Natural selection

In the previous sections we have been looking at some of the ways in which non-adaptive processes can affect the genetic variation of populations. Genetic diversity is affected also by natural selection, a process that leads to 'the differential reproduction of genetically distinct individuals or genotypes within a population' (Li, 1997). Natural selection can alter allele frequencies in a number of different ways that can either increase or decrease overall genetic variation. Both stabilizing selection and directional selection will generally decrease genetic diversity. The former reduces diversity by favouring the average phenotype (and associated genotype) over either extreme. Directional selection generally reduces diversity either through negative or purifying selection, which is selection against any mutation that reduces the fitness of their carriers, or through positive selection, which selects for a particular mutation that increases the fitness of carriers. During the time in which positive selection is increasing the frequency of a particular allele, diversity will increase temporarily; this is known as transient polymorphism. Once the selected allele reaches fixation, that locus becomes monomorphic and genetic variation is lost. The type of selection most likely to increase variation is disruptive (or diversifying) selection, which occurs when two or more pheno-types are fitter than the intermediate phenotypes that lie between them. Figure 3.11 summarizes the three main modes of selection.

Directional selection

Stabilizing selection

Disruptive selection

Parental population

Directional selection

Stabilizing selection

Disruptive selection

Figure 3.11 Three common modes of selection that can influence the level of phenotypic variation -and the variability of underlying genotypes - within a population. In each diagram, phenotype frequency is illustrated by the distribution curve, the dashed line represents the mean value of the phenotypic character in the parental generation and the cross identifies the portion of individuals that have a relatively low fitness in the parental generation. Directional selection favours an extreme phenotype, stabilizing selection favours an intermediate phenotype, and disruptive selection favours two or more phenotypes at the expense of any intermediate phenotypes. Redrawn from Endler (1986)

Offspring

Figure 3.11 Three common modes of selection that can influence the level of phenotypic variation -and the variability of underlying genotypes - within a population. In each diagram, phenotype frequency is illustrated by the distribution curve, the dashed line represents the mean value of the phenotypic character in the parental generation and the cross identifies the portion of individuals that have a relatively low fitness in the parental generation. Directional selection favours an extreme phenotype, stabilizing selection favours an intermediate phenotype, and disruptive selection favours two or more phenotypes at the expense of any intermediate phenotypes. Redrawn from Endler (1986)

Balancing selection is a general term given to selective processes that maintain genetic diversity because particular alleles are selected for in some situations and selected against in others. In large populations, where drift plays a relatively small role in shaping allele frequencies, balancing selection can influence the genetic diversity of some non-neutral loci. One form of balancing selection is heterozygote advantage, also known as overdominance. This occurs when heterozygotes have a higher fitness than homozygotes, and ensures that both alleles will be maintained within a population. One of the best-known examples of single-locus haeterozygote advantage occurs at the ^-haemoglobin locus in some African and Mediterranean human populations. These populations have two alleles at this locus: the normal haemoglobin allele (A) and the sickle-cell haemoglobin allele (S). SS individuals suffer from sickle-cell anaemia and usually die young. SA individuals suffer from slight anaemia but they also have a much higher resistance to the type of malaria caused by the parasite Plasmodium falciparum than AA individuals, who do not suffer from anaemia. Therefore, in regions where malaria is widespread, SA individuals have an advantage over both AA and SS individuals, and for that reason both alleles are maintained within populations.

The second common form of balancing selection that can influence genetic diversity is frequency-dependent selection. This occurs when the fitness of a genotype depends on the frequency of that genotype within a population. Populations of the tropical butterfly Heliconius erato have different-coloured forms that are subject to frequency-dependent selection. The main line of defence that this butterfly has against bird predation is its colouration, which serves as a warning of its unpalatability. If a butterfly from one population is introduced into a new population in which an alternative phenotype predominates, it will appear conspicuous and probably will be eaten despite its unpalatability. This is positive frequency-dependent selection because the fittest genotype is the one that is most common within a given population.

The converse of this is negative frequency-dependent selection, which occurs if a trait is more advantageous when it is relatively rare. An example of this is the maintenance of both yellow and purple flowers in populations of the Elderflower orchid Dactylorhiza sambucina. This species relies on insect pollinators but does not reward them with any nectar. If yellow is the most common colour within a population, insects are likely to visit yellow flowers first. However, when their first visit is not rewarded, their next visit will generally be to a purple flower. As a result, the relatively rare purple flowers will receive a large proportion of insect visits and therefore should have higher fitness values than the yellow flowers. This relatively high fitness means that purple flowers eventually will become more common than yellow flowers, at which point negative frequency-dependent selection will favour yellow flowers until such time as the situation is reversed once more (Gigord, Macnair and Smithson, 2001).

The major histocompatibility complex

The potential effects of balancing selection on genetic polymorphism can be illustrated further using the example of the major histocompatibility complex (MHC). This is a large multigene family in vertebrates that is involved in the immune response and is therefore extremely important in fighting disease.

Molecules encoded by the MHC will bind to antigens (peptides) and transport them to the cell surface membrane. Here, T-cell receptors will initiate an immune response if the antigens are recognized as foreign but will fail to initiate an immune reaction if the antigens are recognized as 'self'. The human MHC, which is usually referred to as the human leucocyte antigen (HLA), is found on chromosome 6 and spans a region more than 4 million bp long that contains over 100 genes. The MHC genes and their products are grouped into three classes on the basis of their chemical structure and biological properties. In general, MHC genes are extremely polymorphic, for example in humans the class I HLA-B locus has 149 alleles and the class II DRB locus has 179 alleles (Hedrick and Kim, 2000).

There is little doubt that MHC polymorphism is a result of selection. For one thing, particular MHC alleles can persist within populations for tens of millions of years, whereas neutral alleles are not expect to persist within a population for more than 2Ne generations (Takahata and Mei, 1990). One outcome of this long-term persistence is that some alleles are maintained in multiple species. This was found in a comparison between Arabian oryx (Oryx leucoryx) MHC sequences and homologous class II DRB sequences from bison, cattle, sheep and goats. Three of the oryx MHC sequences were more closely related to MHC sequences from other species than they were to each other, a finding that is difficult to explain unless selection is maintaining the same alleles for a period of time that transcends the evolution of multiple species (Hedrick et al., 2000).

Evidence that selection maintains a diversity of MHC genes also comes from the pattern of nucleotide substitutions, since non-synonymous substitutions often outnumber synonymous substitutions. In one study a 254 bp region of an MHC class II B gene was sequenced from 666 wild Atlantic salmon (Salmo salar), and 40 nucleotide polymorphisms within 18 different alleles showed that non-synonymous substitutions were nearly three times more common that synonymous substitutions (Landry and Bernatchez, 2001); recall from Chapter 1 that under the neutral theory of evolution, synonymous substitutions are expected to predominate.

Although there is general agreement that balancing selection maintains MHC polymorphism, there is some debate about whether the precise mechanism is frequency-dependent selection or overdominance. An individual that has two alleles at one locus should be able to bind twice the number of foreign peptides as an individual with only one allele, in which case heterozygote individuals will be at an advantage and overdominance will occur. On the other hand, negative frequency-dependent selection would mean that individuals would be at an advantage if they were carrying rare alleles to which pathogens are not adapted.

Support for both mechanisms has come from studies of laboratory and wild populations. One recent study on great reed warblers (Acrocephalus arundinaceus) compared the temporal variance in frequencies of 23 MHC class I alleles and 23 putatively neutral microsatellite loci over nine successive years (Westerdahl et al.,

2004). The variation in allele frequencies was significantly higher for the MHC alleles than for the microsatellite alleles, which provided evidence for selection because MHC and microsatellite allele frequency variations should be comparable if they were both caused by drift. Frequency-dependent selection could be maintaining MHC polymorphism in this species, because the frequencies of different blood parasite strains are known to vary between years.

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