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Equal fitness paradigm explained by a trade-off between generation time and energy production rate

Nature Ecology & Evolutionvolume 2pages262268 (2018) | Download Citation



Most plant, animal and microbial species of widely varying body size and lifestyle are nearly equally fit as evidenced by their coexistence and persistence through millions of years. All organisms compete for a limited supply of organic chemical energy, derived mostly from photosynthesis, to invest in the two components of fitness: survival and production. All organisms are mortal because molecular and cellular damage accumulates over the lifetime; life persists only because parents produce offspring. We call this the equal fitness paradigm. The equal fitness paradigm occurs because: (1) there is a trade-off between generation time and productive power, which have equal-but-opposite scalings with body size and temperature; smaller and warmer organisms have shorter lifespans but produce biomass at higher rates than larger and colder organisms; (2) the energy content of biomass is essentially constant, ~22.4 kJ g−1 dry body weight; and (3) the fraction of biomass production incorporated into surviving offspring is also roughly constant, ~10–50%. As organisms transmit approximately the same quantity of energy per gram to offspring in the next generation, no species has an inherent lasting advantage in the struggle for existence. The equal fitness paradigm emphasizes the central importance of energy, biological scaling relations and power–time trade-offs in life history, ecology and evolution.

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We thank the following individuals for helpful discussions of ideas and/or comments on the manuscript: G. Boyle, J. R. Burger, K. Cummins, J. Damuth, B. J. Enquist, J. F. Gillooly, R. Hengeveld, C. Jordan, A. Kodric-Brown, C. Levitan, J. G. Okie and D. Storch.

Author information

Author notes

    • James H. Brown

    Present address: 636 Piney Way, Morro Bay, CA, USA

    • Charles A. S. Hall

    Present address: 26242 Montana Highway 35, Polson, MT, USA


  1. Department of Biology, University of New Mexico, Albuquerque, NM, USA

    • James H. Brown
  2. Department of Forest and Environmental Biology and Program in Environmental Science, State University of New York – College of Environmental Science and Forestry, Syracuse, NY, USA

    • Charles A. S. Hall
  3. School of Biological Sciences, University of Reading, Reading, UK

    • Richard M. Sibly


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All three co-authors contributed to all aspects of the work.

Competing interests

The authors declare no competing financial interests.

Corresponding authors

Correspondence to James H. Brown or Charles A. S. Hall.

Supplementary information

  1. Life Sciences Reporting Summary

  2. Supplementary Table 1

    Data of Fig. 2 as an Excel file with columns giving taxon, genus, species, dry body mass, temperature in °C, uncorrected mortality rate, log10 dry body mass and log10 generation time at 20 °C, for 2,026 species.

  3. Supplementary Table 2

    Data of Fig. 4 as an Excel file with columns giving taxon, genus and species, dry body mass and ash-free energy content for 74 species.

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