Fitness (biology)

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.

Measures of fitness
There are two commonly used measures of fitness; absolute fitness and relative fitness.

Absolute fitness ($$w_{\mathrm{abs}}$$) of a genotype is defined as the ratio between the number of individuals with that genotype after selection to those before selection. It is calculated for a single generation and may be calculated from absolute numbers or from frequencies. When the fitness is larger than one, the genotype increases in frequency; a ratio smaller than one indicates a decrease in frequency.


 * $${w_{\mathrm{abs}}} = {{N_{\mathrm{after}}} \over {N_{\mathrm{before}}}}$$

Absolute fitness for a genotype can also be calculated as the product of the proportion survival times the average fecundity.

Relative fitness is 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 can therefore take any nonnegative value, including 0.

While researchers can usually measure relative fitness, absolute fitness is more difficult. It is often difficult to determine how many individuals of a genotype there were immediately after reproduction.

The two concepts are related, and both of them are equivalent when they are divided by the mean fitness, which is weighted by genotype frequencies.


 * $${w_{abs} \over \bar{w_{abs}}} = {w_{rel} \over \bar{w_{rel}}}$$

Because fitness is a coefficient, and a variable may be multiplied by it several times, biologists may work with "log fitness" (particularly so before the advent of computers). By taking the logarithm of fitness each term may be added rather than multiplied.

Discussion
An individual's fitness is manifested through its phenotype. As phenotype is affected by both genes and environment, the fitness levels of different individuals with same genotype are not necessarily equal, but depend on the environment in which the individuals live.

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.

A fitness landscape is a way of visualising fitness in terms of peaks, where natural selection will always push uphill but only locally, resulting in suboptimality.

Where there are differences in fitness, a genetic load is exerted on the population.

Richard Dawkins introduced the controversial concept of ethical fitnessism.

History
The British sociologist Herbert Spencer coined the phrase "survival of the fittest" (though originally, and perhaps more accurately, "survival of the best fitted") in his 1851 work Social Statics and later used it to characterise what Charles Darwin had called natural selection. The British biologist J.B.S. Haldane was the first to quantify fitness, in terms of the modern evolutionary synthesis of Darwinism and Mendelian genetics starting with his 1924 paper A Mathematical Theory of Natural and Artificial Selection. The next further advance was the introduction of the concept of inclusive fitness by the British biologist W.D. Hamilton in 1964 in his paper on The Evolution of Social Behavior.