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. 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."