Masking and the evolution of circadian rhythmicity.

In spite of the clear-cut temporal features exhibited by most species, temporal characteristics have merited little attention from evolutionists. However, it is logical to assume that since the environment oscillates cyclically, organisms that adapt to it must also oscillate; that is, cyclic factors have a clear evolutionary role. This article discusses evidence that the timing system is genetically determined; the role of environmental cues such as zeitgebers and masking factors; the temporal basis of mating; and masking as a disorganizing factor.