Читать книгу Principles of Plant Genetics and Breeding - George Acquaah - Страница 175

The focus of this section is on the response to selection (genetic gain or genetic advance). After generating variability, the next task for the breeder is the critical one of advancing the population through selection.

Selection, in essence, entails discriminating among genetic variation (heterogeneous population) to identify and choose a number of individuals to establish the next generation. The consequence of this is differential reproduction of genotypes in the population such that gene frequencies are altered, and subsequently, the genotypic and phenotypic values of the targeted traits. Even though artificial selection is essentially directional, the concept of “complete” or “pure” artificial selection is an abstraction because, invariably, before the breeder gets a chance to select plants of interest, some amount of natural selection would have already been imposed.

The breeder hopes, by selecting from a mixed population, that superior individuals (with high genetic potential) will be advanced, and consequently change the population mean of the trait in a positive way in the next generation. The breeder needs to have a clear objective. The trait to be improved needs to be clearly defined. Characters controlled by major genes are usually easy to select. However, polygenic characters, being genetically and biologically complex, present a considerable challenge to the breeder.

The response to selection (R) is the difference between the mean phenotypic value of the offspring of the selected parents and the whole of the parental generation before selection. The response to selection is simply the change of population mean between generations following selection. Similarly, the selection differential (S) is the mean phenotypic value of the individuals selected as parents expressed as a deviation from the population mean (i.e. from the mean phenotypic value of all the individuals in the parental generation before selection). Response to selection is related to heritability by the following equation: