Principles of Plant Breeding

Selection

• Differential reproduction rates of various genotypes are known as selection. In crop improvement selection is important because it allows the selected genotypes to reproduce. While undesirable genotypes are eliminated. So in random mating population selected genotypes would be fixed and the fitness is zero for remaining genotype. The fitness of a genotype may be defined as its reproduction rate in relation to that of other genotypes.
• It is difficult to identify genotypes for dominance and due to less than 100% heritability especially for quantitative as a result, selection is expected to change gene frequencies rather than to eliminate one or other allele. The selection in a random mating population is highly effective in increasing or decreasing the frequency of allele, but it is unable to either fix or eliminate them. However, in combination with a system of inbreeding, selection is highly efficient in the fixation and elimination of an allele.
• Most of the characters of economic importance are quantitative characters which are governed by many genes. Asthese characters show a continuous distribution selection intensity is measured as the difference between mean of the population and that of the selected individuals. It is expressed in terms of K standard deviation units form which the selection differential ‘K’ is calculated. The value of ‘K’ is related to the percent of extreme individuals saved.
• E.g: When the extreme 5% of the population is saved K=2.06. Selection of the extreme phonotype increases the frequency of desirable alleles in the population. It increasesthe frequency of desirable alleles, the frequency of desirable genotypes would also increase new genotypes would also appears producing more extreme genotypes. Therefore mean of the population would change under selection. Variance may decrease to some extent but selection would show continuous gain for several generations
• In case the quantitative character is governed by one or few genes with large effect the desirable genes may be fixed after some generation of selection. So gain under selection would continue for several generations but in the later generations it will be small and discouraging. The progress under selection is retarded by non-additive gene action, low heritability and limit on permissible selection intensity.
• Heritability of quantitative character is less than 100 percent. Sono perfect correspondence between phonotype and genotype. So identification of desirable genotypes is difficult and reduces the progress under selection. Permissible selection intensity depends on fecundity i.e. reproductive capacity of the species. Another factor that limits selection intensity is the consideration of inbreeding. Inbreeding should be kept to a low level to maintain heterozygosity and to avoid inbreeding depression. So selected plants should be relatively large and cannot be too small.
• The breeder is interested in improvement more than one character. Selection intensity is less for one character and it is released to accommodate selective for many characters