Regressive biological evolution due to environmental change

Simulation results are presented which suggest that regressive evolution (i.e., evolution to a less adapted state) often occurs in response to environmental change, by a process analogous to the stress-induced reversal of aging in glassy materials. The key to this process is the stress-induced disappearance of fitness optima that lead to irreversible changes in the location of a population in genotype space. Even though the population may always evolve to higher fitness states, this irreversible process will often act to bring an initially well-adapted system to a less adapted state upon a return to initial conditions.