Inclusive fitness

Inclusive fitness (IF) is the sum of the direct and indirect fitness effects of an individual’s behaviours, where the direct fitness effect is the impact on the individual’s fitness, and the indirect fitness effect is the impact on the fitness of its social partners, weighted by the degree of relatedness between the individual and its social partners. When social behaviors enhance or diminish the survival or reproduction of other individuals possessing genes that predispose to the same social behaviors, they affect the organism’s indirect fitness. This model is therefore more generalized than kin selection (in the strict sense), which requires that the shared genes are identical by descent; as such, the two models are commonly treated as synonymous, because the most common historical context for modeling inclusive fitness is indeed in groups of closely-related organisms.

From the gene’s point of view, evolutionary success ultimately depends on leaving behind the maximum number of copies of itself in the population. Until 1964 it was generally believed that genes only achieved this by causing the individual to leave the maximum number of viable offspring possible. However, in 1964 W. D. Hamilton showed that because close relatives of an organism are likely to share more genes in common (not to be confused with “common genes,” the opposite of scarce genes), the gene can also increase its evolutionary success by promoting the reproduction and survival of these related individuals. This leads individuals to behave in a manner maximising their inclusive fitness, rather than their individual fitness.

An empirical example of the inclusive fitness principle is provided by the Belding ground squirrel. Here, individuals give alarm calls to warn their group of the presence of a predator. By emitting the alarm, the Belding ground squirrel puts itself in increased danger by giving away its location. In the process, however, the squirrel protects its relatives that live within the population. In further studies, it has been shown that willingness of the squirrel to put itself at risk is directly proportional to how closely related it is to members of its population. Therefore, if protecting the other squirrels in the immediate area will lead to the passing on of more of the squirrel’s own genes than the squirrel could leave by reproducing on its own, the squirrel is willing to risk sacrificing itself, which leads to greater inclusive fitness. Another good example is that a lapwing will fake injury to distract a hawk from its young. By faking injury, it increases its own vulnerability, but increases the inclusive fitness.

Fitness (biology)

Fitness (biology)
From Wikipedia, the free encyclopedia
Jump to: navigation, search

Fitness (often denoted w in population genetics models) is a central concept in evolutionary theory. It describes the capability of an individual of certain genotype to reproduce, and usually is equal to the proportion of the individual’s genes in all the genes of the next generation. If differences in individual genotypes affect fitness, then the frequencies of the genotypes will change over generations; the genotypes with higher fitness become more common. This process is called natural selection.

An individual’s fitness is manifested through its phenotype. As phenotype is affected by both genes and environment, the fitnesses of different individuals with the same genotype are not necessarily equal, but depend on the environment in which the individuals live. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype.

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 arrive in the next generation. That is, for an individual it is equally “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.