Fitness (biology)

Fitness (biology)

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Fitness is a central idea in evolutionary theory
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

. It can be defined either with respect to a genotype
Genotype
The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

 or to a phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

 in a given environment. In either case, it describes the ability to both survive and reproduce, and is equal to the average contribution to the gene pool
Gene pool
In population genetics, a gene pool is the complete set of unique alleles in a species or population.- Description :A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can survive bouts of intense selection...

 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
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

 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
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

.

An individual's fitness is manifested through its phenotype
Phenotype
A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

. The phenotype is affected by the developmental environment as well as by genes, and the fitness of a given phenotype can be different in different environments. The fitnesses of different individuals with the same genotype are therefore not necessarily equal. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype in a given environment or set of environments.

Inclusive fitness
Inclusive fitness
In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...

 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
Kin selection
Kin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/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." 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."

Measures of fitness


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

Absolute fitness


Absolute fitness () of a genotype is defined as the ratio
Ratio
In mathematics, a ratio is a relationship between two numbers of the same kind , usually expressed as "a to b" or a:b, sometimes expressed arithmetically as a dimensionless quotient of the two which explicitly indicates how many times the first number contains the second In mathematics, a ratio is...

 between the number of individuals with that genotype after selection to those before selection. It is calculated for a single generation
Generation
Generation , also known as procreation in biological sciences, is the act of producing offspring....

 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.


Absolute fitness for a genotype can also be calculated as the product of the proportion surviving times the average fecundity
Fecundity
Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

.

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 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
Mean
In statistics, mean has two related meanings:* the arithmetic mean .* the expected value of a random variable, which is also called the population mean....

 fitness, which is weighted by genotype frequencies.


Because fitness is a coefficient
Coefficient
In mathematics, a coefficient is a multiplicative factor in some term of an expression ; it is usually a number, but in any case does not involve any variables of the expression...

, and a variable may be multiplied by it several times, biologists may work with "log fitness" (particularly so before the advent of computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

s). By taking the logarithm
Logarithm
The logarithm of a number is the exponent by which another fixed value, the base, has to be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: More generally, if x = by, then y is the logarithm of x to base b, and is written...

 of fitness each term may be added rather than multiplied.

History



The British
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...

 sociologist Herbert Spencer
Herbert Spencer
Herbert Spencer was an English philosopher, biologist, sociologist, and prominent classical liberal political theorist of the Victorian era....

 coined the phrase "survival of the fittest
Survival of the fittest
"Survival of the fittest" is a phrase originating in evolutionary theory, as an alternative description of Natural selection. The phrase is today commonly used in contexts that are incompatible with the original meaning as intended by its first two proponents: British polymath philosopher Herbert...

" (though originally, and perhaps more accurately, "survival of the best fitted") in his 1851 work Social Statics
Social Statics
Social Statics, or The Conditions essential to Happiness specified, and the First of them Developed is an 1851 book by the British polymath Herbert Spencer...

and later used it to characterise what Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...

 had called natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

.

The British biologist J.B.S. Haldane was the first to quantify fitness, in terms of the modern evolutionary synthesis
Modern evolutionary synthesis
The modern evolutionary synthesis is a union of ideas from several biological specialties which provides a widely accepted account of evolution...

 of Darwinism and Mendelian genetics starting with his 1924 paper A Mathematical Theory of Natural and Artificial Selection
A Mathematical Theory of Natural and Artificial Selection
A Mathematical Theory of Natural and Artificial Selection is the title of a series of scientific papers by the British population geneticist J.B.S. Haldane, published between 1924 and 1934...

. The next further advance was the introduction of the concept of inclusive fitness
Inclusive fitness
In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...

 by the British biologist W.D. Hamilton in 1964 in his paper on The Evolution of Social Behavior
The Evolution of Social Behavior
The Genetical Evolution of Social Behaviour is a 1964 scientific paper by the British evolutionary biologist W.D. Hamilton in which he mathematically lays out the basis for inclusive fitness....

.

Fitness landscape




A fitness landscape
Fitness landscape
In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes and reproductive success. It is assumed that every genotype has a well-defined replication rate . This fitness is the "height" of the landscape...

, first conceptualized by Sewall Wright
Sewall Wright
Sewall Green Wright was an American geneticist known for his influential work on evolutionary theory and also for his work on path analysis. With R. A. Fisher and J.B.S. Haldane, he was a founder of theoretical population genetics. He is the discoverer of the inbreeding coefficient and of...

, 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


Genetic load
Genetic load
In population genetics, genetic load or genetic burden is a measure of the cost of lost alleles due to selection or mutation...

 measures the average fitness of a population of individuals, relative to a hypothetical population in which the most fit genotype has become fixed
Fixation (population genetics)
In population genetics, fixation is the change in a gene pool from a situation where there exist at least two variants of a particular gene to a situation where only one of the alleles remains...

.

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.

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.

See also

  • Gene-centered view of evolution
    Gene-centered view of evolution
    The gene-centered view of evolution, gene selection theory or selfish gene theory holds that evolution occurs through the differential survival of competing genes, increasing the frequency of those alleles whose phenotypic effects successfully promote their own propagation, with gene defined as...

  • Inclusive fitness
    Inclusive fitness
    In evolutionary biology and evolutionary psychology, the inclusive fitness of an organism is the sum of its classical fitness and the number of equivalents of its own offspring it can add to the population by supporting others...

  • Natural selection
    Natural selection
    Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

  • Reproductive success
    Reproductive success
    Reproductive success is defined as the passing of genes onto the next generation in a way that they too can pass those genes on. In practice, this is often a tally of the number of offspring produced by an individual. A more correct definition, which incorporates inclusive fitness, is the...

  • Selection coefficient
    Selection coefficient
    In population genetics, the selection coefficient is a measure of the relative fitness of a phenotype. Usually denoted by the letter s, it compares the fitness of a phenotype to another favored phenotype, and is the proportional amount that the considered phenotype is less fit as measured by...


Further reading

  • Sober, E.
    Elliott Sober
    Elliott Sober is Hans Reichenbach Professor and William F. Vilas Research Professor in the Department of Philosophy at University of Wisconsin–Madison. Sober is noted for his work in philosophy of biology and general philosophy of science. Sober taught for one year at Stanford University and has...

    (2001). The Two Faces of Fitness. In R. Singh, D. Paul, C. Krimbas, and J. Beatty (Eds.), Thinking about Evolution: Historical, Philosophical, and Political Perspectives. Cambridge University Press, pp.309-321. Full text

External links