Fitness

Fitness, often called Darwinian or evolutionary fitness, describes how well an individual or a genotype passes its genes on to later generations. It is not about strength or speed. An organism that lives a long time but leaves no offspring has a fitness of zero, while a small, short-lived one that raises many young that go on to breed has high fitness. Survival matters only because it gives an organism the chance to mate and reproduce.

Biologists usually work with relative fitness, which compares the reproductive success of one genotype with that of others in the same population. Natural selection acts through these differences: alleles carried by fitter individuals become more common over time. Fitness also depends on the environment, so a trait that helps in one setting can be a burden in another. During the Industrial Revolution, dark peppered moths outbred pale ones on soot-darkened trees, where birds found them harder to spot.

Some traits lower survival yet raise fitness, such as the costly tail of a peacock, which attracts mates. Students meet the idea in population genetics problems, where fitness values are used to predict how allele frequencies change.