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Fitness (often denoted w or ω in
population genetics Population genetics is a subfield of genetics that deals with genetic differences within and between populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and pop ...
models) is the
quantitative Quantitative may refer to: * Quantitative research, scientific investigation of quantitative properties * Quantitative analysis (disambiguation) * Quantitative verse, a metrical system in poetry * Statistics, also known as quantitative analysis ...
representation of individual
reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...
. It is also equal to the average contribution to the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surv ...
of the next generation, made by the same individuals of the specified genotype or phenotype. Fitness can be defined either with respect to a
genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...
or to a
phenotype In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
in a given environment or time. The fitness of a genotype is manifested through its phenotype, which is also affected by the developmental environment. The fitness of a given phenotype can also be different in different selective environments. With asexual reproduction, it is sufficient to assign fitnesses to genotypes. With
sexual reproduction Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
, recombination scrambles
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s into different genotypes every generation; in this case, fitness values can be assigned to
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s by averaging over possible genetic backgrounds. Natural selection tends to make
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is a variation of the same sequence of nucleotides at the same place on a long DNA molecule, as described in leading textbooks on genetics and evolution. ::"The chro ...
s with higher fitness more common over time, resulting in
Darwinian Darwinism is a theory of biological evolution developed by the English naturalist Charles Darwin (1809–1882) and others, stating that all species of organisms arise and develop through the natural selection of small, inherited variations that ...
evolution. The term "Darwinian fitness" can be used to make clear the distinction with
physical fitness Physical fitness is a state of health and well-being and, more specifically, the ability to perform aspects of sports, occupations and daily activities. Physical fitness is generally achieved through proper nutrition, moderate-vigorous physic ...
. Fitness does not include a measure of survival or life-span;
Herbert Spencer Herbert Spencer (27 April 1820 – 8 December 1903) was an English philosopher, psychologist, biologist, anthropologist, and sociologist famous for his hypothesis of social Darwinism. Spencer originated the expression "survival of the fittest ...
's well-known phrase " survival of the fittest" should be interpreted as: "Survival of the form (phenotypic or genotypic) that will leave the most copies of itself in successive generations."
Inclusive fitness In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964: * Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports the ...
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 is the evolutionary strategy that favours the reproductive success of an organism's relatives, even when at a cost to the organism's own survival and reproduction. Kin altruism can look like altruistic behaviour whose evolution ...
.


Fitness as propensity

Fitness is often defined as a
propensity The propensity theory of probability is a probability interpretation in which the probability is thought of as a physical propensity, disposition, or tendency of a given type of situation to yield an outcome of a certain kind, or to yield a long- ...
or probability, rather than the actual number of offspring. For example, according to
Maynard Smith John Maynard Smith (6 January 1920 – 19 April 2004) was a British theoretical and mathematical evolutionary biologist and geneticist. Originally an aeronautical engineer during the Second World War, he took a second degree in genetics und ...
, "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." Alternatively, "the fitness of the individual—having an array x of
phenotypes In genetics, the phenotype () is the set of observable characteristics or traits of an organism. The term covers the organism's morphology or physical form and structure, its developmental processes, its biochemical and physiological prop ...
—is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."


Models of fitness

In order to avoid the complications of sex and recombination, the concept of fitness is restricted to an asexual population without
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of genetic material between different organisms which leads to production of offspring with combinations of traits that differ from those found in either parent. In eukary ...
. Thus, fitnesses can be assigned directly to genotypes and measured. There are two commonly used measures of fitness – absolute fitness and relative fitness.


Absolute fitness

The absolute fitness (W) of a genotype is defined as the proportional change in the abundance of that genotype over one generation attributable to selection. For example, if n(t) is the abundance of a genotype in generation t in an infinitely large population (so that there is no
genetic drift Genetic drift, also known as allelic drift or the Wright effect, is the change in the frequency of an existing gene variant (allele) in a population due to random chance. Genetic drift may cause gene variants to disappear completely and there ...
), and neglecting the change in genotype abundances due to
mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA or viral replication, m ...
s, then :n(t+1)=Wn(t). An absolute fitness larger than 1 indicates growth in that genotype's abundance; an absolute fitness smaller than 1 indicates decline.


Relative fitness

Whereas absolute fitness determines changes in genotype abundance, relative fitness (w) determines changes in genotype
frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...
. If N(t) is the total population size in generation t, and the relevant genotype's frequency is p(t)=n(t)/N(t), then :p(t+1)=\fracp(t), where \overline is the mean relative fitness in the population (again setting aside changes in frequency due to drift and mutation). Relative fitnesses only indicate the change in prevalence of different genotypes relative to each other, and so only their values relative to each other are important; relative fitnesses can be any nonnegative number, including 0. It is often convenient to choose one genotype as a reference and set its relative fitness to 1. Relative fitness is used in the standard Wright–Fisher and Moran models of population genetics. Absolute fitnesses can be used to calculate relative fitness, since p(t+1)=n(t+1)/N(t+1)=(W/\overline)p(t) (we have used the fact that N(t+1)=\overline N(t) , where \overline is the mean absolute fitness in the population). This implies that w/\overline=W/\overline, or in other words, relative fitness is proportional to W/\overline. It is not possible to calculate absolute fitnesses from relative fitnesses alone, since relative fitnesses contain no information about changes in overall population abundance N(t). Assigning relative fitness values to genotypes is mathematically appropriate when two conditions are met: first, the population is at demographic equilibrium, and second, individuals vary in their birth rate, contest ability, or death rate, but not a combination of these traits.


Change in genotype frequencies due to selection

The change in genotype frequencies due to selection follows immediately from the definition of relative fitness, :\Delta p = p(t+1)-p(t)=\fracp(t) . Thus, a genotype's frequency will decline or increase depending on whether its fitness is lower or greater than the mean fitness, respectively. In the particular case that there are only two genotypes of interest (e.g. representing the invasion of a new mutant allele), the change in genotype frequencies is often written in a different form. Suppose that two genotypes A and B have fitnesses w_A and w_B, and frequencies p and 1-p, respectively. Then \overline=w_A p + w_B (1-p), and so :\Delta p = \fracp = \fracp(1-p) . Thus, the change in genotype A's frequency depends crucially on the difference between its fitness and the fitness of genotype B. Supposing that A is more fit than B, and defining the
selection coefficient In population genetics, a selection coefficient, usually denoted by the letter ''s'', is a measure of differences in relative fitness. Selection coefficients are central to the quantitative description of evolution, since fitness differences deter ...
s by w_A=(1+s)w_B, we obtain :\Delta p = \fracp = \fracp(1-p)\approx sp(1-p) , where the last approximation holds for s\ll 1. In other words, the fitter genotype's frequency grows approximately logistically.


History

The British sociologist
Herbert Spencer Herbert Spencer (27 April 1820 – 8 December 1903) was an English philosopher, psychologist, biologist, anthropologist, and sociologist famous for his hypothesis of social Darwinism. Spencer originated the expression "survival of the fittest ...
coined the phrase " survival of the fittest" in his 1864 work ''Principles of Biology'' to characterise what
Charles Darwin Charles Robert Darwin ( ; 12 February 1809 – 19 April 1882) was an English natural history#Before 1900, naturalist, geologist, and biologist, widely known for his contributions to evolutionary biology. His proposition that all speci ...
had called
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
.

^ "Herbert Spencer in his ''Principles of Biology'' of 1864, vol. 1, p. 444, wrote: 'This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called "natural selection", or the preservation of favoured races in the struggle for life.'" , citing HERBERT SPENCER, THE PRINCIPLES OF BIOLOGY 444 (Univ. Press of the Pac. 2002.)
The British biologist J.B.S. Haldane was the first to quantify fitness, in terms of the
modern evolutionary synthesis Modern synthesis or modern evolutionary synthesis refers to several perspectives on evolutionary biology, namely: * Modern synthesis (20th century), the term coined by Julian Huxley in 1942 to denote the synthesis between Mendelian genetics and s ...
of Darwinism and
Mendelian genetics Mendelian inheritance (also known as Mendelism) is a type of biological inheritance following the principles originally proposed by Gregor Mendel in 1865 and 1866, re-discovered in 1900 by Hugo de Vries and Carl Correns, and later popularized ...
starting with his 1924 paper ''
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. Haldane outlines the first mathematical models for man ...
''. The next further advance was the introduction of the concept of
inclusive fitness In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964: * Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports the ...
by the British biologist W.D. Hamilton in 1964 in his paper on '' The Genetical Evolution of Social Behaviour''.


Genetic load

Genetic load Genetic load is the difference between the fitness of an average genotype in a population and the fitness of some reference genotype, which may be either the best present in a population, or may be the theoretically optimal genotype. The average ...
measures the average fitness of a population of individuals, relative either to a theoretical genotype of optimal fitness, or relative to the most fit genotype actually present in the population. Consider n genotypes \mathbf _1 \dots \mathbf _n, which have the fitnesses w_1 \dots w_n and the genotype frequencies p_1 \dots p_n respectively. Ignoring
frequency-dependent selection Frequency-dependent selection is an evolutionary process by which the fitness of a phenotype or genotype depends on the phenotype or genotype composition of a given population. * In positive frequency-dependent selection, the fitness of a phenotyp ...
, then genetic load (L) may be calculated as: :L = Genetic load may increase when deleterious mutations, migration,
inbreeding Inbreeding is the production of offspring from the mating or breeding of individuals or organisms that are closely related genetically. By analogy, the term is used in human reproduction, but more commonly refers to the genetic disorders an ...
, or
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing distantly related genetic material into a breeding line, thereby increasing genetic diversity. Outcrossing can be a useful ...
lower mean fitness. Genetic load may also increase when beneficial mutations increase the maximum fitness against which other mutations are compared; this is known as the substitutional load or cost of selection.


See also

*
Gene-centered view of evolution With gene defined as "not just one single physical bit of DNA utall replicas of a particular bit of DNA distributed throughout the world", the gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory hol ...
*
Inclusive fitness In evolutionary biology, inclusive fitness is one of two metrics of evolutionary success as defined by W. D. Hamilton in 1964: * Personal fitness is the number of offspring that an individual begets (regardless of who rescues/rears/supports the ...
* Lineage selection *
Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Cha ...
*
Reproductive success Reproductive success is an individual's production of offspring per breeding event or lifetime. This is not limited by the number of offspring produced by one individual, but also the reproductive success of these offspring themselves. Reproduct ...
*
Selection coefficient In population genetics, a selection coefficient, usually denoted by the letter ''s'', is a measure of differences in relative fitness. Selection coefficients are central to the quantitative description of evolution, since fitness differences deter ...
*
Universal Darwinism Universal Darwinism, also known as generalized Darwinism, universal selection theory, or Darwinian metaphysics, is a variety of approaches that extend the theory of Darwinism beyond its original domain of biological evolution on Earth. Universal ...
* Differential fitness


Notes and references


Bibliography

* Sober, E. (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


Video: Using fitness landscapes to visualize evolution in action

BEACON Blog--Evolution 101: Fitness Landscapes


* ttp://www.blackwellpublishing.com/ridley/a-z/Fitness.asp Evolution A-Z: Fitness
Stanford Encyclopedia of Philosophy entry
{{Authority control Evolutionary biology concepts Genetics concepts Modern synthesis (20th century) Population genetics Sexual selection