 Changes in the pattern of neutral variation in the vicinity of a selected site(s) are also informative because, during an adaptive substitution, neutral variants linked to the piece of selected DNA are carried or “swept” to fixation along with it. This “selective sweep” temporarily reduces the level of neutral variation in the vicinity of selected sites until it can be replaced by mutation. The degree of reduction in neutral variation or the “footprint of selection” depends upon the strength of selection, the frequency of recombination during selection, and the time since the initiation of selection. The footprint is most conspicuous when a selective sweep is initiated by the advent of a single, novel favorable mutation. To the extent that novel selection results from a change of environment and begins to act on existing or standing variation already in the population, the impact on neutral polymorphisms may be quite minimal.