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 IncludedCaucasians Arabs, Armenians, Tristan da Cunhans

Black Africans Bantu, San, U.S. blacksAsians Ainu, Chinese, TurksSouth Asians Andamanese, TamilsAmerinds Aleuts, Navaho, YanomamaOceanians Easter Islanders, MicronesiansAustralians 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

Mating with individuals from the same group. Exogamy

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 OFLACTOSE INTOLERANCE

Population Group Percent

U.S. whites 2–19

Finnish 48

Swiss 12

Swedish 4

FREQUENCIES OFLACTOSE INTOLERANCE

Population Group PercentU.S. blacks 70–77

Ibos 99Bantu 90Fulani 22Thais 99

Asian Americans 95–100Native Americans 85