Background: The concept of homeostasis is often ill-defined, in the scientific literature. The word "homeostasis", literally, indicates the absence of changes and an absolute maintenance of the status quo. The multiplicity of possible examples of homeostasis suggests that it is essentially impossible that all aspects of the composition of the organism and the rate of processes carried out by the organism are simultaneously held constant, when the environment changes are in the non-lethal range. Scope: In attempting to clarify the usage of the term homeostasis, I emphasize the probable contributions to evolutionary fitness of homeostasis main attributes: rate processes and compositions. I also attempted to identify the aspects of homeostasis that are most likely to be subject to natural selection. Conclusions: The tendency to retain the status quo derives from the interplay of functions (among which growth), metabolic pools and elemental stoichiometry. The set points around which oscillations occur in biological system and their control mechanisms are determined by evolutionary processes; consequently, also the tendency of a cell to be homeostatic with respect to a given set point is selectable. A homeostatic response to external perturbations may be selectively favored when the potential reproductive advantage offered by a reorganization of cell resources cannot be exploited. This is most likely to occur in the case of environmental perturbations of moderate intensity and short duration relative to the growth rate. Under these circumstances, homeostasis may be an energetically and competitively preferable option, because it requires no alteration of the expressed proteome and eliminates the requirement for reverse acclimation, upon cessation of the perturbation. This review also intends to be a stimulus to "ad hoc" experiments to assess the ecological and evolutionary relevance of homeostasis.
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