Fitness is a central concept of evolutionary biology. We will consider individual fitness, followed by fitness as applied to alleles or genotypes.

The direct fitness of an individual is related to the number of offspring that that individual produces. Specifically, it is one-half of the number of offspring produced, because in sexual species, only half of an offspring's genes come from either parent. That proportion, one-half, represents the degree of relatedness, or proportion of genes shared, between parent and offspring.

Indirect fitness derives from shared genes with kin other than the direct offspring of an individual. This might include cousins, nieces, nephews, siblings, and so on. The indirect fitness of an individual is calculated by adding the relations of that individual multiplied by the degree of relatedness. Inclusive fitness represents the sum of direct and indirect fitness.

The concept of indirect fitness was developed by the evolutionary biologist...