In coevolutionary processes, the fitness of one organism or species depends on the characteristics of the other organisms or species with which it interacts, while all simultaneously adapt and change." At every moment natural selection is operating to change the genetic composition of populations in response to the momentary environment, but as that composition changes it forces a concommittent change in the environment itself. Organism and environment and environment are both causes and effects in a coevolutionary process." (Lewontin, Triple Helix, p. 126)