fitness

In everyday speech, fitness suggests being strong or healthy. In population genetics it means something narrower and more bookkeeping-like: how many surviving, reproducing offspring a genotype tends to leave compared with others. The currency of fitness is not muscle but descendants.

Formally, fitness is the relative reproductive success of a genotype in a particular environment, combining the chances of surviving to reproduce with the number of offspring produced. It is usually measured relative to a reference genotype, with the fittest scaled to one and others expressed as fractions. The shortfall from one is called the selection coefficient.

Fitness is what natural selection reads. Genotypes with higher fitness leave proportionally more copies of their alleles, so those alleles rise in frequency. Selection is, in a sense, just fitness differences playing out over generations.

Two cautions: fitness is always relative to a specific environment—a genotype that is fit in the cold may be unfit in the heat—and it concerns reproduction, not longevity or visible vigor. An organism that lives long but leaves no offspring has, in this technical sense, zero fitness.