Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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.