chapter 15
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Chapter 15. Modern Human Biology: Patterns of Variation. Chapter Outline. Historical Views of Human Variation The Concept of Race Racism Intelligence Contemporary Interpretations of Human Variation. Chapter Outline. Evolution in Action: Modern Human Populations - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 15
Modern Human Biology:Patterns of Variation
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Chapter Outline• Historical Views of Human Variation• The Concept of Race• Racism• Intelligence• Contemporary Interpretations of Human
Variation
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Chapter Outline• Evolution in Action: Modern Human Populations• Human Biocultural Evolution• Issue: Racial Purity: A False and Dangerous
Ideology
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Historical Views of Human Variation• Biological determinism - cultural and
biological variations are inherited in the same way.
• Eugenics - "race improvement" through forced sterilization of members of some groups and encouraged reproduction among others.
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Traditional Concept of Race• Since the 1600s, race has been used to
refer to culturally defined groups.• Race is used as a biological term, but has
enormous social significance. • In any racial group, there will be
individuals who fall into the normal range of variation for another group for one or several characteristics.
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Examples of Phenotypic Variation Among Africans
• (a) San (South African), (b) West African (Bantu), (c) Ethiopian, (d) Ituri (Central African), (e) North African (Tunisia)
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Racism• Based on false belief that intellect and
cultural factors are inherited with physical characteristics.
• Uses culturally defined variables to typify all members of particular populations.
• Assumes that one's own group is superior.
• A cultural phenomenon found worldwide.
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Intelligence• Genetic and environmental factors contribute to
intelligence.• Many psychologists say IQ scores measure life
experience.• Innate differences in abilities reflect variation
within populations, not differences between groups.
• There is no convincing evidence that populations vary in regard to intelligence.
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Human Polymorphisms• Characteristics with different phenotypic
expressions are called polymorphisms.• A genetic trait is polymorphic if the locus
that governs it has two or more alleles.• Geneticists use polymorphisms as a tool
to understand evolutionary processes in modern populations.
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Clinal Distributions• A cline is a gradual change in the frequency of
a trait or allele in populations dispersed over geographical space. Example: The distribution of the A and B
alleles in the Old World. • Clinal distributions are thought to reflect
microevolutionary influences of natural selection and/or gene flow.
• Consequently, clinal distributions are explained in evolutionary terms.
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Blood Type
• (a) A blood sample is drawn. (b) A few drops of blood are treated with chemicals. The glass slides below the labeled bottles show reactions for the ABO system. The blood on the top slide (at left) is AB; the middle is B; and the bottom is A.
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ABO Blood Group System
• Distribution of the B allele in the indigenous populations of the world.
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Allele Distributions• People in Sardinia, a
large island off the west coast of Italy, differ in allele frequencies at some loci from other European populations.
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Question• Which of the following does not fall
under the study of population genetics?a) allele frequenciesb) genotypesc) phenotypesd) gamete production
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Answer: d• Gamete production does not fall under
the study of population genetics.
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Lewontin’s Study of Population Differences• Harvard population geneticist R. D. Lewontin
calculated population differences in allele frequency for 17 polymorphic characteristics.
• He divided his sample into seven geographical areas, and included several population samples within each region.
• He calculated how much of the total genetic variability within our species could be accounted for by these population subdivisions.
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Lewontin’s Study of Population Differences• Only 6.3% of the total genetic variation was
explained by differences between major population groups.
• The larger population subdivisions within the geographical clusters account for another 8.3%.
• Thus, geographical and local groups together account for just 15% of all human genetic variation, leaving the remaining 85% unaccounted for.
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Population Groupings Used by Lewontin in Population Genetics Study (1972)
Geographic Group Examples of Populations IncludedCaucasians Arabs, Armenians, Tristan da Cunhans
Black Africans Bantu, San, U.S. blacksAsians Ainu, Chinese, TurksSouth Asians Andamanese, TamilsAmerinds Aleuts, Navaho, YanomamaOceanians Easter Islanders, MicronesiansAustralians All treated as a single group
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Polymorphisms at the DNA Level• Molecular biologists have recently uncovered
DNA variability in various regions of the genome.
• Scattered through the human genome are microsatellites, sites where DNA segments are repeated.
• Each person has a unique arrangement that defines their distinctive “DNA fingerprint.”
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Human Biocultural Evolution• Humans live in cultural environments that
are continually modified by their activities.• Evolutionary processes can be
understood only within this cultural context.
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Genetic Polymorphisms Used toStudy Human Variation
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Population Genetics• The study of the frequency of alleles, genotypes, and
phenotypes in populations from a microevolutionary perspective.
• A gene pool is the total complement of genes shared by the reproductive members of a population.
• Breeding isolates are populations that are isolated geographically and/or socially from other breeding groups.
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Population Genetics• Endogamy
Mating with individuals from the same group.
• Exogamy Mating pattern whereby individuals
obtain mates from groups other than their own.
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Hardy-Weinberg Equilibrium• The mathematical relationship expressing
the predicted distribution of alleles in populations; the central theorem of population genetics.
• Provides a tool to establish whether allele frequencies in a human population are changing.
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Hardy-Weinberg Equilibrium• Establishes a set of conditions in a population where no
evolution occurs. • The hypothetical conditions that such a population
would be assumed to meet are as follows: The population is infinitely large to eliminate the
possibility of random genetic drift or changes in allele frequencies due to chance.
There’s no mutation. There’s no gene flow. Natural selection isn’t operating. Mating is random.
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Factors that Act to Change Allele Frequencies 1. New variation (i.e., mutation)2. Redistributed variation (i.e., gene flow or
genetic drift)3. Selection of “advantageous” allele
combinations that promote reproductive success (i.e., natural selection).
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Human Biocultural Evolution• Example: Lactose intolerance
In all human populations, infants and young children are able to digest milk.
In most mammals, including humans, the gene that codes for lactase production “switches off” in adolescence.
The geographical distribution of lactose tolerance is related to a history of cultural dependence on fresh milk products.
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Evolutionary Interactions Affecting the Frequency of the Sickle-cell Allele
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Slash-and-burn Agriculture• A traditional land-clearing practice
involving the cutting and burning of trees and vegetation.
• In many areas, fields are abandoned after a few years and clearing occurs elsewhere.
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Balanced Polymorphism • The maintenance of two or more alleles in
a population due to the selective advantage of the heterozygote.
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Lactose Intolerance• The inability to digest fresh milk products,
caused by the discontinued production of lactase—the enzyme that breaks down lactose, or milk sugar.
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Frequencies ofLactose Intolerance
Population Group Percent
U.S. whites 2–19
Finnish 48
Swiss 12
Swedish 4
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Frequencies ofLactose Intolerance
Population Group PercentU.S. blacks 70–77
Ibos 99Bantu 90Fulani 22Thais 99
Asian Americans 95–100Native Americans 85
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Question• The group that has the highest rate of
lactose tolerance are:a) industrial societies.b) horticulturalists.c) pastoralists.d) hunters and gatherers.
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Answer: c• The group that has the highest rate of
lactose tolerance are pastoralists.