selection with cumulative effects that were usually in the desired direction.

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Because the effectiveness of phenotypic selection depends on the relative heritabilities of the traits, greater progress naturally was made for traits with the greater heritabilities. The effects of phenotypic selection can be illustrated with the history for the development of cultivated maize Zea mays L.