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15

Human Variation and Adaptation

Anthropology:The Exploration of Human Diversity

11th Edition

Conrad Phillip Kottak

McGraw-Hill © 2005 The McGraw-Hill Companies, Inc.

2Human Variation Human Variation and Adaptationand Adaptation• Race: A Discredited Concept in Biology

• Human Biological Adaptation

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3Race: A Discredited Race: A Discredited Concept in BiologyConcept in Biology

– Members of race or subspecies share distinctive physical characteristics based on common ancestry and inheritance of same genes

Human populations have not been isolated enough from one another to develop into discrete races

• Race refers to geographically isolated subdivision of a species

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4Race: A Discredited Race: A Discredited Concept in BiologyConcept in Biology

• Human biological variation distributed gradually between populations along clines

– Human populations vary biologically, but because of extensive gene flow and interbreeding, there are no sharp breaks between populations

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5Race: A Discredited Race: A Discredited Concept in BiologyConcept in Biology

• Race is supposed to reflect shared genetic material

Early scholars used phenotypical traits (skin color) for racial classification

– Scientists have trouble grouping people into distinct racial units

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6Race: A Discredited Race: A Discredited Concept in Biology Concept in Biology

– Obvious problem with “color-based” racial labels is that terms don’t accurately describe skin color

– Another problem with tripartite scheme is that many populations don’t neatly fit into one of the three “great races”

• Races Are Not Biologically Distinct

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7Race: A Discredited Race: A Discredited Concept in Biology Concept in Biology

– Racial classifications based on phenotype lack credibility because similarities and differences don’t necessarily have genetic basis

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8Race: A Discredited Race: A Discredited Concept in Biology Concept in Biology

– Natural selection—process by which nature selects the forms most fit to survive and reproduce in a given environment

• Explaining Skin Color

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9Race: A Discredited Race: A Discredited Concept in Biology Concept in Biology

• Melanin—primary determinant of human skin color; chemical substance manufactured in the epidermis

Prior to 16th century, darker skinned populations closest to the equator

• Explaining Skin Color– Skin color influenced by several genes

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10Race: A Discredited Race: A Discredited Concept in Biology Concept in Biology

• Unprotected humans in tropics face threat of severe sunburn

– Increases susceptibility to disease– Impairs the body’s ability to withstand heat by

reducing the skin’s ability to sweat– Light skin is more susceptible to skin cancer.

Loomis suggested that in tropics, dark skin color protects body against overproduction of vitamin D

• Explaining Skin Color– Skin color influenced by several genes

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11 Human Biological AdaptationHuman Biological Adaptation

• Abundant evidence for human genetic adaptation and evolution through selection working in specific environments

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12 Human Biological AdaptationHuman Biological Adaptation

– According to World Health Report, tropical diseases affect more than 10 percent of world’s population

• Malaria—300 million to 500 million people• Schistosomiasis—more than 200 million• Filarisasis—120 million

• Genes and Disease

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13 Human Biological AdaptationHuman Biological Adaptation

• After food production emerged, infectious diseases pose mounting risk and became foremost cause of human mortality

• ABO blood groups vary in resistance to disease

• Genes and Disease– Microbes major selective agent for

humans, particularly before arrival of modern medicine

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14 Human Biological AdaptationHuman Biological Adaptation

• There is probably genetic variation in susceptibility to HIV

• Genes and Disease– In diseases for which there are no effective

drugs, genetic resistance maintains significance

AIDS could cause large shifts in human gene frequencies

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15 Human Biological AdaptationHuman Biological Adaptation

– Natural selection also affects facial features

• Long noses seem to be adaptive in arid areas and cold environments

• Thomson’s Nose Rule—association between nose form and temperature

• Facial Features

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16 Human Biological AdaptationHuman Biological Adaptation

– Allen’s rule—relative size of protruding body parts—ears, tails, bills, fingers, toes, and limbs—increase with temperature

• Size and Body Build– Bergmann’s rule—smaller of two bodies

similar in shape has more surface area per unit of weight

• Average body size tends to increase and cold areas and decrease in hot ones

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17 Human Biological AdaptationHuman Biological Adaptation

• Size and Body Build– Human populations use different, but

equally effective, biological means of adapting to environmental stresses associated with high altitudes

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18 Human Biological AdaptationHuman Biological Adaptation

– Phenotypical adaptation—adaptive changes that occur during individual’s lifetime

– Genes and phenotypical adaptation work together to produce a biochemical difference between human groups in ability to digest large amounts of milk

Adaptive advantage when other foods are scarce and milk available

• Lactose Tolerance