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Mean Fitness Increases when Fitnesses are Additive


FISHER'S fundamental theorem of natural selection1 states that for a random mating population with fitnesses dependent on the genetic constitution at a single locus, the mean fitness of the population increases with time. This is true for an arbitrary number of possible alleles at the locus and for arbitrary fitness values.


  1. Fisher, R. A., The Genetical Theory of Natural Selection (Oxford University Press, 1930).

    Book  Google Scholar 

  2. Moran, P. A. P., Ann. Hum. Genet., 27, 383 (1964).

    Article  CAS  Google Scholar 

  3. Moran, P. A. P., Proc. Fifth Berk. Symp. Math. Stat. and Prob. (Univ. California Press, 1965).

  4. Kingman, J. F. C., Quart. J. Math., 12, 78 (1961).

    ADS  MathSciNet  Article  Google Scholar 

  5. Moran, P. A. P., Ann. Hum. Genet, (in the press).

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EWENS, W. Mean Fitness Increases when Fitnesses are Additive. Nature 221, 1076 (1969).

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