Fitness (Biology)

In evolutionary biology, fitness is the relative ability of an organism, or of a genotype, to survive and reproduce in a particular environment, contributing its genes to the next generation. It is not the same as strength or health: an animal that lives long but leaves no offspring has a fitness of zero.

Fitness combines survival to reproductive age, the number of offspring produced and the success of those offspring. Absolute fitness is the actual number of offspring, whereas relative fitness compares one type with the others in the population, which is what matters for natural selection. Charles Darwin called the process survival of the fittest, a phrase suggested by Herbert Spencer, though the idea concerns reproduction as much as survival.

W. D. Hamilton showed in 1964 that a gene can also spread by helping relatives who carry copies of it, a result known as inclusive fitness. This explains behaviors such as the sterile workers of ants and bees and warning calls in birds, and it is the basis of kin selection.

Sewall Wright introduced the picture of a fitness landscape in 1932, in which each combination of alleles is a point and its fitness is the height. Populations climb peaks through selection, but genetic drift and valleys between peaks can trap them. The concept is central to population genetics, and the theorem of Ronald Fisher links the rate of change in fitness to genetic variance.

• Natural Selection — the process that acts on fitness
• Genetic Drift — a chance force that works against it
• Population Genetics — the field that models it