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Supervisor: Dr. Justin Gerlach, Evolution and Behaviour Set: Tuesday 28th October 2014 Define “frequency dependent selection,” and describe some of the ways it can arise in nature. Give examples wherever possible. Frequency dependent selection is a very significant form of natural selection. It comes in the form of both negative and positive frequency dependent selection, with the former far more commonly arising in nature. Both of these types will be examined and understood through examples in order to illustrate the differences between the two types, and it will be shown that frequency dependant selection as a whole is incredibly significant, through some of the features of populations that it causes. General definition Frequency dependent selection is defined as when natural selection acts in such a way that fitness of a genotype is dependent on the frequency of that genotype in the population relative to the frequency of other genotypes. Fitness is defined as the average number of offspring produced by an individual relative to the number of offspring produced by an average member of the population, and the frequency of particular alleles means the proportion of a population with those alleles. The two main types of frequency dependent selection are negative and positive frequency dependence. Negative frequency dependence Negative frequency dependence is where the fitness of a particular genotype decreases as its frequency increases. In these cases less common alleles are advantageous because they are less common. An example of negative frequency dependence is in host–parasite interactions. In a population of potential hosts, two host genotypes may confer variable resistance against parasites of two different genotypes, so that each parasite is more able to infect the hosts with one of the genotypes. If one host genotype has a higher frequency, natural selection will select for the parasites with the genotype that are more able to infect these hosts. The higher frequency host genotype is now disadvantageous and has a lower fitness due to its higher frequency, since it is more easily infected by the more common parasites, an example of negative frequency dependence. A specific example of negative frequency dependence in host parasite interactions was studied by Lively & Dybdahl, observing the host snail Potamopyrgus antipodarum, which is infected by the Microphallus parasite. The ability of Microphallus to infect each clone strain depended on the frequency of each clone, as shown by a strain of clones common in one lake being more easily infected by Microphallus from the same lake, but less easily infected by Microphallus from a lake where that snail strain was rare. A related example of how negative frequency dependent selection occurs in nature is in apostatic selection, where predators seek out the more common forms of prey that they recognise more easily (in comparison to those they have evolved to infect more easily). This means that rarer appearances in prey are advantageous, and the alleles that result in more common appearances are selected against. This form of negative frequency dependence is illustrated in an experiment by Tinbergen, who found that great tits ate disproportionately more Supervisor: Dr. Justin Gerlach, Evolution and Behaviour Set: Tuesday 28th October 2014 of the most common type of insect larvae, and less of the rarer insect larvae. Tinbergen hypothesised that predators recognise the prey they have eaten previously through use of a search image, where they have an increased ability to detect familiar prey. Not all negative frequency dependent selection occurs in predator-prey or parasite-host interactions; another way in which it arises in nature is in multiple niche polymorphisms. This occurs where a species contains several genotypes each of which is adapted to different environmental conditions. Where a particular genotype well adapted to a niche is rare, it experiences little competition, and so increases in frequency. This increases competition, resulting in reduced fitness with increased frequency. The fitness of the genotype is negatively frequency dependent. Another polymorphic characteristic maintained by negative frequency dependence are the different sexes, with sex ratios a result of negative frequency dependent selection. The fitness of the male phenotype increases as it becomes less frequent, since they are more able to mate with multiple females and therefore have a greater reproductive success. This would also mean it would also be advantageous to produce sons. Therefore the male phenotype will be selected for until a 1:1 sex ratio is reached. The sex ratio always moves back to this equilibrium position, as if male frequency increased, the male fitness would decrease due to lower reproductive success because of greater competition. This illustrates how negative frequency dependent selection can result in stable polymorphisms, meaning it is a powerful process in maintaining variation. Natural selection favours a genotype when it is rare so it will increase in frequency, but as the frequency of the genotype increases its fitness decreases until it is selected against, therefore an equilibrium frequency of several different genotypes may be reached, where natural selection will not alter their frequencies. Positive frequency dependence Positive frequency dependent selection does not lead to stable polymorphisms, and is less common than negative frequency dependence. Positive frequency dependence is where the fitness of a genotype increases as its frequency increases. Instead of stabalising polymorphisms it results in fixation of particular alleles, which is where the allele is present in all members of the population, with loss of all other alleles for that gene. An example of positive frequency dependence is when insect species have warning colouration, to warm that they are poisonous to eat in order to avoid predation. This works as when birds eat the poisonous insects with warning colouration, they are unpalatable, or the birds become ill and so learn not to eat insects with that appearance. When rare, having warning colouration does not have high fitness because there are fewer other similar insects from which the birds can learn. The fitness of having warning colouration genotypes increases as the frequency of insects with this phenotype increase, since birds will learn not to eat these insects, showing that this is an example of positive frequency dependence. The frequency and fitness of warning colouration increase until the alleles for warning colouration reach fixation, so are found in all members of the population. A specific extended example of this form of positive frequency dependent selection is in Heliconius butterflies, which are all poisonous. Heliconius butterflies have also been Supervisor: Dr. Justin Gerlach, Evolution and Behaviour Set: Tuesday 28th October 2014 associated with negative frequency dependence, in their non-poisonous mimics. Natural selection favours non-poisonous butterflies that have the same colour patterns as the poisonous butterflies, in Batesian mimicry. When the mimics are uncommon, but real Heliconius butterflies have a reasonably high frequency, birds will avoid the mimics as they will have learnt to avoid butterflies with that colour pattern. However, if the non poisonous mimics are common, fewer birds would have learnt to avoid eating butterflies of that colour pattern, so their fitness will be lower. The fitness of alleles that result in warning colouration in mimics is negatively frequency dependent, while the fitness of alleles that result in warning colouration in real Heliconius butterflies is positively frequency dependent. Papillo zagreus (left) a Batsian mimic of Heliconius species, such as Heliconius hecale (right) (image sources: http://en.wikipedia.org/wiki/Papilio_zagreus , http://en.wikipedia.org/wiki/Heliconius ) In conclusion, frequency dependent selection is a form of selection where the fitness of a genotype is dependent on its relative frequency. Positive frequency dependent selection, where fitness of a genotype increases with frequency, leads to the fixation of alleles in populations. The more common negative frequency dependent selection aside from causing 1:1 sex ratios, stable polymorphisms, and multiple niche polymorphisms may result in a greater level of variation, for example in the huge diversity in appearances of insects, which may have been caused by selection for alleles resulting in phenotypes that look as different as possible from other phenotypes nearby. Frequency dependent selection may also be a catalyst for speciation; for example in negative frequency selection on alleles for the appearance of prey, the differences in appearance may result in variable levels of camouflage of members of the same species in different habitats, which could lead to reproductive isolation through habitat isolation. Therefore, frequency dependent selection is a very significant form of natural selection. Bibliography Lectures ‘Evolutionary Genetics’ Dr. John Welch Mark Ridley (2004) ‘Evolution’ 3rd Edition Blackwells: Oxford Supervisor: Dr. Justin Gerlach, Evolution and Behaviour Set: Tuesday 28th October 2014 Lively, C.M. & Dybdahl, M.F. (2000). Parasite adaptation to locally common host geno-types. Nature 405, 679–681. Dennis R. Paulson (Jun., 1973) ‘Predator Polymorphism and Apostatic Selection’ Evolution, Vol. 27, No. 2 pp. 269-277 Alan B. Bond (2007), ‘The Evolution of Color Polymorphism: Crypticity, Searching Images, and Apostatic Selection’ Annual Review of Ecology, Evolution, and Systematics, Vol. 38, pp. 489-514 Olendorf et al. (2006) Frequency-dependent survival in natural guppy populations Nature 441, 633-636 Punzalan et al. (2005) ‘Perceptual processes and the maintenance of polymorphism through frequency-dependent predation’ Evolutionary Ecology