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Maintaining Polymorphisms
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Heterozygote Advantage
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When selection acts against both homozygotes, an equilibrium is achieved, dependent solely on the selection coef cients, that maintains both alleles (see chapter 20). The classic example of heterozygote advantage in human beings is sickle-cell anemia. Sickle-cell hemoglobin (HbS ) differs from normal hemoglobin (HbA) because it has a valine in place of a glutamic acid in position number 6 of the beta chain of the globin molecule. When the availability of oxygen is reduced, the erythrocytes containing sickle-cell hemoglobin change from round to sickleshaped cells (see g. 2.28). There are two unfortunate consequences: (1) sickle-shaped cells are rapidly broken down, which causes anemia as well as hypertrophy of the bone marrow, and (2) the sickle cells clump, which blocks capillaries and produces local losses of blood ow that result in tissue damage. This condition of reduced tness would lead one to predict that the sickle-cell allele would be selected against in all populations and, therefore, would be rare. But this is not the case. The sickle-cell allele is common in many parts of Africa, India, and southern Asia. What could possibly maintain this detrimental allele In the search for an answer to this question, biologists discovered that the distribution of the sickle-cell allele coincided well with the distribution of malaria. The following facts have now been uncovered. The sickle-cell homozygote (HbSHbS ) almost always dies of anemia. The sickle-cell heterozygote (HbAHbS ) is only slightly anemic and has resistance to malaria. The normal homozygote (HbAHbA) is not anemic and has no resistance to malaria. Thus, in areas where malaria is common, the most t genotype of the three appears to be the sickle-cell heterozygote, which has resistance to malaria and only a minor anemia. This conclusion is supported by the changes in allelic frequencies that occur when a population from a malarial area moves to a nonmalarial area. Since the normal homozygote is no longer at risk for malaria, selection acts mainly on the sickle-cell homozygote and, to a slight extent, on the heterozygote. Table 21.1 shows data for African blacks versus African Americans.The African population is, of course, under malarial risk, whereas the American population is not.The sickle-cell hemoglobin allele (HbS ) is reduced in frequency in African Americans. Heterozygote advantage is an expensive mechanism for maintaining a polymorphism. Losses must occur in both homozygous groups in order for the polymorphism to exist. Thus, part of the reproductive output of a population is lost each generation to maintain each polymorphism under heterozygote advantage. In the case of sickle-cell anemia, this means a tragic loss of human life due to either anemia or malaria. (The loss of individuals
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Tamarin: Principles of Genetics, Seventh Edition
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IV. Quantitative and Evolutionary Genetics
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21. Evolution and Speciation
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Genetic Variation
Table 21.1 Sickle-Cell Anemia Frequencies in African Blacks and African Americans
Percentage of Homozygotes (HbAHbA) African Blacks (Midcentral Africa) African Americans 82 92 Percentage of Heterozygotes (HbAHbS) 18 8 Frequency of HbS (q) 0.09 0.04
to maintain genetic variation at a particular locus is called genetic load. In the sickle-cell case, it is due to the segregation of individuals with lowered tness and is therefore called segregational load.) Very few other examples of heterozygote advantage have been documented.
Frequency-Dependent Selection
All the selection models discussed so far (chapter 20) have had selection coef cients that were constants. This is not always the case. For example, L. Ehrman has shown that when a female fruit y has a choice between mates with different genotypes, the female y chooses to mate with a male with a rare genotype. Frequency-dependent selection is selection in which the tnesses of genotypes change according to their frequencies in the population. The population geneticist Bruce Wallace has coined the terms hard selection and soft selection to deal with cases of frequency and density dependence. (Densitydependent selection exists when the tness of a genotype changes as population density changes. We will not deal with that here.) Wallace de ned soft selection as selection in which the selection coef cients depend on the frequency and density of genotypes. Hard selection is selection that is independent of both frequency and density. For example, the low tness of sickle-cell anemia homozygotes involves hard selection because of the objectively deleterious effects of the anemia. Soft selection
could be envisioned as selection that might act on aggressive behavioral genotypes in some lemming and eld mouse species. When population density and frequency of the genotypes are low, these animals survive and reproduce. As population density increases, there can be a selection for more aggressive genotypes because they may be more successful in obtaining resources. As density increases further and the frequencies of the aggressive genotypes increase, they may be selected against because of the preoccupation of these aggressive individuals with territory defense under crowded conditions. This has been suggested as a mechanism of wildlife s lemming cycle, rapid declines in the density of lemming and eld mouse populations every three to ve years. A model for frequency-dependent selection can be constructed by assigning tnesses that are not constants. One way to do this is to assign tnesses that are a function of allelic frequencies. Thus, the assigned tnesses for one locus with two alleles could be (1.5 p), 1, and (1.5 q) for the AA, Aa, and aa genotypes, respectively (table 21.2). An interesting outcome of this model is that at p q 0.5, the system is in equilibrium, and no selection takes place because all the tnesses are equal to 1. Another way of looking at frequency-dependent selection is to look at the situation in which each genotype exploits a slightly different resource. As a genotype becomes rare, competition for the resource that genotype uses will likely decrease, and the genotype will thus have an advantage over the common genotypes, which are competing for resources.This type of selection is probably very common.
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