How phenotypic plasticity made its way into molecular biology.

Phenotypic plasticity has been fashionable in recent years. It has never been absent from the studies of evolutionary biologists, although the availability of stable animal models has limited its role. Although opposed by the reductionist and deterministic approach of molecular biology, phenotypic plasticity has nevertheless recently made its way into this discipline, in particular through the limits of the molecular description. Its resurrection has been triggered by a small group of theoreticians, the rise of epigenetic descriptions and the publicized discovery of stem cell plasticity. The notion of phenotypic plasticity remains vague. History shows that too strong a belief in plasticity can be an obstacle to the development of biology. Two important questions are still pending: the link between the different forms of plasticity present at different levels of organization, and the relation, if any, between the modular organization of organisms and phenotypic plasticity. Future research will help to discriminate between possible and actual mechanisms of phenotypic plasticity, and to give phenotypic plasticity its real place in the living world.