The general selection model was developed for diploid organisms. We will consider the simplest case where only two alleles are present at a single, selected locus. Assume also that the Hardy-Weinberg equilibrium exists before selection occurs. If w 11 , w 12 , and w 22 are the fitnesses associated with genotypes A 1 A 1 , A 1 A 2 , and A 2 A 2 respectively, and their respective selection coefficients are s, h, and t, then we can predict the change in frequency of the A 2 allele after one generation of selection as follows:.
And the net change of frequency in allele A 2 after one generation of selection is:. This final formula represents the "general selection formula" that can be applied to any case of selection involving diploid organisms.
Let's apply the general selection formula to the case of selection against a recessive allele. This is the form of selection that occurs against the susceptible allele at a resistance locus when resistance is dominant in the host when disease is present; or against a virulence allele when avirulence is dominant if the pathogen encounters a susceptible host and the virulence allele has a fitness cost.
Plugging these fitness values into the general selection model and solving will yield the following solution:. Selection against recessive alleles is very efficient at first, but becomes progressively slower because a larger proportion of the recessive allele is protected in heterozygotes as the allele frequency decreases Figure Therefore, natural selection alone cannot entirely eliminate the recessive allele, even if it is lethal.
This is an example of directional selection. The following example illustrates selection against a dominant allele. This form of selection operates against an avirulence allele when the pathogen encounters a resistance gene that recognizes the elicitor encoded by the avirulence allele if avirulence is dominant. Selection against dominant alleles is more efficient than selection against recessive alleles. It takes fewer than generations to eliminate a dominant deleterious allele with an initial frequency of 0.
Compare this to how long it took to remove recessive deleterious alleles. Overdominance is known to occur in plants and animals. One famous example is the sickle-cell anemia allele for hemoglobin, which offers a survival advantage to heterozygous humans who live in malaria-infested areas of the world. Overdominance has not yet been demonstrated in plant pathogens, but it is not difficult to imagine how it could occur. For example, consider a diploid oomycete pathogen with two alleles at a pectinase locus, where one allele works well at high temperature, and the other allele works well at low temperature.
The heterozygotes can colonize host tissue over a wider temperature range than either homozygote. This could lead to overdominance in a population exposed to wide temperature fluctuations. While overdominance cannot occur for haploid pathogens such as bacteria or ascomycetous fungi, it is very important for the plant hosts. Overdominance is the basis of F1 hybrid crops such as maize and sorghum.
After many generations of selection, an equilibrium will be achieved. The equilibrium allele frequency will be:. Overdominance maintains both alleles in the population to achieve the maximum overall fitness for a population Figure The equilibrium frequencies depend on the values of the selection coefficients, s and t, regardless of the initial allele frequencies. The equilibrium at this point is stable. These equilibria are unstable and the equilibrium will be broken when allele frequencies deviate slightly from the equilibrium points 0 or 1 , whereupon the population moves toward the stable equilibrium point.
This is an example of stabilizing selection. Fitness is a complicated concept. It is a measurement of the total output of viable progeny by an individual in its lifetime. If no offspring are produced, the fitness of an individual is zero. Fitness increases with increasing life span also called viability and the number of progeny produced also called fecundity. As an example in plant pathology, the most fit pathogen genotype is the one that infects the most host plants in the shortest period of time and produces the most spores for pathogens that utilize asexual reproduction.
Populations of pathogens undergoing selection will constantly evolve to increase their overall level of fitness. The type of selection that operates will determine the direction of change in allele frequency to achieve the optimum overall fitness for the population Figure Figure The effects of selection on allele frequency in a one-locus, two allele fitness model.
In case 1, the A allele has a fitness advantage over the a allele. In case 2, the a allele has a fitness advantage over the A allele. According to Fisher's Fundamental Theorem of Natural Selection the mean fitness of a population always increases in a fluctuating environment.
The change in fitness of a selected population will be proportional to the additive genetic variation for genes affecting fitness in the population. Therefore, populations will move to the nearest local optimum of allele frequencies that maximize fitness, which is not necessarily the global optimum.
As the amount of genetic variation in populations increases, the rate of change in fitness of the population increases proportionally. As a result, populations with the greatest genetic diversity have the greatest potential for evolution. Natural selection is the driving force for boom and bust cycles and for the evolution of fungicide resistance in plant pathogens.
Selection can occur on genes increasing and decreasing the frequencies of specific alleles or on genotypes increasing and decreasing the frequencies of specific clones. Both types of selection are well documented in pathogen populations. As an example, selection on genes alleles occurs when mutants that lost an avirulence allele encounter a plant with a resistance gene.
This process has been documented dozens of times for cereal rusts and mildews. Selection on genotypes occurs for Fusarium oxysporum formae speciales and probably explains the displacement of the "old" clone of Phytophthora infestans by the "new" clones of P.
Lompat ke Halaman. Cari di dalam dokumen. Fluck 1 Alysa Fluck Mrs. What is the percentage of heterozygous tasters 2pq in your class? The percentage of heterozygous tasters in our class is approximately 0. What percentage of the North American population is heterozygous for the taster trait? The percentage of heterozygous tasters in North America is approximately 0. Exercise B Allele Frequency: The allele frequencies, p and q, should be calculated for the population after five generations of random mating.
What does the Hardy-Weinberg equation predict for the new p and q? According to the Hardy-Weinberg equation, our p and q values should equal 0. Do the results you obtained in this simulation agree? If not, why? Our results vary from the expected results, mostly because we did not fulfill all of the assumptions for the Hardy-Weinberg equation.
What major assumptions were not strictly followed in this simulation? It was assumed that the population size would be much larger than the one that we sampled. Therefore, the percentages do not completely come into play.
Case II 1. How do the new frequencies of p and q compare to the initial frequencies in Case I? These numbers are very different than the initial frequencies. The assumption that there is no natural selection was violated because when the aa genotypes were received they died. This selection greatly changed the data and that is why there is not a genetic equilibrium. Predict what would happen to the frequencies of p and q if you simulated another five generations.
I think that the data would become heavier in the AA genotypes because a homozygous recessive genotype is much harder to continue through generations. The p value would increase and the q value would decrease directly. In a large population would it be possible to completely eliminate a deleterious recessive allele? The natural selection can not destroy that trait entirely because the carrier does not express it and is not weekend by the trait.
Case III 1. Dominant traits are not always the most common. Some people may think that dominant trait is the most likely to be found in the population, but the term "dominant" only refers to the fact that the allele is expressed over another allele. An example of this is Huntington's disease.
Are all alleles dominant or recessive? In the real world, genes often come in many versions alleles. Alleles aren't always fully dominant or recessive to one another, but may instead display codominance or incomplete dominance.
Does disruptive selection increase genetic variation? Disruptive Selection Increases Variation Because the values for the trait in the resulting population are further from the population mean, the variation of the trait is increased. How is genetic drift different from natural selection quizlet? Natural selection and genetic drift both result in a change in the frequency of alleles in a population, so both are mechanisms of evolution.
Genetic drift causes evolution by random chance due to sampling error, whereas natural selection causes evolution on the basis of fitness. What spray kills codling moth? What are the names of Santa's 12 reindeers?
Co-authors It is almost impossible to totally eliminate recessive alleles from a population, because if the dominant phenotype is what is selected for, both AA and Aa individuals have that phenotype. Individuals with normal phenotypes but disease-causing recessive alleles are called carriers. The dominant phenotype is selected against. In this model, the recessive phenotype is selected for, and the dominant phenotype is selected against. Perhaps the dominant allele causes a rare and lethal disease.
The result : each of the five times that the model was run, the frequency of the dominant allele dropped to zero. The dominant allele was eliminated from the population even when its frequency was very high to begin with. When the dominant phenotype is selected against, any individual with even one dominant allele will have the undesirable trait, and so will have few or no offspring.
In a relatively short time, only the aa individuals with the selected for recessive trait will be left. This is called purifying selection.
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