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Define “frequency dependent selection”, and describe some of the ways it can arise in
nature. Give examples whenever possible.
In the natural habitats of populations of organisms, there are always many different reasons
behind a change in the frequency of a particular phenotype, whether there are specific selection
pressures such as competition, predation, environmental conditions or simply due to random drift. This
can often make it hard for us to predict which phenotypes are most likely to be selected for.
For some characteristics, we cannot judge which allele will be selected for without collecting
data on the whole population. Frequency-dependent selection describes an evolutionary process
whereby the fitness of individual phenotypes depends on the frequency of other alleles in that
population. The two different forms of frequency dependent selection are positive and negative. The
positive form is when the fitness of the phenotype increases as it becomes more common. This is usually
because there is no significant benefit of the phenotype until this point, forming a stabilising selection
curve where the mean phenotype is most favoured. This can be taken to the extreme and be compared
to the Allee effect, which describes how new species or sub species may be steadily reduced in number
until extinction. Conversely, with negative frequency dependent selection, the fitness of a phenotype
decreases as it becomes more common; rare phenotypes are favoured, forming a disruptive selective
curve, where phenotypes further away from the mean increase in frequency.
Due to the high rate of mutation of genes and randomly occurring drift, new phenotypes can
occur very frequently. These new phenotypes are not usually advantageous to the organism and
sometimes they can decrease the fitness of the individual. In this case it would be to the detriment of
the population to allow the frequency of this disadvantageous allele to increase. Positive frequencydependency selection ensures that only new alleles which increase the fitness of the organism will
increase to a high frequency.
A common example of this is Batesian mimicry, where a harmless species takes on the
characteristics of a harmful species in order to dissuade predators from attacking them. This is seen in
many organisms such as the Monarch (Danaus plexippus) and Viceroy (Limenitis archippus) butterflies.
Monarch butterflies are poisonous to their predators due to the presence of cardiac glycosides in their
bloodstreams. They are also very brightly coloured and this is thought to be a warning sign to predators
who will associate those particular colours with danger; they are the model species. The Viceroy
butterflies have become the mimic species as they are not themselves harmful to any predators but
have evolved to mimic the bright 'warning' colours of the Monarch butterflies. Their predators still
associate the colouring with danger and so less of the Viceroy butterflies are eaten. Following the steps
of this evolution, we can see how positive frequency-dependent selection has achieved this. Originally,
the Viceroy would have looked completely different to the Monarch. However, there was a random
mutation which made an individual Viceroy look more similar to a Monarch. This would already have
been advantageous, as it would have slightly decreased the chance of the individual being eaten. This
rare phenotype would have increased in abundance slowly due to the evolutionary advantage. As the
new phenotype became more common, positive frequency-dependent selection would have acted upon
the phenotype and rapidly increased its frequency. This was because the new phenotype had a much
higher fitness than the old phenotype. Eventually the whole population become the new mimic
phenotype. Often it is the case that the positive form shifts the phenotype of the population to a much
fitter version.
By contrast, the negative form can arise in nature as a method of improving the survival rates of
the whole population by decreasing intra-specific competition. It can maintain two or more separate
phenotypes in the population. An example of negative frequency-dependent selection is found in the
African cichlid (Perissodus microlepis), a species of fish from Lake Tanganyika, which feeds on the scales
of a larger species of fish. The African cichlid appear to have two distinct phenotypes: one has their
mouth bent to the left and the other to the right. The former can only feed on the right side scales of
their prey and vice versa. Over a period of time, the ratio of the genotypes averages at 1:1. This is
attributed to negative frequency-dependent selection. If we consider a sudden increase in left sided fish,
the prey will be attacked more frequently on their right sides. Not only does this mean a decrease in the
amount of scales available to the African cichlids, but also the prey will become much more wary of
predators approaching from the right and are therefore more likely to spot the oncoming predators and
escape. Overall, there will be much less food available to the left sided fish; they will be less likely to
survive and begin to decrease in numbers. Meanwhile, the right sided fish are able to continue attacking
the prey on their left sides, as the prey is only wary of predators approaching from the right.
Consequentially, more food is available to the right sided fish, so they have a higher fitness and are
more likely to survive, reproduce and pass on the allele for the right sided phenotype. Over time, the
frequency of right sided fish returns to around 50% and the left sided fish will reduce in number to
around 50% and so the 1:1 stable population ratio is maintained over time. The African cichlids are a
good example of these, as the phenotypes are controlled by only two alleles at a single locus.
Negative frequency-dependent selection can also arise in a totally different way in a population.
Instead of maintaining a stable population of just two phenotypes, it can give rise to an extremely high
variety of phenotypes which are constantly changing. The advantage to this is that predators can often
associate a certain appearance with their prey. They attempt to seek out this learned appearance
repeatedly for their food source. Therefore, it is advantageous for the prey to have high variety in its
appearance as they are less likely to be eaten. Bryan Clarke described this phenomenon in his paper
based on frequency-dependent selection in 1962. He observed that the high colour and band pattern
polymorphism, in British snails, seemed to enable a greater chance of survival.
As seen in the previous example, we can see that frequency-dependent selection is extremely
important in the continual survival of many species. At first glance, it appears as though the positive and
negative forms are complete opposites. However, I would challenge that statement. I believe that both
forms have arisen in all modern day organisms. If this were not the case, their lineage would have
become extinct. The positive form appears to be the major cause of evolutionary branching, by changing
the phenotype of whole populations to a more evolutionarily advantageous, fitter phenotype. However,
the initial stage of these evolutionary changes began with an individual with a mutated phenotype. This
means that originally the phenotype was extremely rare and could not have increased via positive
frequency-dependent selection. Instead the negative form must have acted first. If this individual had a
fitter phenotype than the rest of the population, it would have easily been able to thrive compared to
the rest of the organisms. Therefore, it would have gone on to survive longer, successfully reproduce
more and so increase the frequency of the phenotype. Therefore, I believe that both the negative and
positive forms of frequency-dependent selection are vital for the evolution of all organisms.