Human Evolution, Dr. Chiu Lecture Feb. 15, 2012 Human Adaptations 1. As we've discussed already, Homo sapiens is a large population with a great deal of genetic and phenotypic diversity. Ethnic groups exist. But in general the amount of genetic variation within any ethnic group is higher than between two different ethnic groups; hence the concept of ‘race’ is not valid. 2. In this lecture, we focus on human adaptations to different global environments. A. Example 1: Sickle cell anemia, the heterozygote advantage, and resistance to malaria. 1. Sickle cell anemia is a single gene trait that is inherited in an autosomal recessive manner. The S allele (normal) is dominant to the s allele (sickling, disease causing). There are three genotypes: SS, Ss, ss and two phenotypes: normal vs. disease. Individuals that are heterozygous, Ss, are phenotypically normal but are carriers of the disease causing allele, s. Use a punnet square to figure out the % chance that two Ss parents will produce an ss child. 2. If two copies of the s allele cause this horrible disease, why is the s allele maintained at fairly high frequencies? This is called Heterozygote Advantage. Individuals who are Ss are also resistant to malaria. Natural selection for resistance to malaria outweighs the ‘costs’ of having children with sickle cell anemia. 3. From this, one would predict that areas of the world that have a lot of malaria also will have a lot of carriers and individuals who have sickle cell anemia. This is the case. This type of relationship between geography and either a genotype or phenotype is called a cline. 3. In the sickle cell anemia example, the mutation is in the protein coding sequence. But variation (mutation) can arise and be fixed in regulatory noncoding DNA sequences as well (we discussed this in the previous lecture). 4. Humans (and other animals) also can adapt to temperature. a. Allen’s Rule and Bergmann’s rules have determined body size and shape in human evolution over the past 1.5 million years. 5. Human skin color is another adaptation that corresponds to a cline. The regions of the world that are closest to the equator have people with the darkest skin; the regions furthest from the equator have people with the lightest skin. a. Skin color is influenced by two competing forces: UV penetration and Vitamin D synthesis.
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