Negative selection, in natural 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.
Negative selection, in artificial selection, is where negative, rather than positive, traits of a species are selected for. It is generally not desirable, but may be caused by man-made conditions such as bad management, such as when 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.
