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| Biological anthropologists use information on biological variation to make inferences concerning the evolutionary history of populations and of specific traits. Many studies have used biological data to examine questions about a population's history, such as where they came from and how they interacted with other populations. Case studies in this Chapter focus on the Asian origin of Native Americans, the genetic impact of population settlement and invasion in Ireland, and the biological history of African Americans. Data on biological variation can also inform us about natural selection in recent human history. Perhaps the best-documented example of natural selection in human populations is the relationship between hemoglobin alleles and two selective forces: sickle cell anemia and malaria. In environments where malaria is common, selection has led to an increase in the sickle cell allele because the heterozygotes are the most fit—they show greater resistance to malaria but do not suffer from the adverse effects of sickle cell anemia. Studies of blood groups and other genetic markers also suggest a role for natural selection in human variation. Skin color is another example of a trait that shows a strong environmental correlation, in this case with latitude. This distribution, combined with other evidence, suggests that dark skin is selected for near the equator primarily to protect against the harmful effects of excess ultraviolet radiation (photodestruction of folate). Light skin may have evolved farther from the equator to facilitate sufficient vitamin D synthesis. Natural selection in human populations is often affected by culture change. In the case of the evolution of the sickle cell allele in Africa, the introduction of horticulture by humans altered the physical environment, making it a more hospitable environment for the reproduction of mosquitoes that transmit malaria. As a consequence, higher frequencies of the sickle cell allele were selected for. The distribution of lactase persistence in human populations also shows the impact of culture change; higher frequencies are found in populations that have a long history of dairy farming. |