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.
Selection can work only with available variation.
If appropriate mutations do not arise, selection cannot operate. As used here, genetic constraint refers to the failure of a possible mutation to occur. Developmental constraints may cause adaptive imperfection. Natural selection can work only with available material. Because of the nature of a complex developmental program, some forms of variation may not be possible.