Transcript Chapter 12

Chapter 12
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
Chapter Outline
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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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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.
Traditional Concept of Race
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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.
The characteristics used to define races are
influenced by several genes and exhibit a
continuous range of expression.
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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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.
Clinal Distributions
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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.
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.”
Human Biocultural Evolution
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Humans live in cultural environments that
are continually modified by their activities.
Evolutionary processes can be
understood only within this cultural
context.
Human Biocultural Evolution
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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.
Evolutionary Interactions Affecting the
Frequency of the Sickle-cell Allele
Frequencies of
Lactose Intolerance
Population Group
Percent
U.S. whites
2–19
Finnish
48
Swiss
12
Swedish
4
Frequencies of
Lactose Intolerance
Population Group
U.S. blacks
Ibos
Bantu
Fulani
Thais
Asian Americans
Native Americans
Percent
70–77
99
90
22
99
95–100
85
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.
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).
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.
Homeostasis
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A condition of stability within a biological
system, maintained by the interaction of
physiological mechanisms that compensate for
changes.
Stress is the body’s response to any factor that
threatens its ability to maintain homeostasis.
Homeostasis
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Acclimatization is physiological responses to
changes in the environment.
 Such responses may be temporary or
permanent, depending on the duration of the
environmental change and when it occurs.
 Because it is under genetic influence,
acclimatization is subject to evolutionary
factors such as natural selection or genetic
drift.
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
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Long-term adaptations to heat evolved in
our ancestors:
 Sweat Glands
 Vasodilation
Bergmann's rule - body size tends to be
greater in populations that live in cold
environments.
Bergmann’s Rule
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In mammalian species, body size tends to be
greater in populations that live in colder
climates.
As mass increases, the relative amount of
surface area decreases proportionately.
Because heat is lost at the surface, it follows
that increased mass allows for greater heat
retention and reduced heat loss.
Allen’s Rule
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In colder climates, shorter appendages,
with increased mass-to-surface ratios, are
adaptive because they are more effective
at preventing heat loss.
Conversely, longer appendages, with
increased surface area relative to mass,
are more adaptive in warmer climates
because they promote heat loss.
Human Response to Cold
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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 was the number one limiting
factor to human populations.
Since the 1940s, the use of antibiotics
has reduced mortality resulting from
infectious disease.
Impact of Infectious Disease
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In the late 1960s, the surgeon general
declared the war against infectious
disease won.
Between 1980 and 1992 deaths from
infectious disease increased by 58%.
Increases in the prevalence of infectious
disease may be due to overuse of
antibiotics.
Environmental Factors.
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Global warming may expand the range of
tropical diseases.
The spread of disease is associated with
encountering people; this includes crossing
borders and penetrating remote areas.
The increasingly large human population leads
to overcrowding, unsanitary conditions and the
spread of communicable disease.
Quick Quiz
1. Which of the following is a true statement?
a) Polygenic traits are usually more
straightforward than polymorphic traits.
b) Comparing allele frequencies between
populations can tell us nothing about
evolutionary events.
c) Distributions of alleles for a single genetic
trait do not conclusively demonstrate
genetic relationships between populations.
d) The best way to understand patterns of
population relationships is to follow a single
polymorphic trait.
Answer: c
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The following is a true statement:
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Distributions of alleles for a single
genetic trait do not conclusively
demonstrate genetic relationships
between populations.
2. One of the results of mitochondrial DNA
(mtDNA) research has been the discovery
that:
a) the variation of mtDNA within Homo
sapiens is much less than found in other
species.
b) the variation of mtDNA within Homo
sapiens is much more than found in other
species.
c) chimpanzees have much less variation in
their mtDNA than humans do.
d) the length of the mtDNA is as long as
nuclear DNA, about 3 billion nucleotides.
Answer: a
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One of the results of mitochondrial DNA
(mtDNA) research has been the
discovery that the variation of mtDNA
within Homo sapiens is much less
than found in other species.
3. The total complement of genes shared by
reproductive members of a population,
is that population's
a) gene flow.
b) gene drift.
c) gene pool.
d) bottleneck effect.
Answer: c
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The total complement of genes shared
by reproductive members of a
population, is that population's gene
pool.
4. The pigment which helps protect against
ultraviolet radiation by absorbing it is
a) carotene.
b) lactose.
c) hemoglobin.
d) melanin.
Answer: d
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The pigment which helps protect against
ultraviolet radiation by absorbing it is
melanin.
5. Inuits have a large "globular" body, while
the body structure of the Kalahari !Kung
is thin and linear. This is explained by
a) Bergmann's rule.
b) Allen's rule.
c) Gloger's rule.
d) Kleiber's rule.
Answer: a
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Inuits have a large "globular" body, while
the body structure of the Kalahari !Kung
is thin and linear. This is explained by
Bergmann's rule.