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Fitness (biology)
Fitness (often denoted in population genetics models) is a central idea in evolutionary and sexual selection theories. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes individual reproductive success and is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. The term "Darwinian fitness" can be used to make clear the distinction with physical fitness.〔Wassersug, J. D., and R. J. Wassersug, 1986. Fitness fallacies. Natural History 3:34-37.〕 Where fitness is affected by differences between various alleles of a given gene, the relative frequency of those alleles will change across generations by natural selection and alleles with greater positive effect on individual fitness will become more common over time; this process is known as natural selection. Fitness does not include a measure of survival or life-span; the well known phrase Survival of the fittest should be interpreted as: "Survival of the form (phenotypic or genotypic) that will leave the most copies of itself in successive generations."
Fitness can only measure heritable differences, and these can then be chosen in mate choice, causing sexual selection. An individual's fitness is manifested through its phenotype, which is affected by the developmental environment as well as by genes, and the fitness of a given phenotype can be different in different environments. The fitnesses of different individuals with the same genotype are therefore not necessarily equal. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype in a given environment or set of environments.
Inclusive fitness differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele. One mechanism of inclusive fitness is kin selection.
==Fitness is a propensity==
Fitness is often defined as a propensity or probability, rather than the actual number of offspring. For example, according to Maynard Smith, "Fitness is a property, not of an individual, but of a class of individuals — for example homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky." 〔Maynard-Smith, J. (1989) ''Evolutionary Genetics'' ISBN 0-19-854215-1〕 Equivalently, "the fitness of the individual - having an array x of phenotypes — is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."〔Hartl, D. L. (1981) ''A Primer of Population Genetics'' ISBN 0-87893-271-2〕
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