1. Direct fitness (absolute fitness). Symbol W. A measure of the contribution of an individual to the genetic composition of subsequent generations through its own offspring. Hence, individuals that contribute the most offspring to the next generation are the fittest. Fitness therefore reflects how well an organism is adapted to its environment, which determines its survival.

2. Indirect fitness. A measure of the reproductive success of an individual’s indirect descendants, e.g. siblings and cousins, which share a proportion of the individual’s alleles. The sum of direct and indirect fitness is called inclusive fitness. See also selection coefficient.

3. Relative fitness. Symbol w. The contribution of an individual’s genotype or phenotype to a population relative to the contributions of other individuals. Mathematically it can be expressed as the individual’s absolute fitness divided by the absolute fitness of the fittest individual, such that the latter has a relative fitness of 1.