Adaptations in nature are not perfect. Adaptation is necessarily limited by the variants which actually occur and by the population-genetic processes by which gene frequencies change. Adaptation is often discussed as if any useful trait will be found by natural selection. In fact, adaptations evolve because variation appears which happens to be favored by selection. Natural selection is not some irresistable force.
Natural selection is simply the consequence of heritable variation in traits that affect fitness.
Factors which limit or prevent perfect adaptation are often referred as constraints . Each of the following sections 13.7 - 13.11 describes one category of constraint, one basic reason why adaptation may not be perfect. Adaptations may be imperfect because of time lags. Genetic constraint may cause imperfect adaptation. As effective as it clearly is, natural selection is not impotent.