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Chapter 15
Modern Human Biology:
Patterns of Variation
Chapter Outline
Historical Views of Human Variation
The Concept of Race
Contemporary Interpretations of Human
Chapter Outline
Evolution in Action: Modern Human Populations
Human Biocultural Evolution
Issue: Racial Purity: A False and Dangerous
Historical Views of Human
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
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.
Examples of Phenotypic
Variation Among Africans
(a) San (South African), (b) West African
(Bantu), (c) Ethiopian, (d) Ituri (Central African),
(e) North African (Tunisia)
Based on false belief that intellect and
cultural factors are inherited with physical
Uses culturally defined variables to typify
all members of particular populations.
Assumes that one's own group is
A cultural phenomenon found worldwide.
Genetic and environmental factors contribute to
Many psychologists say IQ scores measure life
Innate differences in abilities reflect variation
within populations, not differences between
There is no convincing evidence that
populations vary in regard to intelligence.
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.
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.
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.
ABO Blood Group System
Distribution of the B allele in the indigenous populations
of the world.
Allele Distributions
People in Sardinia, a
large island off the
west coast of Italy,
differ in allele
frequencies at some
loci from other
Which of the following does not fall
under the study of population genetics?
a) allele frequencies
b) genotypes
c) phenotypes
d) gamete production
Answer: d
Gamete production does not fall under
the study of population genetics.
Lewontin’s Study of Population
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.
Lewontin’s Study of Population
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.
Population Groupings Used by Lewontin in
Population Genetics Study (1972)
Geographic Group
Examples of Populations Included
Arabs, Armenians, Tristan da Cunhans
Black Africans
Bantu, San, U.S. blacks
Ainu, Chinese, Turks
South Asians
Andamanese, Tamils
Aleuts, Navaho, Yanomama
Easter Islanders, Micronesians
All treated as a single group
Polymorphisms at the DNA
Molecular biologists have recently uncovered
DNA variability in various regions of the
Scattered through the human genome are
microsatellites, sites where DNA segments are
Each person has a unique arrangement that
defines their distinctive “DNA fingerprint.”
Human Biocultural Evolution
Humans live in cultural environments that
are continually modified by their activities.
Evolutionary processes can be
understood only within this cultural
Genetic Polymorphisms Used to
Study Human Variation
Population Genetics
The study of the frequency of alleles, genotypes, and
phenotypes in populations from a microevolutionary
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
Population Genetics
 Mating with individuals from the same
Mating pattern whereby individuals
obtain mates from groups other than
their own.
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
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.
Factors that Act to Change
Allele Frequencies
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).
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.
Evolutionary Interactions Affecting the
Frequency of the Sickle-cell Allele
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
Balanced Polymorphism
The maintenance of two or more alleles in
a population due to the selective
advantage of the heterozygote.
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.
Frequencies of
Lactose Intolerance
Population Group
U.S. whites
Frequencies of
Lactose Intolerance
Population Group
U.S. blacks
Asian Americans
Native Americans
The group that has the highest rate of
lactose tolerance are:
a) industrial societies.
b) horticulturalists.
c) pastoralists.
d) hunters and gatherers.
Answer: c
The group that has the highest rate of
lactose tolerance are pastoralists.