Chapter 15

Modern Human Biology:

Patterns of Variation

Chapter Outline

• Historical Views of Human Variation• The Concept of Race• Racism• Intelligence• Contemporary Interpretations of Human

Variation

Chapter Outline• Evolution in Action: Modern Human Populations• Human Biocultural Evolution• Issue: Racial Purity: A False and Dangerous

Ideology

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 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)

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.

• A cultural phenomenon found worldwide.

Intelligence• 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

• 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 European populations.

Question

• 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 Differences

• 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 Differences

• 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

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.”

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.

Genetic Polymorphisms Used toStudy Human Variation

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.

• Breeding isolates are populations that are isolated geographically and/or socially from other breeding groups.

Population Genetics

• Endogamy Mating with individuals from the same

group.• Exogamy

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 changing.

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

1. New variation (i.e., mutation)

2. Redistributed variation (i.e., gene flow or genetic drift)

3. 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 elsewhere.

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 ofLactose Intolerance

Population Group Percent

U.S. whites 2–19

Finnish 48

Swiss 12

Swedish 4

Frequencies ofLactose Intolerance

Population Group Percent

U.S. blacks 70–77

Ibos 99

Bantu 90

Fulani 22

Thais 99

Asian Americans 95–100

Native Americans 85

Question

• 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.