Fitness inbiology is the relative ability of anorganism to survive and pass on itsgenes to the next generation.[1]p160 It is a central idea inevolutionary theory. Fitness is usually equal to the proportion of the individual'sgenes in all the genes of the next generation.
Like all terms inevolutionary biology, fitness is defined in terms of an interbreedingpopulation, which might or might not be a wholespecies. If differences in individualgenotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This is the process callednatural selection.
An individual's fitness is caused by itsphenotype, and passed on by itsgenotype. The fitness of different individuals with the same genotype are not necessarily equal. It depends on theenvironment in which the individuals live, and on accidentalevents. However, since the fitness of the genotype is anaveraged quantity, it reflects the reproductive outcomes ofall individuals with that genotype.
Fitness measures the number of thecopies of the genes of an individual in the next generation. It doesn't really matter how the genes arrive in the next generation. For an individual, it is equally "beneficial" to reproduce itself, or to help relatives with similar genes to reproduce,as long as similar number of copies of individual's genes get passed on to the next generation. Selection which promotes this kind of helperbehaviour is calledkin selection.
Our closest relatives (parents, siblings, and our own children) share on average 50% (half) of our genes. One step further removed are grandparents. With each of them we share on average 25% (a quarter) of our genes. That is a measure of our relatedness to them. Next come first cousins (children of our parents' siblings). We share 12.5% (1/8) of their genes.[2]p100
William Hamilton added various ideas to the notion of fitness. His rule suggests that a costly action should be performed if:
Fitness costs and benefits are measured infecundity.[3]
Inclusive fitness is a term which is essentially the same as fitness, but emphasises the group of genes rather than individuals.
Biological fitness says how well an organism can reproduce, and spread its genes to its offspring. The theory of inclusive fitness says that the fitness of an organism is also increased to the extent that its close relatives also reproduce. This is because relatives share genes in proportion to their relationship.
Another way of saying it is:the inclusive fitness of an organism is not a property of itself, but a property of its set ofgenes. It is calculated from the reproductive success of the individual, plus the reproductive success of its relatives, each one weighed by an appropriate coefficient of relatedness.[4]
TheBritishsocialphilosopherHerbert Spencer coined the phrasesurvival of the fittest in his 1864 workPrinciples of biology to mean whatCharles Darwin callednatural selection.[5] The original phrase was "survival of the best fitted".
