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Home » Fitness » Fitness (biology) – Wikipedia, the free encyclopedia

Fitness (often denoted in population genetics models) is a central idea in evolutionary theory. 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.[1] 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 evolution by 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."

An individual's fitness is manifested through its phenotype. The phenotype 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 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." [2] 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."[3]

There are two commonly used measures of fitness; absolute fitness and relative fitness.

Absolute fitness () of a genotype is defined as the ratio between the number of individuals with that genotype after selection to those before selection. It is calculated for a single generation and must be calculated from absolute numbers. When the absolute fitness is larger than 1, the number of individuals bearing that genotype increases; an absolute fitness smaller than 1 indicates an absolute fall in the number of individuals bearing the genotype. If the number of individuals in a population stays constant, then the average absolute fitness must be equal to 1.

Absolute fitness for a genotype can also be calculated as the product of the probability of survival multiplied by the average fecundity. Absolute fitness is used in Fisher's fundamental theorem.

Relative fitness is quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation, i.e. one genotype is normalized at and the fitnesses of other genotypes are measured with respect to that genotype. Relative fitness can therefore take any nonnegative value, including 0. Relative fitness is used in the standard Wright-Fisher and Moran models of population genetics.

The two concepts are related, as can be seen by dividing each by the mean fitness, which is weighted by genotype frequencies.

The British sociologist Herbert Spencer coined the phrase "survival of the fittest" (though originally, and perhaps more accurately, "survival of the best fitted")[citation needed] in his 1864 work Principles of Biology to characterise what Charles Darwin had called natural selection.

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Fitness (biology) - Wikipedia, the free encyclopedia

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Home » Fitness » Fitness (biology) – Wikipedia, the free encyclopedia