Lecture 14 human variation


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Lecture 14 - Human Variation

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Lecture 14 human variation

  1. 1. Anthro 101: Human Biological Evolution Lecture 14: Human Variation
  2. 2. Human Variation • These charts tell us that Homo sapiens evolved relatively recently, and expanded rapidly • In addition, they tell us that biological race does not exist…
  3. 3. But there seems to be variation between people in different parts of the world. How can we understand this?
  4. 4. Molecular data tell us that non-Africans are all descendants of people that moved out of Africa about 50 kya
  5. 5. Over the last 50ky humans have diversified • Human morphology varies • Height and body proportions • Skin color • Hair color and texture • Facial features • Human physiology varies • Lactose absorption • Hemoglobin types • Susceptibility to diseases • Human behavior varies • Subsistence strategies • Social organization • Beliefs, values, etc.
  6. 6. Variations among humans can be due to: genetic differences or environmental differences • Genetic variation • Differences caused by variation in genes inherited from parents • Environmental variation • Differences caused by environmental factors • Differences caused by culture • Both of these interact to generate our variation • Difficult to distinguish between these two causes because parents and children share genes and environment
  7. 7. Natural selection contributes to some of this genetic variation between groups • Different environments favor different traits (genes) • Lactose absorption • Hemoglobin types • Genetic diseases • Sickle-cell anemia • Skin color • Body shape
  8. 8. Natural Selection & Heterozygote advantage can produce genetic variation between groups • Heterozygotes for sickle cell allele (AS) are partially protected against malaria • Heterozygotes more likely to survive than homozygotes of either type • Selection maintains balanced polymorphism • Variation in genetic composition of population will be maintained
  9. 9. Presence of sickling gene is associated with the rate of malaria in different parts of the world
  10. 10. Natural selection can shape differences in ability to digest milk between populations • Mother’s milk contains large amounts of lactose (large sugar) • Lactose too big to be absorbed through intestinal walls • Enzyme lactase breaks lactose into two smaller sugars that can be absorbed • Infants produce large amounts of lactase • All normal mammalian infants can digest milk
  11. 11. Most mammals lose ability to digest lactose after weaning • In most mammals, lactase production tapers to low levels after weaning • Most humans are lactose intolerant as adults • Drinking milk causes cramps, diarrhea • can eat processed milk products like yoghurt & cheese • But in some populations, adults CAN drink milk • Controlled by dominant allele, LAC*P
  12. 12. Rate of Lactose Intolerance among Indigenous Peoples E. Asia 80-100% S.E. Asia 70-80% China 70-80% Greenland 72% S. America 100% 40% 30-50% 12-20% 18% 20% 38% 61% N. America 50-70% 94% 17%
  13. 13. In most of world, ability to digest lactose associated with dairy production • In Africa, pastoralists who use fresh milk have higher proportion of lactose tolerance than pastoralists who use fermented milk products Orma women milk cows
  14. 14. Lactose absorption is also common in Northern Europe, but not because of dairy culture • Calcium necessary for skeletal maintenance • Vitamin D necessary to absorb calcium • Sunlight passes through skin and produces Vitamin D • In northern latitudes, little sunlight  Vitamin D deficits • Milk is an alternate source of calcium and Vitamin D • Ability to digest milk may help prevent Vitamin D deficiency Sami reindeer herder
  15. 15. ABO blood groups • The proportion of each blood group follows a geographical cline • Type A & B more common where bacterial disease common • Type O more common where viral disease common http://anthro.palomar.edu/vary/vary_3.htm Type A • 10 - 35% of people in most populations • increased risk of small pox, some cancers • absent in native Central & South Americans
  16. 16. ABO blood groups • The proportion of each blood group follows a geographical cline • Type A & B more common where bacterial disease common • Type O more common where viral disease common http://anthro.palomar.edu/vary/vary_3.htm Type B • Rarest type • 20 - 30 % in some Asian populations • lowest in native populations of Americas & Australia
  17. 17. ABO blood groups • The proportion of each blood group follows a geographical cline • Type A & B more common where bacterial disease common • Type O more common where viral disease common http://anthro.palomar.edu/vary/vary_3.htm Type O • most common type - universal donor • Esp. in Native populations of the Americas
  18. 18. Skin color varies across the globe
  19. 19. Variation in skin color may reflect adaptations to local environment • Chimpanzees have light-colored skin, covered by hair • Pigment on faces and hands darkens with exposure to sun • When H. ergaster moved out into the savanna, body proportions adapted to hot climate • may have lost hair and increased sweat glands to keep cool • Like chimps, skin may have darkened as it was exposed to sunlight
  20. 20. Sunlight is necessary, but potentially harmful • Sunlight necessary to start production of Vitamin D • Vitamin D is necessary for absorption of calcium • Vitamin D deficiencies cause rickets and other skeletal disorders • Sunlight also has harmful effects • UV radiation damages DNA and causes skin cancer • UV destroys Vitamin B folate
  21. 21. Melanin is a natural sunscreen • Skin contains melanocytes that synthesize brown melanin pigment • Melanin prevents and repairs damage from UV rays • Absorbs UV rays and causes them to lose energy • Neutralizes free radicals that form after damage by UV rays • Skin color may reflect adaptation to balance beneficial and harmful effects of sunlight
  22. 22. Skin color may be adaptation to preserve B vitamin folate levels • Often suggested that high concentrations of melanin were favored as adaptation against skin cancer • But, serious forms of skin cancer are uncommon and occur late in life • Selection against traits that occur after reproductive years is weak • Sunlight causes reduction in folate levels • Folate deficits in pregnant women linked to neural tube defects, like spina bifida • Folate essential for DNA synthesis, new cell production, and sperm production • Where sun is strong, dark skin protects folate levels
  23. 23. Skin color is related to exposure to UV • Map shows exposure to UV radiation (red = high, blue = low) • In Zone 1, people can have dark skin and still produce enough Vitamin D • In Zone 2, people need lighter skin during some parts of the year to produce sufficient Vitamin D, and people’s skin color changes seasonally • In Zone 3, people need light skin year-round to absorb enough Vitamin D, and do not tan Zone 1 Zone 2 Zone 3 Zone 3 Zone 2
  24. 24. Some exceptions to general pattern are informative • Inuit are darker than expected based on their location • Migrated to N. America fairly recently, and/or • Diet very rich in Vitamin D from fish • Peoples on western side of Red Sea have very dark skin, while people on the eastern side have lighter skin • Peoples on the western side have been there longer than peoples on the eastern side • Peoples on the eastern side have adapted culturally (garments that cover bodies, tents for shelter from sun)
  25. 25. Variation within groups Women’s basketball team Range = 5’2” to 6’7” Mean = 5’10” Women’s gymnastics team Range = 5’0” to 5’6” Mean = 5’2” Variation between groups
  26. 26. Sources of variation within & among groups are often different Waters, weeds, fertilizes Watches TV, studies Variation within each lawn = genetic Variation between lawns = environmental
  27. 27. Children’s height correlated with Parent’s height
  28. 28. People living in colder climates tend to be bigger Mean annual temperature 20 40 60 80 50 40 60 70 Weight (kg)
  29. 29. What does this tell us about the variation between groups? • Much of the variation in height within populations is genetic • The variation in height between populations is more likely to be adaptive (but not always!) • You might conclude that variation between groups is due to genetic variation between groups • This argument is wrong!
  30. 30. Environment can explain differences between populations more often than genetics • In early 20th century, Japanese migrated to Hawaii to work on sugar plantations • Migrants were similar to those who stayed in Japan (in height) • The children of immigrants were 2.5% taller than their parents • Rapid change must reflect environmental variation (diet, disease, etc) 156 157 158 159 160 161 162 163 164 Immigrants Residents Immigrants' Offspring Height(cm)
  31. 31. Environment affects height differences between groups more strongly than genetics • Compared the height of Mayan children, Mayan-American children, and white-American children. • Mayan-American children were more similar to white- American children B. Bogin & L. Rios 2003 Mayan-American kids Mayan kids American kids
  32. 32. What about variation that we associate with different races? • Long history of classification • mostly used for racist purposes • Linnaeus’ 5 subspecies or races of Homo sapiens (NOT valid) • Afer Ferus • Americanus Europaeus • Asiaticus The five human racial divisions proposed in Carleton Coon's The Origin of Races (1962)
  33. 33. Unpacking the definition of racism • Racism = prejudicial belief that members of one ethnic group are superior to those of another • Assumes that important qualities of an individual: • Intellect • Physical ability • Temperament are biologically determined by his or her racial group = stereotyping
  34. 34. Assumption 1: People fall naturally into a small number of distinct groups • Americans tend to classify people into 3 groups • People of European ancestry • People of Asian ancestry • People of African ancestry • These schemes ignore the complexity of human variation
  35. 35. Classifications based on morphology link together people from Africa, Australia, & New Guinea Morphology Genes Africa New Guinea
  36. 36. Morphology Genes Similarity based on morphology doesn’t reflect similarity based on genes
  37. 37. But I can see races… However, these discontinuities are an illusion • Suppose you parachute into Stockholm • Immediately know you are not in Nairobi
  38. 38. Discontinuities are an illusion • Suppose you parachute into Stockholm • Immediately know you are not in Nairobi • So, you’d think races are real • But imagine you bicycle from Nairobi to Stockholm • Boundaries would be impossible to detect
  39. 39. Discontinuities are an illusion • Suppose you parachute into Stockholm • Immediately know you are not in Nairobi • So, you’d think races are real • But imagine you bicycle from Nairobi to Stockholm • Boundaries would be impossible to detect
  40. 40. Assumption 2: Differences between races are due to biological heritage • Implies members of each group are more similar to one another than to members of other groups • Differences within local human groups account for 85-95% of total variation • Differences between local human groups account for 5-15% of total variation • If we wiped out everyone on the planet, except the Inuit, the Aché, and the Efe, we would maintain about 85-95% of the total variation in the human species
  41. 41. Racial classification schemes explain little of the genetic variation among humans • There is more variation within each racial category than between them
  42. 42. Amerindians, West Africans, & Portuguese in Brazil • Brazilians define “color” by • Skin color • Hair type • Nose shape • Lip shape • Brazilians identify many categories of color • (mestiço, mulato, caboclo, cafuzo, moreno, pardo) • based on phenotype not descent • Siblings can be classified differently • Do phenotypic categories match genotypic traits?
  43. 43. Racial categories are not natural biological entities • Recall that we argued that biological species are natural units • They have discrete boundaries • There are no (rarely) intermediate cases • Racial groups are not natural units • They are based on an arbitrary set of traits • There are intermediate cases • Categorizations based on different traits produce different groupings…lots of overlap between populations • Blood type • Skin color • Eye shape • Body proportions
  44. 44. Assumption 3: Race tells us something important about a person • Is race important in diagnosis and treatment of disease? • African Americans higher rates of hypertension than White Americans • FDA approved specific drug that’s more effective in African Americans
  45. 45. Race is not a meaningful biological category, but racism exists Brown vs Topeka Board of Education US Supreme Court 1954 “Separate educational facilities are inherently unequal”
  46. 46. Housing discrimination in the USA 0 5 10 15 20 25 30 35 40 White Asian Latino Black ProportionofLoanApplicationsDenied USA, 1993 Sidanius & Pratto 1999
  47. 47. Retail discrimination in USA $0 $200 $400 $600 $800 $1,000 $1,200 White women Black Women Black Men DifferencefromPricePaidbyWhiteMen Price differential for Buying New Car, USA 1990’s Sidanius & Pratto 1999
  48. 48. These kinds of social experiences could contribute to differences in health • People act on perceptions about race • These have real consequences for individuals For example: • Discrimination  • education & job opportunities  • Socioeconomic status  • dietary options & preferences   stress  hypertension
  49. 49. So, we have to deal with a contradiction between the biology and the culture of rac(ism) • There is more genetic variation within human groups than between them • Racial categories are based on a small number of superficial traits • Phenotype is poor measure of ancestry • Racial and ethnic categories are culturally constructed • Racism and ethnic prejudice have real consequences on people’s lives