Phenotypic plasticity explains violation of Dollo’s law

Dollo’s law of irreversibility states that once a complex adaptation has been lost in evolution, it will not be regained. Recently, various violations of this principle have been described. Here, we argue that the logic underlying Dollo’s law only applies to traits that are constitutively expressed, while it fails in case of ‘plastic’ traits that are up- or downregulated according to needs. We tested this hypothesis for an archetypal violation of Dollo’s law, the loss and regain of fat synthesis in parasitic wasps. Wasps from lineages that supposedly had lost lipogenic ability more than 200 million years ago were grown under various conditions. In line with our hypothesis, it turned out that fat synthesis had not been lost but was only switched on in low-fat environments. Such plasticity cannot only explain supposed violations of Dollo’s law, but also the maintenance of adaptations to rarely occurring extreme events.