Fitness (biology)

Fitness (often denoted ${\displaystyle w}$ or ω in population genetics models) is a quantitative representation of individual reproductive success. It is also equal to the average contribution to the gene pool of the next generation, made by the same individuals of the specified genotype or phenotype. Fitness can be defined either with respect to a genotype or to a phenotype in a given environment or time. The fitness of a genotype is manifested through its phenotype, which is also affected by the developmental environment. The fitness of a given phenotype can also be different in different selective environments.

With asexual reproduction, it is sufficient to assign fitnesses to genotypes. With sexual reproduction, recombination scrambles alleles into different genotypes every generation; in this case, fitness values can be assigned to alleles by averaging over possible genetic backgrounds. Natural selection tends to make alleles with higher fitness more common over time, resulting in Darwinian evolution.

The term "Darwinian fitness" can be used to make clear the distinction with physical fitness. Fitness does not include a measure of survival or life-span; Herbert Spencer's 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."

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. To avoid double counting, inclusive fitness excludes the contribution of other individuals to the survival and reproduction of the focal individual. 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."

Alternatively, "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."

In order to avoid the complications of sex and recombination, the concept of fitness is presented below in the restricted setting of an asexual population without genetic recombination. Thus, fitnesses can be assigned directly to genotypes. There are two commonly used operationalizations of fitness – absolute fitness and relative fitness.

The absolute fitness (${\displaystyle W}$) of a genotype is defined as the proportional change in the abundance of that genotype over one generation attributable to selection. For example, if ${\displaystyle n(t)}$ is the abundance of a genotype in generation ${\displaystyle t}$ in an infinitely large population (so that there is no genetic drift), and neglecting the change in genotype abundances due to mutations, then