Energetics and evolutionary fitness.

It has long been recognized that energy is the currency of evolution, but contrasting conceptions of the relationship between energy and adaptation have yielded different interpretations. In the equal fitness paradigm (EFP), fitness (defined as the energetic equivalent of surviving offspring per generation) is held to be a constant within and between species in a steady-state, zero-sum closed system of constant energy availability. The fossil record, however, indicates that living space and energy availability and access have increased over time in response to pervasive natural selection that favors traits conferring as much power (energy per unit time) as possible, given the constraints imposed by external conditions and other organisms. Through various collaborative relationships and power-enhancing innovations, the inevitable tradeoffs among reproductive parameters envisioned in the EFP are relaxed enough that allocation to offspring can vary in space and time. We suggest that the EFP applies only under highly specific conditions of local constancy of energy availability and not to the biosphere as a whole. We note that fitness, however defined, need therefore not be constant and is difficult to measure in extant species and to infer with proxies in fossil taxa.