Fitness (biology)
Fitness is a very central concept in evolutionary theory. It measures the capability of an individual of certain genotype to reproduce, and usually it equals to the proportion of the individual's genes of all the genes of the next generation. If there are differences in individual genotypes which affect the fitness, then the frequencies of the genotypes will change over generations, the genotypes with higher fitness becoming more common. This process is called natural selection.
There are two commonly used measures of fitness:
- Absolute fitness: is defined for a given genotype for one generation as
- wabs = (number of individuals of given genotype before selection)/(number of individuals of given genotype after selection)
- Relative fitness: quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation, i.e. one genotype is normalized at w = 1 and the fitnesses of other genotypes are measured with respect to that genotype.
Relative fitness we can generally measure, absolute fitness is more difficult to measure: we can't easily know what the number of individuals of the genotype there were immediately after reproduction.
An individual's fitness is manifested through its phenotype. Since phenotype is affected by both genes and environment, it follows that the fitnesses of different individuals with same genotype are not necessarily equal, but they depend on the environment in which the individuals live.
It should be noted that as fitness measures the quantity of the copies of the genes of an individual in the next generation, it doesn't really matter how the genes actually get copied. That is, for an individual it is just as beneficial to reproduce itself, or to help relatives with similar genes to reproduce, as long as similar amount of copies of individual's genes get passed on to the next generation. Selection which promotes this kind of helper behaviour is called kin selection.
See also: fitness landscape, ethical fitnessism