Fitness (biology)
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.
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.
[1] 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."
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.
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] 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.
"[3] 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.
in an infinitely large population (so that there is no genetic drift), and neglecting the change in genotype abundances due to mutations, then[4] An absolute fitness larger than 1 indicates growth in that genotype's abundance; an absolute fitness smaller than 1 indicates decline.
is the mean relative fitness in the population (again setting aside changes in frequency due to drift and mutation).
It is often convenient to choose one genotype as a reference and set its relative fitness to 1.
Relative fitness is used in the standard Wright–Fisher and Moran models of population genetics.
Assigning relative fitness values to genotypes is mathematically appropriate when two conditions are met: first, the population is at demographic equilibrium, and second, individuals vary in their birth rate, contest ability, or death rate, but not a combination of these traits.
[5] The change in genotype frequencies due to selection follows immediately from the definition of relative fitness, Thus, a genotype's frequency will decline or increase depending on whether its fitness is lower or greater than the mean fitness, respectively.
In the particular case that there are only two genotypes of interest (e.g. representing the invasion of a new mutant allele), the change in genotype frequencies is often written in a different form.
In other words, the fitter genotype's frequency grows approximately logistically.
The British sociologist Herbert Spencer coined the phrase "survival of the fittest" in his 1864 work Principles of Biology to characterise what Charles Darwin had called natural selection.
Haldane was the first to quantify fitness, in terms of the modern evolutionary synthesis of Darwinism and Mendelian genetics starting with his 1924 paper A Mathematical Theory of Natural and Artificial Selection.
The next further advance was the introduction of the concept of inclusive fitness by the British biologist W.D.
Hamilton in 1964 in his paper on The Genetical Evolution of Social Behaviour.
Ignoring frequency-dependent selection, then genetic load (
) may be calculated as: Genetic load may increase when deleterious mutations, migration, inbreeding, or outcrossing lower mean fitness.
Genetic load may also increase when beneficial mutations increase the maximum fitness against which other mutations are compared; this is known as the substitutional load or cost of selection.
