# Relative Fitness

∞ generated and posted on 2016.12.23 ∞

Number of offspring that survive to reproduce as compared to the average for a population.

 Relative Fitness describes the ability of one organism to reproduce as compared with the same ability in another organism.

Relative fitness is the absolute fitness of an organism as divided by the average fitness of the population within which that organism is found. Thus, if an organism produces 3 offspring but the population as a whole tends to produce only two, then the relative fitness of the focus organism is 3/2 = 1.5 (or the other 0.67).

Figure legend: Reproduction of blue-eyed cats (hypothetically) is three per generation versus two per generation for green-eyed cats. The absolute fitness therefore is 3 versus 2, respectively. The relative fitness, in turn, is 50% higher for the blue-eyed cats versus the green-eyed cats.

The absolute fitness of an organism often is considered in terms of that organism's genotype. Relative fitness thus is typically calculated as

absolute fitnessspecific genotype ÷ mean absolute fitnesspopulation

It can be difficult to measure absolute fitness for individual organisms, and this is particularly so for populations that are more or less constant in size over time, that is, which are at an equilibrium population size. The relative change in the frequency of certain alleles or traits over time, however, often can be easier to assess, and such changes deterministically occur as a function of relative fitness.