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CHAPTER 15
Modern Human Biology:
Patterns of Variation
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.

TRADITIONAL CONCEPT OF RACE
Since the 1960s, 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)
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.

INTELLIGENCE
 Genetic
and
environmental factors
contribute to
intelligence.
 Innate differences in
abilities reflect
variation within
populations, not
differences between
groups.
HUMAN POLYMORPHISMS
Characteristics with different phenotypic
expressions are called polymorphisms.
 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.

Distribution of the B allele in the indigenous populations of the world.
PATTERNS OF POLYMORPHIC VARIATION
 Analyzing
single
traits can be
confusing
 Lewontin’s
study
People in Sardinia, a large island
off the west coast of Italy, differ in
allele frequencies at some loci
from other European populations.
POPULATION GROUPINGS USED BY LEWONTIN IN
POPULATION GENETICS STUDY (1972)
Geographic Group
Examples of Populations Included
Caucasians
Arabs, Armenians, Tristan da Cunhans
Black Africans
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
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.”
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.

HARDY-WEINBERG EQUILIBRIUM

Hardy-Weinberg Equilibrium
The mathematical relationship expressing the
predicted distribution of alleles in populations; the
central theorem of population genetics.
 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.

EVOLUTION IN ACTION:MODERN HUMAN
POPULATIONS
Nonrandom mating
 Endogamy



Exogamy


Mating with individuals from the same group.
Mating pattern whereby individuals obtain mates
from groups other than their own.
Interbreeding
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.


HbS allele
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.
FREQUENCIES OF
LACTOSE INTOLERANCE
Population Group
Percent
U.S. whites
2–19
Finnish
48
Swiss
12
Swedish
4
FREQUENCIES OF
LACTOSE INTOLERANCE
Population Group
Percent
U.S. blacks
70–77
Ibos
99
Bantu
90
Fulani
22
Thais
99
Asian Americans
95–100
Native Americans
85