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1913 L. J. Henderson Fitness of Environment p. v Darwinian fitness is compounded of a mutual relationship between the organism and the environment. Of this, fitness of environment is quite as essential a component as the fitness which arises in the process of organic evolution.

1930 R. A. Fisher Genetical Theory Nat. Selection ii. 46 The vital statistics of an organism in relation to its environment provide a means of determining a measure of the relative growth-rate of the population, which may be termed the Malthusian parameter of population increase... The Malthusian parameter will in general be different for each different genotype, and will measure the fitness to survive of each... The rate of increase of fitness of any species is equal to the genetic variance in fitness.]

1953 Symp. Soc. Exper. Biol. 7 112 Fitness for survival cannot be completely defined except as applied to a ‘unit of evolution’... The fitness of such a unit is its probability of leaving descendants after a given long period of time.

1955 T. Dobzhansky Evolution, Genetics & Man vi. 122 Adaptive value, or Darwinian fitness, is not the same thing as bodily strength, vigor, or bravery.

1960 D. S. Falconer Introd. Quantitative Genetics ii. 26 Individuals..contribute different numbers of offspring to the next generation. The proportionate contribution of offspring to the next generation is called the fitness of the individual.

1973 B. J. Williams Evol. & Human Origins xiv. 257/2 If we remember to stick to our definition of fitness as Darwinian fitness, it becomes apparent that advances in medical genetics have improved the fitness of those who suffer from PKU.

1983 J. R. S. Fincham Genetics xviii. 530 AA homozygotes have a fitness of 1 and aa and Aa both have fitnesses of 1—s.