1 bi3010h08 chapter 12 natural selection ii balanced selection can occur in many different forms,...

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BI3010H08 Chapter 12 Natural selection II

Balanced selection can occur in many different forms, which common trait is that they maintain genetic variability in the populations. Single locus overdominance is one of theseforms. When this form was discussed in chapter 5, the assumption was two alleles, threegenotypes, and that the fitness of the genotypes was constant. While that model was veryuseful for studying basic phenomena, it is probably quite unrealistic because;

- many loci have more than two alleles- fitness of a genotype may vary geographically- fitness of a genotype may vary with time- fitness of a genotype may vary between sexes- fitness of a genotype may be frequency dependent- fitness of a genotype may vary with population size- fitness of a genotype may depend on genotype at other loci (pleiotrophy)

This chapter treats this and other types of natural selection. One of the goals is to get an overview of the diverse ways in which of natural selection may act in natural populations. Another goal is to evaluate the importance of the various types in the real world, and theirpotential in maintaining genetic variability.

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12.1 Natural selection with multiple alleles at a locus

In chapter 5 it was concluded that genetic variability would be maintained only if the heterozygote had higher fitness than both of the homozygotes. What we observe in natural populations is that many allozyme, microsatellite, and other loci have much more than two alleles. (MHC, the major histocompatibility complex in humans have more than 100).

Can natural selection explain these multi-allele polymorphisms?For example, what would the assumptions be for a stable 3-allele poly-morphism? Or in general; a number of k alleles? Studies have shown that the assumption of heterozygote superiority which is valid for a 2-allele polymorphism is not necessary, nor suffient for k-allele plymorphism. Neither is it necessary or sufficient that all heterozygotes have higher fitness than the homozygotes.

The insight generated by many model simulations of quite high complexity is that natural selection probably is NOT the cause for most multi-allele polymorphisms (though it might be in some cases; cf the MHC).

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12.2 Two-locus models of selectionGenes interact with other genes, both linked and unlinked. For example, the genes that interact in the glucolysis co-operate in order to coordinate the end product. Gene interaction must therefore be widespread in nature, and therefore be affected by natural selection. In the following, we treat the interaction of genes at two polymorphic loci with alleles A, a, B, and b, with frequencies pA, pa, pB, and pb. The recombination frequency between the two loci is r. There are 4 combined genotypes: AB, Ab, aB, og ab, with frequencies g1, g2, g3, and g4.

Without selection, the disequilibrium coefficient (D) (Chapter 4.2) is: D = g1g4 - g2g3 , and the recursion equiation for D is Dt+1 = (1-r)D.

Thus, in the absence of selection, D will approach zero at a rate which depends on the recombination frequency r, and each of the two loci will approach Hardy-Weinberg equilibrium.

How would the presence of selection change these conclusions?

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A general two-locus model with selection for survival

No general solution is known for this model, but some special cases may give some insight:Generally: If the recombination frequency r = 0, the four gamete-types will act as single alleles at one locus (cf. kap. 12.1). Additive model (the mathematically simplest): If survival is additive over loci, i.e. no interaction (epistasis), the following is valid:

Conclusions in Chapter 12.2 (2-locus model with selection):1. The prerequisite for a stable two-locus polymorphic equilibrium is that there is overdominance at both loci.2. If such an equilibrium exists, it is the only one, and globally stable. 3. At such an equilibrium D=0, and the gamete frequencies are equal to the products of the involved allele frequencies.4. Mean fitness of the populations will then be maximized.5. These results are independent of r.

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How common is gene interaction in nature?

As we have seen above the gametic disequilibrium approaches zero unless there is interaction between loci. Therefore, the demonstration of stable and repeatable non-zero values of D will be signs of gene interaction (epistasis) if the other evolutionary forces can be excluded.

However, non-zero values of D are not necessarily indications of epistasis. Other evolutionary forces; genetic drift, mutations, population substructure, bottlenecks, or directional selection due to ”hitch-hiking” may generate gametic disequilibrium. Tight linkage with reduced recombination delays the breakdown of gametic disequilibria.

Nonetheless there is solid evidence that epistasis is common in natural populations. Fisher (1930) launched the ”reduction principle”; namely that evolution would favour chromosome-arrangements where such genes are in close proximity of each other (reduced re-combination). It is assumed that this is the way ”supergenes” have evolved.

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BI3010H08

Chapter 12 Natural selection II

Gene interactions beyond two loci:

Just going from one to two loci subject to selection resulted in enormously increased complexity, because the unit of study is no longer allele frequencies, but gamete frequencies. Most organisms have hundreds and thousands of polymorphic loci that may interact. The consequences of potential interactions of higher order is not known.This is an important recognition. If multi-locus interactions is a limited selection factor in nature, evolution can be ascribed to changes in allele frequences. However, if they are frequent, allele frequencies are unsufficient for understanding and describing evolution, and one has to look at more complex units of selection (maybe gametes, chromosomes or even whole genomes). It is therefore important to know something about the existence and strength of multilocus interactions in natural selection. One possible approach is to study phenotypic variation, which forms the basis for quantitative genetics (Chapter 13).

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BI3010H08Chapter 12 Natural selection II

When environment varies in time and spaceWhen environment varies in time and space

Selection regimes which varies geographically (12.3):

It is intuitive that the fitness of a genotype can vary with its actual environment. In chapter 9 it was shown that when selection is strong compared to gene flow, the equilibrium allele frequencies in different sub-populations mainly depend on the local selection forces. Increasing gene flow lowers the efficiency of local selection.What happens with the polymorphisms when subgroups in a panmictic population lives in niches with different selection regimes?

Levene model:(one locus, two alleles A1 and A2, n niches)Levene (1953) showed that in each niche, both alleles will be preserved in each separate niche if the allele-frequency change for A1 due to selection obeys the following:

∆p is positive for p near 0 ∆p is negative for p near 1

Prout (1968) called this ”protected polymorphisms”. Generally such a polymorphism will survive if the arithmetic mean value for heterozygote fitness is higher than for the homozygotes ("marginal overdominans").

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12.4 Selection regimes which vary over time :

Time variation can be either seasonal or over longer periods, and results in changes in selection regimes. Intuitively, one would expect that such variations favour heterozygotes. Two scenarios can be imagined: between generation or within generations.

Between generations:A sufficient prerequisite for maintaining a plymorphism is that the geometric mean for fitness for the homozygotes is lower than for the heterozygote.Generally, however, it can be concluded that the maintenance of a polymorphism is only marginally more probable in a milieu that varies between generations than in one that is constant.

Within generation:Borash et al. (1998) showed that, like for the between-generation variation, polymorphisms will be favoured in within-generation milieu changes if the weighted mean fitness for heterozygotes is larger than for homozygotes, e.g. a kind of "mean overdominance".

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12.5 Selection in randomly varying environments

These models are very complex, and the textbook only summarizes some of the main conclusions:

1. These models predict maintenance of a large amount of genetic variation

2. Spatial subdivision does mot necessarily lead to increased poly-morphism, but does so under some selection regimes

3. Temporal milieu variation increases the amount of polymorphism, but redunding milieu conditions decreases it.

4. Under some circumstances, the predicted distribution of allele frequencies is identical to, or very similar to, that under the neutral theory (cf. Section 10.4).

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12.6 Frequency dependent selection

Occurs when a genotype's fitness depends on its frequency and the frequencies of other genotypes in the population. Usually in form of an increase in fitness when the genotype becomes rare. Levin (1988) called this stabilising frequency dependent selection. This type can lead to a "protected polymorphism" (cf light/dark moths Fig. 5.7 page 147).

If fitness is reduced when the genotype becomes rare, we talk about disruptive frequency dependent selection, which can lead to loss of rare genotypes.

Frequency dependent selection can lead to surprising results, e.g. stable equilibria where the heterozygote has the lowest fitness! Generally the dynamics of this type of selection is very complex in form of cycluses, chaos and periodicity, but it is meant to be very common because it is evolutionary favourable with respect to preserving genetic variation (through fixation and hence conservation of different alleles). In addition to the "industrial melanism" in moths, it is meant demonstrated in plants, Drosophila, fish (Atlantic silverside), host-parasite co-evolution, MHC in vertebrates, and in the 1:1 sex proportion.

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12.7 Antagonistic pleiotrophy

Selection can work at any ontogenetic (developmental) stage. If genes have pleiotrophic effects which are favourable at one stage but not at another, we talk about antagonistic pleiotrophy. The principle is meant to be very common in nature. A classical example is larval development speed in Drosophila (Halliburton side 507 ff). [also, in salmon parr; rapid initial growth gives precoccious mails, which then grow more slowly after maturation].

For a simple model with one locus, two alleles, it is indicated that if a selection regime with antagonistic pleiotrophy shall preserve a polymorphism, the relative fitness of the heterozygote must be higher than for both of the homozygotes.

12. 8 Opposite selection in males and femalesGenes with opposite effects in sexes are called sexually antagonistic genes (e.g. plumage in birds). The prerequisite for a protected polymorphism here is similar to that of Levene's model of a two-niche "spatial heterogeneity" (slide 7), namely that the geometric mean of homozygote fitness is lower than that of the heterozygote. [also; PanI sexual difference in allele frequencies in cod of the Trondheimsfjord; Karlsson & Mork 2005]. Such a scheme produces an excess of heterozygotes each generation].

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12. 9 Sexual selection

Darwin (1871) wondered why the males in so many species possess morphological and behavioural traits which probably reduce their fitness. He meant the answer is that these traits mean an advantage for the possesor in the competition for females. He defined sexual selection as an individual competition for mating partners. The investment in offspring differs widely in type and strength.

Males increase their fitness by mating with many females. Batesman (1948) tested and confirmed this hypothesis. The relative success of the males is often based on pronounced morphological traits (plumage in birds, body size in elephant seals, adipose fin in salmon), but also on "sperm competition" in insects. In vertebrates, behaviour is often involved (dance and posing in birds, killing other males' offspring in lions).Many types of sexual selection can be dangerous, and there must be a trade-off between reproductive success and e.g. survival. Endler (1978) confirmed such a trade-off in guppies (small fishes).

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Females increase their fitness by making the right choise of males for mating. There are three hypotheses for how the evolution of the females' ability to choose has evolved:1. Females choose males which offer the best resources (large reviers)2. Females prefer males that appears healthy and strong (favourable genes)3. Females are predisposed by preferring specific traits in males (sounds etc)

These hypotheses are not mutually exclusive.

Fisher (1915) launched the term "runaway selection", where e.g. in birds the males' even more impressing plumage and the females' preferences could lead to even more exaggerated sexual characters. The now extinct Irish moose, with enormous antlers, could be an example of this.

Reversed sexual rolesIn some species the sexual roles are reversed (e.g. in sandpipers), so that the males nurse the offspring and invest most in them. In such cases, it is observed that the females compete for the favour of the males.

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The message in Chapter 12

is above all that natural selection is not a constant force. It can vary in time, space and between sexes. It may act on many traits simultaneously, and can act in different directions at different life stages. To understand how selection works in natural populations we must study them in their natural habitats, and also observe the complex interactions between different genes and genotypes.

A long-term study have been performed on sheep (Ovis varies), where a local, wild population on a Scottish island (St. Kilda) was intensively followed since 1960. The results have demonstrated how complex natural selection actually can be. (Halliburton page 519).

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A few memos (extracted from Chapter 12 Summary)A few memos (extracted from Chapter 12 Summary)

1. Individual fitness is not constant, it may vary in time and space, genotype frequencies, and population size.

3. Interaction between loci (epistasis) is common. Permanent genetic (gametic) disequilibrium indicates strong interaction or close linkage between loci.

6. The probability for preservation of a polymorphism in a spatially variable milieu (common in nature) increases under habitat-preferences and non-random mating.

9. Frequency-dependent selection is common in nature (e.g. in maintaning 1:1sex proportion).

11. Antagonistic pleiotrophy is very common in nature, but is maybe not a very important factor for preservation of polymorphisms.

12. Natural selection can work oppositedly in males and females, and can be a very common type of selection.

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1 bi3010h08 chapter 12 natural selection ii balanced selection can occur in many different forms,...

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