Evolutionary fitness
Evolutionary fitness is a measure of reproductive success: how well a trait, genotype, or organism contributes genes to future generations in a particular environment.
What evolutionary fitness is
Evolutionary fitness, often called Darwinian fitness, is about reproductive success. A trait, genotype, or organism has higher fitness when it leaves more genetic representation in future generations than alternatives in the same population and environment.
Not just survival
Survival can contribute to fitness, but only if it affects reproduction or the survival of related gene copies. An organism that lives a long time without reproducing has low direct fitness. Another that lives briefly but leaves many successful offspring may have high fitness.
Relative fitness
Biologists often compare fitness values rather than treating fitness as an absolute score. A genotype may be assigned a relative fitness compared with the most successful genotype in a setting. That comparison helps model how allele frequencies might change under selection.
Traits, genotypes, and environments
Fitness is not a permanent property of a trait by itself. The same phenotype can be useful, neutral, or harmful depending on predators, climate, mates, food, disease, competitors, and social conditions. A genotype's fitness can also depend on other genes and developmental context.
Tradeoffs
Fitness often involves tradeoffs. Growing faster may reduce repair. Producing many offspring may reduce care for each one. A bright display may attract mates and predators. A drought-tolerant plant may grow more slowly in wet conditions. Selection works through these whole-life consequences.
Inclusive fitness
Direct fitness comes from an individual's own offspring. Inclusive fitness also considers effects on relatives who share genes. This idea helps explain some social behaviors, such as alarm calls, cooperative breeding, and sterile worker behavior in some social insects, when benefits to relatives can outweigh costs.
Fitness landscapes
A fitness landscape is a metaphor that maps genotypes or traits to reproductive success. Peaks represent combinations with higher fitness, while valleys represent lower fitness. The metaphor is useful, but real landscapes can shift as environments, competitors, and genetic backgrounds change.
Why it matters
Evolutionary fitness matters because it is the bridge between individual differences and population change. It helps explain adaptation, sexual selection, disease evolution, crop and animal breeding, conservation, and why evolution does not always produce the strongest, biggest, or most complex form.