Fitness (biology)

Fitness (often denoted w in population genetics models) is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype. In either case, it describes the ability to both survive and reproduce, and is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. If differences between alleles at a given gene affect fitness, then the frequencies of the alleles will change over generations; the alleles 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.

Inclusive fitness differs from individual fitness by including the ability of an allele in one individual to promote the survival and/or reproduction of other individuals that share that allele, in preference to individuals with a different allele. One mechanism of inclusive fitness is kin selection.

## Fitness is a propensity

Fitness is often defined as a propensity or probability, rather than the actual number of offspring. For example, according to Maynard Smith, "Fitness is a property, not of an individual, but of a class of individuals – for example homozygous for allele A at a particular locus. Thus the phrase ’expected number of offspring’ means the average number, not the number produced by some one individual. If the first human infant with a gene for levitation were struck by lightning in its pram, this would not prove the new genotype to have low fitness, but only that the particular child was unlucky." [1] Equivalently, "the fitness of the individual - having an array x of phenotypes - is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."[2]

## Measures of fitness

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

### Absolute fitness

Absolute fitness (wabs) 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 1.0, the genotype increases in frequency; a ratio smaller than 1.0 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 surviving times the average fecundity.

### Relative fitness

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.

The two concepts are related, as can be seen by dividing each by the mean fitness, which is weighted by genotype frequencies.

${\frac{w_{abs}}{\overline{w}_{abs}} = \frac{w_{rel}}{\overline{w}_{rel}}}$

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.

## History

Herbert Spencer (27 April 1820 - 8 December 1903) was an English philosopher.

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.

## Fitness landscape

Natural selection pushes fitness towards nearby peaks, but lacks the foresight to select the highest peak.

A fitness landscape, first conceptualized by Sewall Wright, is a way of visualising fitness in terms of a high-dimensional surface, in which height indicates fitness, and each of the other dimensions represents allele identity for a different gene. Peaks correspond to local fitness maxima; it is often said that natural selection always progresses uphill but can only do so locally. This can result in suboptimal local maxima becoming stable, because natural selection cannot return to the less-fit "valleys" of the landscape on the way to reach higher peaks.

Genetic load measures the average fitness of a population of individuals, relative to a hypothetical population in which the most fit genotype has become fixed.

Genetic load is the probability that an average individual will die or fail to reproduce because of its harmful genes. It is a number between 0 and 1 that measures the extent to which the average individual is inferior to the best individual.[3]

If there are a number of genotypes in a population, each with its characteristic fitness; the genotype with the highest fitness is called Wopt. The average fitness of the whole population is the fitness of each genotype multiplied by its frequency: this is called mean fitness. V symbolizes mean fitness. The formula for genetic load (L) is as follows:

L = (Wopt-V)/(Wopt)

If all the individuals in the population have the optimal genotype, then v = Wopt and the load is zero. If all but one have a genotype of zero fitness then v = 0 and L = 1.[3]

## Notes

1. ^ Maynard-Smith, J. (1989) Evolutionary Genetics ISBN 0198542151
2. ^ Hartl, D. L. (1981) A Primer of Population Genetics ISBN 0878932712
3. ^ a b Ridley, Mark. "Evolution A-Z". Genetic load. Blackwell Publishing. Retrieved April 17, 2011.

Wikimedia Foundation. 2010.

### Look at other dictionaries:

• Fitness — may mean: The state of being physically active on a regular basis to maintain good physical condition.* Physical fitness, a general state of good health, usually as a result of exercise and nutrition * Cardiorespiratory fitness * Fitness… …   Wikipedia

• Fitness landscape — In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes (or phenotypes) and reproductive success. It is assumed that every genotype has a well defined replication rate (often… …   Wikipedia

• Biology (Philosophy of) in the nineteenth century — Philosophy of biology in the nineteenth century Jagdish Hattiangadi THE PHILOSOPHY OF BIOLOGY The emergence of biology as a unified subject Students of history and of biology share a common delight: as they study the details of any subject, they… …   History of philosophy

• fitness — noun 1》 the state of being fit. 2》 Biology an organism s ability to survive and reproduce in a particular environment …   English new terms dictionary

• Darwinian fitness — fitness (def. 3). * * * Darwinian fitness (biology) The number of offspring of an individual that live to reproduce in the next generation • • • Main Entry: ↑fit …   Useful english dictionary

• List of biology topics — Biology is the study of life and its processes. Biologists study all aspects of living things, including all of the many life forms on earth and the processes in them that enable life. These basic processes include the harnessing of energy, the… …   Wikipedia

• Polymorphism (biology) — Light morph Jaguar (typical) Dark morph or melanistic Jaguar (about …   Wikipedia

• Inclusive fitness — There are a few definitions of Inclusive fitness (IF), but one (that, according to Oli, 2003 is not consistent with W. D. Hamilton s first description), is the sum of the direct and indirect fitness effects of an individual s behaviors, where the …   Wikipedia

• List of evolutionary biology topics — This is a list of topics in evolutionary biology and evolution. See also: List of biology topics, List of biochemistry topics, NOTOC A abiogenesis adaptation adaptive radiation allele allele frequency allopatric speciation altruism Archaeopteryx… …   Wikipedia

• Mutualism (biology) — Hummingbird Hawkmoth drinking from Dianthus. Pollination is a classic example of mutualism. Mutualism is the way two organisms of different species biologically interact in a relationship in which each individual derives a fitness benefit (i.e.,… …   Wikipedia