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. Population genetics is the study of the distribution of and change in allele frequencies under the influence of the five evolutionary forces: natural selection, genetic drift, mutation, migration and nonrandom mating. ... A speculatively rooted phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains, bacteria, archaea, and eukaryotes, as described initially by Carl Woese. ... The genotype is the specific genetic makeup (the specific genome) of an individual, usually in the form of DNA. It codes for the phenotype of that individual. ... This stylistic schematic diagram shows a gene in relation to the double helix structure of DNA and to a chromosome (right). ... Natural selection is the name Charles Darwin gave to the principal process through which new species emerge, or evolve. ...

Measures of fitness

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

Absolute fitness (w_abs) is defined for a given genotype for one generation as the ratio of organisms with that genotype after selection to those before. It may be calculated from absolute numbers or from frequencies, and as a ratio, is a value between 0 and 1. Generation, also known as procreation, is the act of producing offspring. ... In algebra, a ratio is the relationship between two quantities. ...

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 positive 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. In population genetics, the genotype frequency is the frequency (or proportion i. ...

Because fitness is a coefficient, and that coefficient may be multiplied by 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. In mathematics, a coefficient is a multiplicative factor of a certain object such as a variable (for example, the coefficients of a polynomial), a basis vector, a basis function and so on. ... A drawing of the everyday computer. ... Logarithms to various bases: red is to base e, green is to base 10, and purple is to base 1. ...

Discussion

An individual's fitness is manifested through its phenotype. As phenotype is affected by both genes and environment, the fitnesses of different individuals with same genotype are not necessarily equal, but depend on the environment in which the individuals live. The phenotype of an individual organism is either its total physical appearance and constitution, or a specific manifestation of a trait, such as size or eye color, that varies between individuals. ...

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. Kin selection was first suggested by Darwin as an explanation of the sterile castes of social insects and has later been mathematically defined by W. D. Hamilton as a mechanism for the evolution of apparently altruistic acts. ...

A fitness landscape is a way of visualising fitness in terms of peaks, where natural selection will always push uphill but , resulting in suboptimality. In evolutionary biology, fitness landscapes or adaptive landscapes are used to visualize the relationship between genotypes (or phenotypes) and replicatory success. ...

Where there are differences in fitness, a genetic load is exerted on the population. In population genetics, genetic load or genetic burden is a measure of the cost of lost alleles due to selection (selectional load) or mutation (mutational load). ...

Richard Dawkins introduced the controversial concept of ethical fitnessism. Richard Dawkins Clinton Richard Dawkins, FRS (born March 26, 1941), known as Richard Dawkins, is a British ethologist and popular science writer. ... Ethical fitnessism, or fitnessism for short, is the ethic whose behaviour tends to be maximized as a result of natural selection, i. ...

History

The British economist Herbert Spencer coined the phrase "survival of the fittest" 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. An economist is someone who studies Economics. ... Herbert Spencer. ... Herbert Spencer coined the phrase survival of the fittest Survival of the fittest is a phrase which is a shorthand for a concept relating to competition for survival or predominance. ... 1851 was a common year starting on Wednesday (see link for calendar). ... Social Statics is a 1851 book by the British economist Herbert Spencer. ... In his lifetime Charles Darwin gained international fame as an influential scientist examining controversial topics. ... Natural selection is the name Charles Darwin gave to the principal process through which new species emerge, or evolve. ... John Burdon Sanderson Haldane (November 5, 1892 - December 1, 1964), who normally used J.B.S. as a first name, was a geneticist born in Scotland and educated at Eton and Oxford University. ... The modern evolutionary synthesis (often referred to simply as the modern synthesis or the evolutionary synthesis), neo-Darwinian synthesis or neo-Darwinism, generally denotes the combination of Charles Darwins theory of the evolution of species by natural selection, Gregor Mendels theory of genetics as the basis for biological... Mendelian inheritance (or Mendelian genetics or Mendelism) is a set of primary tenets that underlie much of genetics developed by Gregor Mendel in the latter part of the 19th century. ... 1924 (MCMXXIV) was a leap year starting on Tuesday (link will take you to calendar). ... 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. ... Inclusive fitness encompasses conventional Darwinian fitness with the addition of behaviors that contribute to an organism’s individual fitness through altruism. ... This article is about the British biologist Bill Hamilton. ... For the Nintendo 64 emulator, see 1964 (Emulator). ... The Evolution of Social Behavior is a 1964 scientific paper by the British evolutionary biologist W.D. Hamilton in which he lays out a kin selection. ...

References

• Haldane, J.B.S. (1924) "A mathematical theory of natural and artificial selection" Part 1 Transactions of the Cambridge philosophical society: 23: 19-41 link (pdf file)
• Hamilton, W.D. (1964) "The evolution of social behavior" Journal of Theoretical Biology 1:...

External links

• Evolution A-Z: Fitness
• Stanford Encyclopedia of Philosophy entry