The adaptive advantage of phenotypic memory in changing environments.

The adaptive value of carry-over effects, the persistence of induced phenotypes for several generations despite the change in the conditions that first induced these phenotypes, is studied in the framework of a simple model. Three different organismal strategies-non-inducible (genetic), completely inducible (plastic), and intermediate (carry-over)-are compared in fitness terms within three different environments. Analytical results and numerical simulations show that carry-over effects can have an advantage in stochastic environments even over organisms with high adaptive plasticity. We argue that carry-over effects represent an adaptive mechanism on the ecological timescale that fills the gap between short-term individual adaptations and long-term evolutionary adaptations. An extension of the concept of plasticity to incorporate the time dimension and include the stability of induced phenotypes through both clonal and sexual generations, is suggested.