Irreversibility of Evolution

in biology, a law of the historical development of organisms that states that organisms, even when they return to their former habitat, cannot resemble their earlier forms in all respects. In the modern view, the irreversibility of evolution is governed by random processes. The recurrence of certain mutations that sometimes results in the reappearance of certain traits in phylogeny is statistically probable. The repetition of common directions of selection is also probable, but the reproduction of the gene complexes that disappeared or changed in the course of evolution is statistically improbable. Also improbable is the absolute immutability in time of the nonliving and living environment to which evolving organisms adapt.