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Fitness is a measure of the relative performance or adaptedness of an organism represented by its genotype in a given environment. The term fitness is sometimes also used to describe other biological units, such as the gene or the population. Classically fitness is used to describe differences in survival (viability selection as described by Charles Darwin (1809–1882) with the phrase "survival of the fittest"), mating success (sexual selection), and reproductive output (fecundity selection) between individuals characterized by their genotypes and measured as their relative contribution to the next generation in terms of the number of offspring a genotype succeeds in producing and rearing to sexual maturity. A genotype that leaves more offspring will thus have a higher fitness.

In the field of classical population genetics theory, evolutionary changes are exemplified by the change in gene frequency at a single gene locus with two alleles, A1 and A2, in a diploid organism. The modes of selection depend on the fitness of the heterozygote A1A2 compared to that of the homozygotes A1A1 and A2A2. If one homozygote (e.g., A1A1) has the highest fitness, directional selection will favor that genotype and eventually lead to fixation of allele A1. A famous example of directional selection is the industrial melanism of the peppered moth (Biston bitularia) in England, where the black or melanic morph increased in frequency after the industrial revolution, then decreased in the 1950s when "smokeless zones" were established and tree trunks became lighter, thus giving the black morph a disadvantage due to increased risk of predation by birds.

If the heterozygote has the highest fitness, stabilizing selection or heterozygote advantage will usually maintain both alleles in the population (an example is variation at the beta–globin gene in humans, where heterozygotes have an advantage in regions with malaria, while one type of homozygotes gets sickle cell disease), while heterozygote disadvantage will lead to disruptive selection favoring both homozygotes. This simple theory was developed for one locus in infinite populations and for constant fitness coefficients by, among others, R. A. Fisher (1890–1962) and J. S. B. Haldane (1892–1964) in the 1920s and 1930s. The theory was later modified and expanded to include multiple loci and variable environments, as well as population substructure and finite populations.

The shifting balance theory of Sewall Wright (1889–1988) describes the fitness landscape of more complex multilocus genotypes, where the fitness of certain genotypes has local peak values, while simple changes in genotype will lead to a fitness decrease. Shifts from one peak to another in that landscape require more complex changes with intermittent genotypes of reduced fitness. In small populations random genetic drift may counteract the selective forces that are driven by fitness differences and push populations from one peak to another.

Darwin considered fitness to be a property of the individual; later biologists sometimes use the term to refer to lower levels of organization, such as, for example, a property of the gene (the idea of the selfish gene is based on this unit) or of higher levels, such as, for example, the population. Socalled group selection is based on higher units, and the concept of inclusive fitness includes contributions of related individuals who share genes. This concept of fitness has been used to explain the evolution of altruistic behaviors, such as warning calls in birds, which may bring the altruistic individual to higher risk but may benefit its genes by improving the chance of survival of relatives.

See also ADAPTATION; ALTRUISM; EVOLUTION; SELECTION, LEVELS OF; SELFISH GENE; SOCIOBIOLOGY

VOLKER LOESCHCKE