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1. Selection operates on existing variability:
The main function of the selection exercise is to discriminate between individuals. This is possible only when sufficient variation is present in the material subjected to selection pressure. Thus, selection acts on the existing variation it cannot create new variation.
2. Selection acts only through heritable differences:
Only the selected individuals are permitted to contribute to the next generation/progenies. Therefore, should there be greater influence of non-heritable agencies on the individuals selected; the parent-progeny correlation will be greatly vitiated. Hence the variation among individuals to be selected must be genetic in nature, since it is the genetic variation that tends to close the gap between phenotype and genotype. Environmental variability cannot be of any use under selection.
3. Selection works because some individuals are favoured in reproduction at the expense of others: As a consequence of its past evolutionary history and breeding structure, a population or a crop consists of highly genetically variable individuals with regards to such diverse phenomena as differential viability, differential maturity, differences in mating tendencies, fecundity, and duration of reproductive capacity. Hence some individuals tend to become superior to others for some or other traits desirable under domestication. These superior individuals are retained for reproduction while others discarded under selection.
Dullayo answered the question on August 14, 2018 at 05:29
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