Fitness Directory

Fitness is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual's genes in all the genes of the next generation. If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.

An individual's fitness is manifested through its phenotype. As phenotype is affected by both genes and environment, the fatnesses of different individuals with same genotype are not necessarily equal, but depend on the environment in which the individuals live.