Bio Anth 1001 Week 6 of Notes
Bio Anth 1001 Week 6 of Notes Anth 1001
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This 4 page Class Notes was uploaded by Jaimee Kidd on Monday February 22, 2016. The Class Notes belongs to Anth 1001 at George Washington University taught by Shannon C. McFarlin in Spring 2016. Since its upload, it has received 29 views. For similar materials see Biological Anthropology in anthropology, evolution, sphr at George Washington University.
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Date Created: 02/22/16
Human Variation Subspecies Group of local populations sharing part of the geographic range of a species, differentiated from other subspecies based on one or more phenotypic trait. Populations Smaller groups within species, temporally and geographically restricted, within which the majority of mating occurs. • Humans are phenotypically diverse, although our perception of phenotypic diversity can have a subjective bias • Human genetic diversity is surprisingly low ◦ Depending on the locus examined, chimpanzees show 24X more genetic diversity than humans ◦ Possible Reasons: 1 (1) A relatively recent bottleneck event in human evolutionary history 2 (2) Gene flow between different human populations due to migration • "Race" ◦ A group of populations sharing certain characteristics that make them distinct from other groups of populations; often incorporates biological and cultural criteria ◦ A typological and arbitrary concept which assumes that human variation can be classified into a number of distinct groups 1 Skin Color 2 Clinal Distribution 1 patterned distribution across a geographic gradient 1 Skin Color, Sickle Cell Anemia, ABO Blood Types • Some variation is geographically structured ◦ Because people tend to choose their mates among those they live close to ◦ The relationship between geographic and genetic proximity reflects gene flow ◦ Apportionment of Human Diversity 1 10% of human diversity is accounted for by different geographic regions 2 85% of the variation is accounted for by individual differences WITHIN populations 3 5% among local populations within geographic regions • To Summarize.. ◦ Human variation is poorly described by racial classifications ◦ More genetic variation is attributed to differences among populations ◦ Distribution of many traits historically used to define "racial types" is continuous ◦ Poor concordance among traits Biology of Skin Color Dermis • Protective and nonprotective functions ◦ Thermorregulation ◦ Metabolic excretion ◦ Sensation • Structure ◦ Blood Vessels ◦ Glands ◦ Nerve Endings ◦ Hair Follicles • Epidermis ◦ Function ▪ Protection of underlying tissue from abrasion and drying ▪ Protection against UV Radiation damage ▪ Protection against pathogens ▪ Synthesis of Vitamin D ◦ Keratinocytes ▪ Produce keratin which affords protection against water and abrasion ◦ Melanocytes ▪ Produce the pigment melanin ▪ Melanin ▪ Number of melanocytes is approximately the same in all individuals, but the amount of melanin deposited in keratinocytes varies 1 Dark skin = synthesis of large amounts of melanin 2 Light skin = synthesis of less melanin ▪ Polygenic trait influenced by a number of different genes • The color of skin depends on ◦ Hemoglobin ◦ Keratin ◦ Melanin • Skin Color shows a cinal distribution ◦ Intensity of UV is greatest at the equator and decreases towards the poles ◦ Melanin blocks the UV portion of sunlight from penetrating through the skin ◦ The degree to which UV penetrates the skin is inversely proportional to the amount of melanin in the skin • Natural Selection and Skin Color ◦ Three main theories: ▪ Skin damage and skin cancer ▪ Nutrient photolysis: Folic Acid ▪ Vitamin D Synthesis ◦ Natural Selection: Skin Damage ▪ Sunburn ▪ Facilitates infection, impairs thermoregulation, permanent damage ▪ Skin Cancer ▪ Highest incidence in lightskinned people living in tropical zones; low incidence in dark skinned population ▪ Melanoma incidence ▪ The rate of UV penetration in the skin is inversely proportional to the amount of melanin in skin ▪ Overexposure to UV radiation: ▪ Skin cancer by damaging DNA ▪ Selection for dark skin near the equator where UV is high ◦ Natural Selection: Nutrient Photolysis ▪ Chemicals essential for metabolism are sensitive to excess UV radiation and will undergo decomposition (photolysis) ▪ Folic Acid ▪ Important for central nervous system development; DNA replication and cell division ▪ Deficiencies can lead to pregnancy complications, neural tube defects, impaired sperm production all of which can negatively impact reproductive success ▪ Photo sensitivity of these nutrients suggests that darker skin affords protection from the depleting effects of UV radiation ◦ Natural Selection: Vitamin D Synthesis ▪ Essentials for calcium metabolism, and normal bone and tooth development ▪ Difficult to obtain from diet ▪ Scarce in most foods, except for fish oils, egg yolk, butter, cream, and liver ▪ Primary source of Vitamin D is synthesis within the skin ▪ Synthesized through the interaction of UV light and a cholesterol like substance found in epidermal cells ▪ Dark skin requires six times as long to make the same amount of vitamin D as light skin ▪ Effects of underexposure to UV ▪ Rickets: ▪ A disease affecting children, caused by vitamin D deficiency. Results in impaired mineralization of developing bones and teeth ▪ Legs and pelvis are particularly affected: fitness can decrease due to impaired movement and problems in childbearing ▪ Negatively impact reproductive success and drive selection for lighter skin color in areas with reduced UV exposure ◦ Antagonist selective pressures ▪ More melanin to protect from skin damage and photolysis as you get closer to the equator ▪ Less melanin to facilitate vitamin d synthesis as you get farther from the equator ▪ result: clinal distribution or gradient ▪ Modern humans originated in Africa and ancestral populations were probably dark skinned ▪ Only recently populations in norther high latitudes evolved lighter skin color (due to relaxed selection on dark skin and direct selection to absorb more UV to make vitamin D)
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