Power and Efficiency as Indexes of Fitness in Metabolic Organization

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Abstract
Bioenergetics, the study of energy flow through organisms, is a useful context for the evolutionary studies of metabolic organization. The far-from-equilibrium nature of living systems requires criteria of metabolic function not drawn from inappropriate equilibrium-machine analogies. Energy processing by organisms is divided into the production of metabolic "currencies" from food input (i.e., metabolic processing) and the allocation of such currency to fitness-related activities. A careful analysis of the nature of, and connections among, these processes can identify character states in metabolism, which are distinguishable by natural selection. An analysis of mechanisms affecting metabolic processing power (rate of production of currency) and efficiency (currency produced per unit input) shows that, although selection can act on each of these measures independently, these measures will often covary positively, since they are both promoted by the same kinetic adjustments. Previous claims of a forced trade-off between metabolic power and efficiency, as analyzed here, have failed to separate thermodynamic constraint from adaptive kinetic options, or have misapplied equilibrium or near-equilibrium reasoning to far-from-equilibrium situations. Metabolic power and efficiency relate to fitness increases via the biochemical mechanisms of energy-currency allocation to fitness-related uses. The power and the efficiency of allocation may or may not differ in nature from the power and the efficiency of metabolic processing, but may also covary positively. All four of these measures are thus effective indexes of fitness. A clarification of the distinct, but often covarying, roles of power and efficiency as metabolic-level fitness indexes potentiates the development of an evolutionary theory of metabolic organization. Such a theory may provide the essential connecting links between foraging theory and life-history-strategy theory, may provide and effective context for evolutionary study of natural genetic variation, and may yield effective insight into the interactions among adapative change, mechanistic constraint, and historical constraint in the evolution of energy-flow patterns in living systems.