Sickle Cell Anemia As Example Of Natural Selection

One of the textbook classics of natural selection, selection at the human j -Hb locus that codes for the j chain of adult hemoglobin, illustrates the fact that selection is not survival of the fittest and the importance of understanding natural selection in terms of average excesses. Although this story is usually told starting in Africa, we will start the story thousands of kilometers away, in Southeast Asia.

The people living in the coastal areas of Southeast Asia and the islands that now make up Indonesia were perhaps the best sailors of ancient times (Wiesenfeld 1967). About 2000 years ago, these people established a colony on the island of Madagascar, which lies just off the eastern coast of Africa and is many thousands of kilometers away from modern-day Indonesia. The colonists brought not only their language (the language of the present-day people of Madagascar has its closest affinities to the language of the Ma'anyan of Southeast Borneo) but also their agricultural system. Agriculture originated at several locations and times during human history, and the system brought by these ancient colonists is called the Malaysian agricultural complex. The Malaysian agricultural complex includes many root and tree crops such as yams and bananas, plants that are adapted to wet, tropical conditions. In contrast, the cereal-based agricultural system that arose in the Middle East is centered on crops such as wheat and barley that do not do well under wet, tropical conditions. Consequently, the cereal-based agricultural system penetrated in only a limited way into the wet, tropical portions of Africa. However, shortly after the Malaysian agricultural complex was introduced to Madagascar, Bantu-speaking peoples on mainland Africa also took it up. The Bantus rapidly expanded into much of wet, tropical Africa with this new agricultural complex and in so doing greatly changed the environment in which they lived. In particular, this new slash-and-burn agricultural system created breeding sites and optimal habitat for the mosquito Anopheles gambia, the primary vector for transmitting the deadly malaria parasite Plasmodium falciparum. Moreover, the agricultural system allowed human populations to become much more dense in this area. These two factors combined to allow falciparum malaria to become a sustained epidemic disease in this part of Africa and a major source of mortality, particularly in children. We now examine how these Bantu populations adapted to their new environment, and in particular to malaria, through natural selection operating upon genetic variation at the human j-chain hemoglobin autosomal locus (3 - Hb).

In Chapter 8, we noted that the sickle cell allele (S) at this locus is a dominant allele for the phenotype of malarial resistance. In contrast, another allele at this locus, C (a mutation in the same codon as S), is a recessive allele for malarial resistance (Cavalli-Sforza and Bodmer 1971). [Modiano et al. (2001) suggest that C is not completely recessive, but for now, we will treat it as a recessive allele before turning our attention to the possibility that it is not.] Moreover, both the S and Calleles are associated with hemolytic anemia, another source of mortality. Hence, these alleles are expected to influence the phenotype of viability in both the premalarial and the malarial environment of wet, tropical Africa. Cavalli-Sforza and Bodmer (1971) estimated these viabilities for a West African population under malarial conditions, as shown in Table 11.1. Table 11.1 also shows the estimated viabilities under nonmalarial conditions by removing the beneficial effects of the malarial-resistant phe-notypes. Table 11.1 gives what is known as relative fitness, in which the fitness of one genotype is set to 1 and all other fitnesses are measured relative to this standard. Such a rescaling of fitness values has no effect on equation 11.5 because the impact of natural selection depends only upon (wy - w)/w, which is invariant to scale transformations. Hence, we can scale the fitnesses for mathematical or biological convenience without altering the evolutionary impact of natural selection through equation 11.5.

Table 11.1 makes explicit that fitness phenotypes emerge from interactions between genotypes and environments. The relative fitnesses of the various genotypic classes are altered in going from a nonmalarial to a malarial environment. The AS and CC genotypes will be our focus with regard to the human adaptation to the malarial environment because both have malarial resistance without serious hemolytic anemia. Interestingly, Table 11.1

Table 11.1. Phenotypic Attributes and Relative Fitnesses (Viabilities) of Six Genotypes Formed by A, S, and C Alleles at ß-Hb Locus in Humans in Wet, Tropical Africa

Fitness in Nonmalarial Fitness in Malarial

Genotype Phenotypic Attributes Environment Environment

Table 11.1. Phenotypic Attributes and Relative Fitnesses (Viabilities) of Six Genotypes Formed by A, S, and C Alleles at ß-Hb Locus in Humans in Wet, Tropical Africa

Fitness in Nonmalarial Fitness in Malarial

Genotype Phenotypic Attributes Environment Environment

AA Malarial susceptibility

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Responses

  • Gavin
    How is sickle cell anemia an example of natural selection?
    2 years ago
  • gemma
    Is sickle cell anemia a example of microevolution?
    6 months ago

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