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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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Two schools of thought developed to explain
diversity:
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Monogenists believed that all humans were
descended from a original pair of humans.
Polygenists believed that all humans were
descended from a number of pairs of humans.
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.
"Racial traits" are not the only phenotypic expressions
that contribute to social identity: sex and age are also
critically important.
In the 1950's the use of the term "race" was replaced
with "ethnicity "
Allele Frequencies
Within and Between Populations
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After World War II, the study of human variation shifted
to the study of differences in allele frequencies within
and between populations.
The application of evolutionary principles to human
variation has replaced the view that was based solely
on observed phenotype.
Races are no longer viewed as fixed biological entities,
composed of individuals fitting a particular type.
Groupings Used by Lewontin in
Population Genetics Study (1972)
Geographical Group
Caucasians
Black Africans
Examples of Populations Included
Arabs, Armenians, Tristan da Cunhans
Bantu, San, U.S. blacks
Asians
Ainu, Chinese, Turks
South Asians
Andamanese, Tamils
Amerinds
Aleuts, Navaho, Yanomama
Oceanians
Easter Islanders, Micronesians
Australians
All treated as a single group
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.
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:
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Long-term (genetic) evolutionary changes
characterize all individuals within a population or
species.
Short-term, temporary physiological response is
called acclimatization.
Pigmentation
and Geographical Divisions
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Before 1500, skin color in populations followed a
geographical distribution, particularly in the Old World.
– 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
Influenced by three substances:
1. Hemoglobin, when it is carrying oxygen, gives a
reddish tinge to the skin.
2. Carotene, a plant pigment which the body
synthesizes into vitamin A, provides a yellowish
cast.
3. 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:
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Sweat Glands
Vasodilation
Bergmann's rule - body size tends to be
greater in populations that live in cold
environments.
Human Response to Cold
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Short-term responses to cold:
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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:
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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.
Small Pox
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The only disease considered to be eliminated
as a result of medical technology
Smallpox has a higher incidence in those with
type A or AB than in those type O blood.
The immune systems of individuals with type A
antigen may not recognize the small pox
antigen as a threat.
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.