Chapter 15 & 16

Human Variation and Adaptation

Chapter Outline Historical Views of Human Variation The Concept of Race Racism & Intelligence Contemporary Interpretations of Human

Variation Human Biocultural Evolution Population Genetics The Adaptive Significance of Human Variation The Continuing Impact of Infectious Disease

Historical Views of Human Variation

Early human classification Biological determinism Eugenics - "race improvement“

“Three generations of imbeciles is enough”

The Mismeasure of Man

by S.J. Gould

Traditional Concept of Race Since the 1600s, race = ethnicity. Biological fallacy but social reality. The characteristics that define races are

influenced by several genes, and exhibit a continuous range of expression.

More genetic variation within one race than other “different” races.

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 Polymorphisms Genetic trait with 2+ alleles. Used as a tool to understand evolutionary

processes. 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 Indigenous Populations

Polymorphisms at theDNA 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.”

Evolutionary Interactions Affecting the Frequency of the Sickle-cell Allele

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.

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 (see appendix C, p.391). Formula= p2 + 2pq + q2

p= frequency of dominant allele q= frequency of recessive allele

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, sexual selection)

Adaptive Significance of Human Variation Human variation is the result of adaptations to

environmental conditions. Physiological response to the environment

operates at two levels:1. Long-term evolutionary changes

characterize all individuals within a population or species.

2. Short-term, temporary physiological response is called acclimatization.

UV Light and Vitamin D Synthesis

Pigmentation and Geographical Divisions 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:

Hemoglobin, when it is carrying oxygen, gives a reddish tinge to the skin.

Carotene, a plant pigment which the body synthesizes into vitamin A, provides a yellowish cast.

Melanin, has the ability to absorb ultraviolet radiation preventing damage to DNA.

Thermal Environment 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 Long-term adaptations to heat evolved in our

ancestors: Sweat Glands Vasodilation

Human Response to Cold

Short-term responses to cold: 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 Multiple factors produce stress on the

human body at higher altitudes: Hypoxia (reduced available oxygen) Intense solar radiation Cold Low humidity Wind (which amplifies cold stress)

Infectious Disease 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.

Impact of Infectious Disease Before the 20th century, infectious disease #1

limiting factor. Since the 1940s,antibiotics reduced mortality

rates. In the late 1960s,war against infectious

disease won. Between 1980 and 1992 deaths from infectious

disease increased by 58%. Increases may be due to overuse of

antibiotics.