Evolutionary biology is poised for a third major synthesis. The first presented Darwin’s evidence from natural history. The second incorporated genetic mechanisms. The third will be based on energy and biophysical processes. It should include the equal fitness paradigm (EFP), which quantifies how organisms convert biomass into surviving offspring. Natural selection tends to maximize energetic fitness, E=P_coh GFQ, where P_coh is mass-specific rate of cohort biomass production, G is generation time, F is fraction of cohort production that is passed to surviving offspring, and Q is energy density of biomas. At steady state, parents replace themselves with an exactly equal mass-specific energy content, E ≈ 22.4 kJ/g, and biomass, M ≈ 1 g/g, of offspring. The EFP highlights: i) the energetic basis of survival and reproduction; ii) how natural selection acts directly on the parameters of M; iii) why there is no inherent intrinsic fitness advantage for higher metabolic power, ontogenetic or population growth rate, fecundity, longevity, or resource use efficiency; and iv) the role of energy in animals with a variety of life histories. Underlying the spectacular diversity of living things is pervasive similarity in how energy is acquired from the environment and used to leave descendants offspring in future generations.