Developmental plasticity involves environmentally triggered change in the growth or development of a bodily or behavioral trait, and is generally nonreversible. It is an important source of human variation, and allows adaptation to diverse environments while also modifying risk for many diseases. The most plastic systems generate adaptations via an initial overproduction of redundant variants, which are retained if they are used, and removed if not. The central nervous and immune systems, and to a lesser extent traits like bone and muscle, develop from this principle. Fine-tuning of developing structures in response to use and disuse allows few genes to generate many possible phenotypes in response to an individual's unique experiences. Other systems show heightened sensitivity during narrow windows of early development called critical periods, during which factors like nutrients or hormones permanently alter trait development. In mammals many critical periods occur during fetal life, creating opportunities for an intergenerational form of plasticity in which maternal experiences alter, and possibly guide, offspring development. Although developmental plasticity may slow the pace of genetic adaptation, recent work recognizes that new traits may first appear through plasticity, which leads the way for the gradual fixation or fine-tuning of the trait through genetic adaptation.