Fitness

Definition

In evolutionary biology, the relative reproductive success of an individual or genotype, measured by its contribution of genes to subsequent generations. Fitness encompasses survival to reproductive age, mating success, fecundity, and offspring viability; it is always context-dependent and defined relative to alternative phenotypes or genotypes in a specific environment. Natural selection acts on variation in fitness, favoring heritable traits that increase it.

Etymology

From English fit ('suitable, adapted') + -ness; adopted into evolutionary biology from Herbert Spencer's phrase "survival of the fittest" (1864), later refined by population geneticists to emphasize differential reproductive success rather than mere physical vigor.

Example

In a population of peppered moths (Biston betularia), the carbonaria melanic morph has higher fitness in soot-darkened industrial habitats because improved crypsis reduces bird predation, while the typical pale morph has higher fitness in unpolluted rural woodlands.

Synonyms

• Darwinian fitness
• reproductive success
• adaptive value

Related Terms

• Natural selection
• selection coefficient
• inclusive fitness
• Kin selection
• Life history
• trade-off
• phenotype
• Genotype
• population genetics
• evolutionary ecology

Usage Notes

Fitness is strictly relative: a genotype has high fitness only in comparison to alternatives in the same environment. It is not synonymous with strength, health, or longevity unless these directly enhance reproduction. The term is operationalized differently across subfields—absolute fitness counts total offspring, relative fitness scales to a reference genotype, and inclusive fitness includes effects on relatives. In insects and arachnids, fitness components often measured include egg-to-adult survival, development time, adult longevity, and lifetime fecundity. Avoid confusing with physical fitness in exercise physiology.