Download evolution-frequency-dependent-selection

Jessica Griffiths
03/11/2013
Define “frequency dependent selection”, and describe some of the ways it can arise in nature.
Give examples whenever possible.
Determining and understanding the mechanisms by which genetic diversity is generated and
maintained is one of the main focuses of evolutionary biology. Frequency-dependent selection
occurs where the fitness of a genotype is dependent upon the frequency of the allele or genotype
within the population. Positive frequency-dependent selection and negative frequency-dependent
selection can be considered as two extremes of a continuous scale. Frequency-dependent selection
is one of the most fundamental principles in population genetics.
Negative frequency-dependent selection promotes genetic variation in populations of organisms as
it favours rare genotypes, thereby helping to maintain large numbers of alleles at loci. Negative
frequency-dependent selection has found to be present in a range of different systems, including:
mimicry, search images in predator-prey systems and host-pathogen coevolution.
One of the key features of negative frequency-dependent selection is the fact that the point of
neutrality is a stable equilibrium. If the population deviates from this neutral position then the
fitness difference will cause it to return to its original position. Negative frequency-dependent
selection therefore helps to maintain genetic polymorphism.
Batesian mimicry is where a harmless species has evolved resemblance to a harmful species. This
can include the evolution of aposematic colouration, although warnings are not limited to visual
signals. The hoverfly, for example, like bees, feeds on nectar and has a striped black and yellow
colouring. Unlike bees, however, hoverflies lack a sting and so are harmless. The hoverfly therefore
benefits from increased protection against predation at a lesser energetic expense to themselves. If
the population size of the mimic species increases too much, however, then the model species – the
bees – will experience increased predation, resulting in a decreasing population size. This is a result
of the predator learning that the aposematic signal does not always have lethal or harmful
consequences. This is therefore an example of negative frequency-dependent selection.
The search image hypothesis suggests that predators are better at detecting familiar prey because
they have learnt to see them. This causes the predators to target common prey varieties as opposed
to rarer ones. Recognition of monomorphic prey is easier than polymorphic prey as there is less
variation in their physical appearance. The fitness of rare genotypes in prey is therefore greater than
common genotypes in low-density populations – this is an example of negative frequencydependent selection. Non-apostatic selection by predators can be observed in high-density
populations, resulting in increased predation of rarer organisms with more unusual appearances.
This shift in selection restores the stable equilibrium of the point of neutrality – this results in the
maintenance of polymorphism among prey.
Trypanosoma brucei, more commonly known as the African sleeping sickness parasite, changes the
antigens present on its surface sequentially, as its host develops antibodies against them. The host
and pathogen exert reciprocal selective pressures on each other, leading to rapid host-pathogen
coevolution. The dynamics of this host-pathogen relationship are often described using the ‘Red
Queen’ hypothesis – both the pathogen and host have to continually change in order to keep up
with each other’s adaptations. The host antibodies target the most common antigens; therefore
parasites with rarer antigens have a selective advantage and greater fitness.
Jessica Griffiths
03/11/2013
Negative frequency-dependent selection acts here to preserve the genetic diversity of the parasite’s
antigens and of the hosts’ antibodies. As rare alleles increase in frequency a stable polymorphism of
host and parasite genotypes is reached. Newly-arising rare alleles are favoured, but become
increasingly less fit as they become more common. This demonstrates the dynamic nature of
negative frequency-dependent selection.
In positive frequency-dependence, the fitness of a phenotype increases as it becomes more common
in a population. It favours common alleles, increases the rate at which rare alleles are lost, and
decreases genetic variation. The equilibrium point reached by positive frequency-dependent
selection is unstable. This means that any changes in the population, due to genetic drift, for
example, can lead to fixation/the convergence of alleles at loci, resulting in monomorphism.
Heliconius butterflies are poisonous, have distinctive wing colourings. They mimic other local,
unpalatable butterfly species – both within the same genus and in other groups. This reduces the
overall cost of educating predators, so is therefore beneficial. This type of mimicry is referred to as
Müllerian mimicry, and it leads to convergent evolution. Müllerian mimicry is a stable process since
selection is highest against less common genotypes. This selection against rare forms clearly
demonstrates positive frequency-dependent selection.
Frequency-dependent selection is integral to determining the roles and interactions of selection and
neutral processes, such as genetic drift, in maintaining the astonishingly large amounts of genetic
variation present in nature. Many models of natural selection assume that the selection coefficient
remains constant, for practical reasons. However, realistically, selection varies with many aspects of
its context. Frequency-dependent selection is a conceptually simple yet effective and more accurate
means of modelling genetic variability in natural populations. In combination with analysis of other
forms of selection, such as overdominance and environmental variation, we can start to understand
the relationships and principles underpinning evolution.