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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