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In biology, negative selection has two different definitions depending on if it is due to natural selection or artificial selection.
In natural selection negative selection is the selective removal of rare alleles that are deleterious. This is also called purifying selection and can result in the maintenance of conserved gene sequences between species over long periods of evolutionary time. The ongoing process of purging of deleterious alleles, due to the constant occurrence of new deleterious mutants, is referred to as background selection.
In artificial selection negative selection is where negative, rather than positive traits of a species are selected for. It is generally not desirable, but caused by bad management, wherein humans eat the best plants or animals they have, leading to worse and worse stock; it occurred frequently, for example, in primitive beekeeping. Another, more modern example, are regulations on fish and game, where fish below a certain sized had to be released, and deer below a certain size could not be hunted, leading to undersized deer and fish populations. Some proponents of eugenics argue that medicine and other technological, societal, and cultural practices cause negative selection in humans. According to them, man as a species manifests more and more negative traits as a result of this. In the case of microbiology and molecular biology, negative selection is used to screen for microorganisms that fail to possess a certain phenotype. An example of this is through replica plating.
In politics, negative selection is a process that occurs in rigid hierarchies, most notably dictatorships.
The person on the top of the hierarchy, wishing to remain in power forever, chooses his associates with the prime criterion of incompetence - they must not be competent enough to remove him from power. The associates do the same with those below them in the hierarchy, and the hierarchy is progressively filled with more and more incompetent people.
If the dictator sees that he is threatened nonetheless, he will remove those that threaten him from their positions - "purge" the hierarchy. Emptied positions in the hierarchy are normally filled with people from below - those who were less competent than their previous masters. So, over the course of time, the hierarchy becomes less and less effective. As this happens relatively often, once the dictator dies, or is removed by some external influence, what remains is a grossly ineffective hierarchy.
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