In genetics, truncation selection can be a form that natural selection takes, but it's such an extreme form that it's unlikely to occur in practice. Truncation selection occurs when there is a threshold in a quantitative trait (height, for example); all individuals that fall above the threshold survive and reproduce, while none of those below the threshold do. Natural selection is much more likely be partly random: more fit individuals have a higher probability of reproducing, but the probability is always less than 1.0. (On the other hand, the converse is not true: there are genotypes that give the individual zero chance of reproducing.)
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