Frequency dependent selection
Encyclopedia
Frequency-dependent selection is the term given to an evolutionary process where the fitness
of a phenotype
is dependent on its frequency relative to other phenotypes in a given population
. In positive frequency-dependent selection (or purifying frequency-dependent selection) the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection (or diversifying frequency-dependent selection) the fitness of a phenotype increases as it becomes rarer. Negative frequency-dependent selection is an example of balancing selection
. Frequency-dependent selection is usually the result of interactions between species (predation, parasitism, or competition) or between genotypes within species (usually competitive or symbiotic), and has been especially frequently discussed with relation to anti-predator adaptations. Frequency-dependent selection can lead to polymorphic equilibria which result from interactions among genotypes within species in the same way that multi-species equilibria require interactions between species in competition (e.g. where αij parameters in Lotka-Volterra competition equations are non-zero).
of apostatic selection
(a synonym of negative frequency-dependent selection). In this paper, Clarke discussed frequency-dependent selection particularly with regard to predator attacks on polymorphic British snails, citing Luuk Tinbergen
's classic work on searching images as support that predators such as birds would tend to specialize on common forms of palatable species. Clarke later became a major promoter of the idea that frequency-dependent balancing selection might explain abundant molecular polymorphisms (often in the absence of heterosis
) in opposition to the neutral theory of molecular evolution
.
Another example of negative frequency-dependent selection is in the case of plant self-incompatibility allele
s. When two plants share the same incompatibility allele, they are unable to mate. Thus, a plant with a new (and therefore, rare) allele has more success at mating, and its allele spreads quickly through the population .
Negative frequency-dependent selection also operates in the interaction of many human pathogens, such as the flu virus . Once a particular strain has been common in a human population, most individuals would have developed an immune response to that strain. But a rare, novel strain of the flu virus would be able to spread quickly to almost any individual. This advantage of genetic novelty causes continual evolution of viral strains, with new versions common each year. Another immune-related example of negative frequency-dependent selection is the major histocompatibility complex
, which is involved in recognition of foreign antigens ). Frequency-dependent selection as an important factor may explain the high degree of polymorphism of MHC (Borghans et al., 2004).
, in which if a species is too rare, it may decline to extinction. This means that new alleles can have a difficult time invading a population, since they don't experience significant benefit until they become common. This has been proposed as a difficulty in the evolution of aposematic (or warning) coloration in toxic or distasteful organisms. The presumed advantage of the aposematic coloration is that predators have learned to avoid potential prey with that color pattern. But when the pattern is rare, the predator population does not become 'educated' and the pattern has no benefit. Therefore the warning color is only advantageous once it has become common. Warning coloration, if it involves more than one species, is known as Müllerian mimicry
, a form of convergent evolution
where multiple species share the same advantageous pattern.
Fitness (biology)
Fitness is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment...
of 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...
is dependent on its frequency relative to other phenotypes in a given population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...
. In positive frequency-dependent selection (or purifying frequency-dependent selection) the fitness of a phenotype increases as it becomes more common. In negative frequency-dependent selection (or diversifying frequency-dependent selection) the fitness of a phenotype increases as it becomes rarer. Negative frequency-dependent selection is an example of balancing selection
Balancing selection
Balancing selection refers to a number of selective processes by which multiple alleles are actively maintained in the gene pool of a population at frequencies above that of gene mutation. This usually happens when the heterozygotes for the alleles under consideration have a higher adaptive value...
. Frequency-dependent selection is usually the result of interactions between species (predation, parasitism, or competition) or between genotypes within species (usually competitive or symbiotic), and has been especially frequently discussed with relation to anti-predator adaptations. Frequency-dependent selection can lead to polymorphic equilibria which result from interactions among genotypes within species in the same way that multi-species equilibria require interactions between species in competition (e.g. where αij parameters in Lotka-Volterra competition equations are non-zero).
Negative frequency-dependent selection
The first explicit statement of frequency dependent selection appears to have been by E.B. Poulton in 1884 with reference to the way that predators could maintain color polymorphisms in their prey. Perhaps the best known early modern statement of the principle in the twentieth century was the discussion by Bryan ClarkeBryan Clarke
Professor Bryan Campbell Clarke FRS, born in 1932, is a British geneticist. He is professor emeritus of genetics at the University of Nottingham. Clarke is particularly noted for his work on apostatic selection and other forms of frequency-dependent selection, and work on polymorphism in snails,...
of apostatic selection
Apostatic selection
Apostatic selection is frequency-dependent selection by predators, particularly in regard to prey that are different morphs of a polymorphic species that is not a mimic of another species. It is closely linked to the idea of prey switching, however the two terms are regularly used to describe...
(a synonym of negative frequency-dependent selection). In this paper, Clarke discussed frequency-dependent selection particularly with regard to predator attacks on polymorphic British snails, citing Luuk Tinbergen
Luuk Tinbergen
Luuk Tinbergen was a Dutch ornithologist and ecologist.Tinbergen was the youngest of three eminent brothers — both Jan and Nikolaas won Nobel Prizes, for economics and physiology or medicine, respectively.He was appointed in 1949 by Gerard Baerends to the University of Groningen, where he...
's classic work on searching images as support that predators such as birds would tend to specialize on common forms of palatable species. Clarke later became a major promoter of the idea that frequency-dependent balancing selection might explain abundant molecular polymorphisms (often in the absence of heterosis
Heterosis
Heterosis, or hybrid vigor, or outbreeding enhancement, is the improved or increased function of any biological quality in a hybrid offspring. The adjective derived from heterosis is heterotic....
) in opposition to the neutral theory of molecular evolution
Neutral theory of molecular evolution
The neutral theory of molecular evolution states that the vast majority of evolutionary changes at the molecular level are caused by random drift of selectively neutral mutants . The theory was introduced by Motoo Kimura in the late 1960s and early 1970s...
.
Another example of negative frequency-dependent selection is in the case of plant self-incompatibility allele
Allele
An allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s. When two plants share the same incompatibility allele, they are unable to mate. Thus, a plant with a new (and therefore, rare) allele has more success at mating, and its allele spreads quickly through the population .
Negative frequency-dependent selection also operates in the interaction of many human pathogens, such as the flu virus . Once a particular strain has been common in a human population, most individuals would have developed an immune response to that strain. But a rare, novel strain of the flu virus would be able to spread quickly to almost any individual. This advantage of genetic novelty causes continual evolution of viral strains, with new versions common each year. Another immune-related example of negative frequency-dependent selection is the major histocompatibility complex
Major histocompatibility complex
Major histocompatibility complex is a cell surface molecule encoded by a large gene family in all vertebrates. MHC molecules mediate interactions of leukocytes, also called white blood cells , which are immune cells, with other leukocytes or body cells...
, which is involved in recognition of foreign antigens ). Frequency-dependent selection as an important factor may explain the high degree of polymorphism of MHC (Borghans et al., 2004).
Positive frequency-dependent selection
Where negative frequency-dependent selection gives an advantage to rare phenotypes, positive frequency-dependent selection gives an advantage to common phenotypes. In the between-species analogue, this is equivalent to an Allee effectAllee effect
The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita population growth rate in very small populations.-Description:...
, in which if a species is too rare, it may decline to extinction. This means that new alleles can have a difficult time invading a population, since they don't experience significant benefit until they become common. This has been proposed as a difficulty in the evolution of aposematic (or warning) coloration in toxic or distasteful organisms. The presumed advantage of the aposematic coloration is that predators have learned to avoid potential prey with that color pattern. But when the pattern is rare, the predator population does not become 'educated' and the pattern has no benefit. Therefore the warning color is only advantageous once it has become common. Warning coloration, if it involves more than one species, is known as Müllerian mimicry
Müllerian mimicry
Müllerian mimicry is a natural phenomenon when two or more harmful species, that may or may not be closely related and share one or more common predators, have come to mimic each other's warning signals...
, a form of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
where multiple species share the same advantageous pattern.
See also
- Evolutionary game theoryEvolutionary game theoryEvolutionary game theory is the application of Game Theory to evolving populations of lifeforms in biology. EGT is useful in this context by defining a framework of contests, strategies and analytics into which Darwinian competition can be modelled. It originated in 1973 with John Maynard Smith...
- Evolutionarily stable strategyEvolutionarily stable strategyIn game theory and behavioural ecology, an evolutionarily stable strategy , which is sometimes also called an evolutionary stable strategy, is a strategy which, if adopted by a population of players, cannot be invaded by any alternative strategy that is initially rare. An ESS is an equilibrium...
- Mimicry
- Tit for tatTit for tatTit for tat is an English saying meaning "equivalent retaliation". It is also a highly effective strategy in game theory for the iterated prisoner's dilemma. It was first introduced by Anatol Rapoport in Robert Axelrod's two tournaments, held around 1980. An agent using this strategy will initially...
- Apostatic selectionApostatic selectionApostatic selection is frequency-dependent selection by predators, particularly in regard to prey that are different morphs of a polymorphic species that is not a mimic of another species. It is closely linked to the idea of prey switching, however the two terms are regularly used to describe...