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Fitness (often denoted w or ω in
population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredient in the of the . Its pri ...
models) is the quantitative representation of
natural Nature, in the broadest sense, is the natural, physical, material world or universe The universe ( la, universus) is all of space and time and their contents, including planets, stars, galaxies, and all other forms of matter and ...
and
sexual selection Sexual selection is a mode of in which members of one of the other to with (intersexual selection), and compete with members of the same sex for access to members of the opposite sex (intrasexual selection). These two forms of selection me ...
within
evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interacti ...

evolutionary biology
. It 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 or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a specific ...
or to a
phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen ...

phenotype
in a given environment. In either case, it describes 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. Reproductive ...
and is 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 surviv ...
of the next generation that is made by individuals of the specified genotype or phenotype. 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 Asexual reproduction is a type of that does not involve the fusion of s or change in the number of . The offspring that arise by asexual reproduction from either unicellular or s inherit the full set of genes of their single parent. Asexual rep ...
, it is sufficient to assign fitnesses to genotypes. With
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
, genotypes have the opportunity to have a new frequency in the next generation. In this case, fitness values can be assigned to
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s by averaging over possible genetic backgrounds. Natural selection tends to make
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s with higher fitness more common over time, resulting in
Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as obser ...
evolution. The term "Darwinian fitness" can be used to make clear the distinction with
physical fitness Physical fitness is a state of health Health, according to the World Health Organization, is "a state of complete physical, Mental health, mental and social well-being and not merely the absence of disease and infirmity".World Health Organiza ...
. 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, biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has speciali ...

Herbert Spencer
's well-known phrase "
survival of the fittest "Survival of the fittest" is a phrase that originated from Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemp ...
" 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 Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
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 evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes that ...
.


Fitness is a propensity

Fitness is often defined as a
propensity The propensity theory of probability is Probability interpretations, one interpretation of the concept of probability. Theorists who adopt this interpretation think of probability as a physical propensity, or disposition, or tendency of a given type ...
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 Evolutionary biology is the subfield of biology that studies the evolutionary processes ( natural selection, common ...
, "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 right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The letters B and b represent genes for color, and the pictures show the resultant phenotypes. Ill ...
—is the probability, s(x), that the individual will be included among the group selected as parents of the next generation."


Models of fitness: asexuals

To avoid the complications of sex and recombination, we initially restrict our attention to an asexual population without
genetic recombination Genetic recombination (also known as genetic reshuffling) is the exchange of between different s which leads to production of offspring with combinations of traits that differ from those found in either parent. In s, genetic recombination during ...
. Then fitnesses can be assigned directly to genotypes rather than having to worry about individual alleles. 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 (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, M ...

genetic drift
), and neglecting the change in genotype abundances due to
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
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 A unit of time is any particular time Time is the indefinite continued sequence, progress of existence and event (philosophy), events that occur in an apparen ...
. 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 (biology), fitness. Selection coefficients are central to the quantitative description of evolution, since fitness ...
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 British may refer to: Peoples, culture, and language * British people The British people, or Britons, are the citizens of the United Kingdom of Great Britain and Northern Ireland The United Kingdom of Great Britain and Northern Ir ...

British
sociologist
Herbert Spencer Herbert Spencer (27 April 1820 – 8 December 1903) was an English philosopher, biologist Francesco Redi, the founder of biology, is recognized to be one of the greatest biologists of all time A biologist is a professional who has speciali ...

Herbert Spencer
coined the phrase "
survival of the fittest "Survival of the fittest" is a phrase that originated from Darwinian Darwinism is a theory A theory is a reason, rational type of abstraction, abstract thinking about a phenomenon, or the results of such thinking. The process of contemp ...
" in his 1864 work ''Principles of Biology'' to characterise what
Charles Darwin Charles Robert Darwin (; ; 12 February 1809 – 19 April 1882) was an English naturalist Natural history is a domain of inquiry involving organism In biology, an organism () is any organic, life, living system that fu ...

Charles Darwin
had called
natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
.

^ "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 of Darwinism and
Mendelian genetics Mendelian inheritance is a type of biological inheritance Inheritance is the practice of passing on private property, titles A title is one or more words used before or after a person's name, in certain contexts. It may signify either ...
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 paper : ''For a broader class of literature, see Academic publishing Academic publishing is the subfield of publishing Publishing is the ac ...
''. The next further advance was the introduction of the concept of
inclusive fitness In evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
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 Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal colour in a pea plant. The letters B and b represent a ...
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 selectionFrequency-dependent selection is an evolutionary process by which the fitness of a phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal color in pea plants. The l ...
, then genetic load (L) may be calculated as: :L = Genetic load may increase when deleterious mutations, migration,
inbreeding Inbreeding is the production of offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspri ...
, or
outcrossing Out-crossing or out-breeding is the technique of crossing between different breeds. This is the practice of introducing unrelated genetic material into a breeding line. It increases genetic diversity, thus reducing the probability of an individua ...
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 The gene-centered view of evolution, gene's eye view, gene selection theory, or selfish gene theory holds that adaptive evolution Evolution is change in the Heredity, heritable Phenotypic trait, characteristics of biological populations o ...
*
Inclusive fitness In evolutionary biology Evolutionary biology is the subfield of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...
*
Lineage selectionLineage selection occurs when the frequency of one biological Lineage (evolution), lineage changes relative to another lineage. Lineage selection is a generalization of individual based natural selection; the stating that an allele is favored by natu ...
*
Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype right , Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of peta ...
*
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. Reproductive ...
*
Selection coefficient In population genetics, a selection coefficient, usually denoted by the letter ''s'', is a measure of differences in relative Fitness (biology), fitness. Selection coefficients are central to the quantitative description of evolution, since fitness ...
*
Universal Darwinism Universal Darwinism (also known as generalized Darwinism, universal selection theory, or Darwinian metaphysics) refers to a variety of approaches that extend the theory of Darwinism Darwinism is a scientific theory, theory of Biology, biologic ...
* 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