Fitness (biology)

Fitness is a very central concept in evolution 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.

Individual's fitness is manifested through its phenotype. Since phenotype is affected by both genes and enviroment, it follows that the fitnesses of different individuals with same genotype are not necessarily equal, but they depend on the enviroment 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.