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Chapter 15 & 16
Human Variation and Adaptation
Chapter Outline
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Historical Views of Human Variation
The Concept of Race
Racism & Intelligence
Contemporary Interpretations of Human
Variation
Human Biocultural Evolution
Population Genetics
The Adaptive Significance of Human Variation
The Continuing Impact of Infectious Disease
Historical Views of Human
Variation
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Early human classification
Biological determinism
Eugenics - "race improvement“
 “Three generations of imbeciles is
enough”
 The Mismeasure of Man
by S.J. Gould
Traditional Concept of Race
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Since the 1600s, race = ethnicity.
Biological fallacy but social reality.
The characteristics that define races are
influenced by several genes, and exhibit a
continuous range of expression.
More genetic variation within one race
than other “different” races.
Racism
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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.
Intelligence
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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.
Human Polymorphisms
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Polymorphisms
Genetic trait with 2+ alleles.
Used as a tool to understand evolutionary
processes.
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.
Distribution of the B Allele in
Indigenous Populations
Polymorphisms at the
DNA Level
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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.”
Evolutionary Interactions Affecting the
Frequency of the Sickle-cell Allele
Population Genetics
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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.
Hardy-Weinberg Equilibrium
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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 (see appendix C, p.391).
 Formula= p2 + 2pq + q2
p= frequency of dominant allele
q= frequency of recessive allele
Factors that Act to Change
Allele Frequencies
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New variation (i.e., mutation)
Redistributed variation (i.e., gene flow or
genetic drift)
Selection of “advantageous” allele
combinations that promote reproductive
success (i.e., natural selection, sexual
selection)
Adaptive Significance of Human
Variation
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Human variation is the result of adaptations to
environmental conditions.
Physiological response to the environment
operates at two levels:
1. Long-term evolutionary changes
characterize all individuals within a
population or species.
2. Short-term, temporary physiological
response is called acclimatization.
UV Light and Vitamin D
Synthesis
Pigmentation
and Geographical Divisions
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Before 1500, skin color in populations followed
a geographical distribution, particularly in the
Old World.
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Populations with the greatest amount of
pigmentation are found in the tropics.
Populations with lighter skin color are
associated with more northern latitudes.
Skin Color
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Influenced by three substances:
 Hemoglobin, when it is carrying oxygen,
gives a reddish tinge to the skin.
 Carotene, a plant pigment which the body
synthesizes into vitamin A, provides a
yellowish cast.
 Melanin, has the ability to absorb ultraviolet
radiation preventing damage to DNA.
Thermal Environment
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Mammals and birds have evolved complex
physiological mechanisms to maintain a
constant body temperature.
Humans are found in a wide variety of thermal
environments, ranging from 120° F to -60° F.
Human Response to Heat
 Long-term adaptations to heat evolved in our
ancestors:
 Sweat Glands
 Vasodilation
Human Response to Cold
Short-term responses to cold:
 Metabolic rate and shivering
 Narrowing of blood vessels to reduce
blood flow from the skin,
vasoconstriction.
 Increases in metabolic rate to release
energy in the form of heat.
High Altitude
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Multiple factors produce stress on the
human body at higher altitudes:
 Hypoxia (reduced available oxygen)
 Intense solar radiation
 Cold
 Low humidity
 Wind (which amplifies cold stress)
Infectious Disease
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Caused by invading organisms such as
bacteria, viruses, or fungi.
Throughout evolution, disease has
exerted selective pressures on human
populations.
Disease influences the frequency of
certain alleles that affect the immune
response.
Impact of Infectious Disease
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Before the 20th century, infectious disease #1
limiting factor.
Since the 1940s,antibiotics reduced mortality
rates.
In the late 1960s,war against infectious
disease won.
Between 1980 and 1992 deaths from infectious
disease increased by 58%.
Increases may be due to overuse of
antibiotics.