Selection with Regard to a Trait with Quantitative Variation

In Chapter 6 the topic of selection was introduced. The contrast between natural selection and artificial selection was elaborated. Methods for artificial selection with regard to a trait with qualitative variation were considered in an order of decreasing efficiency. Special attention was given to the question whether the trait under selection is expressed before or after pollen distribution. The present chapter pays attention to selection with regard to a trait with quantitative variation. The selection is aimed at improving the average genotypic value, but the actual selection among the entries (plants, clones, lines, hybrids, families) is based on the phenotypic values of the candidates. In order to improve the efficiency of such indirect selection the breeder should make efforts to evaluate the candidates in such a way that the coefficient of correlation between the phenotypic value (the auxiliary trait) and the genotypic value (the target trait) is as high as possible. Procedures to promote this are elaborated in the present chapter. Such procedures can be classified according to the way of evaluation of the candidate genotype. Section 14.2 gives attention to procedures concerning the evaluation of entries on the basis of single plants (single plant evaluation); Section 14.3 considers procedures for the evaluation of candidates on the basis of plots such that the plants occurring in a certain plot represent a certain candidate (plot evaluation).