Chapter 12

Human Variation and Adaptation

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

Variation

Chapter Outline Human Biocultural Evolution Population Genetics The Adaptive Significance of Human

Variation The Continuing Impact of Infectious

Disease

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.

The characteristics used to define races are influenced by several genes and exhibit a continuous range of expression.

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.

Distribution of the B Allele in Indigenous Populations

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.

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

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

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.

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).

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.

Homeostasis A condition of stability within a biological

system, maintained by the interaction of physiological mechanisms that compensate for changes.

Stress is the body’s response to any factor that threatens its ability to maintain homeostasis.

Homeostasis Acclimatization is physiological responses to

changes in the environment. Such responses may be temporary or

permanent, depending on the duration of the environmental change and when it occurs.

Because it is under genetic influence, acclimatization is subject to evolutionary factors such as natural selection or genetic drift.

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

Bergmann's rule - body size tends to be greater in populations that live in cold environments.

Bergmann’s Rule In mammalian species, body size tends to be

greater in populations that live in colder climates.

As mass increases, the relative amount of surface area decreases proportionately.

Because heat is lost at the surface, it follows that increased mass allows for greater heat retention and reduced heat loss.

Allen’s Rule In colder climates, shorter appendages,

with increased mass-to-surface ratios, are adaptive because they are more effective at preventing heat loss.

Conversely, longer appendages, with increased surface area relative to mass, are more adaptive in warmer climates because they promote heat loss.

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 was the number one limiting factor to human populations.

Since the 1940s, the use of antibiotics has reduced mortality resulting from infectious disease.

Impact of Infectious Disease In the late 1960s, the surgeon general

declared the war against infectious disease won.

Between 1980 and 1992 deaths from infectious disease increased by 58%.

Increases in the prevalence of infectious disease may be due to overuse of antibiotics.

Environmental Factors. Global warming may expand the range of

tropical diseases. The spread of disease is associated with

encountering people; this includes crossing borders and penetrating remote areas.

The increasingly large human population leads to overcrowding, unsanitary conditions and the spread of communicable disease.

Quick Quiz

1. Which of the following is a true statement?a) Polygenic traits are usually more

straightforward than polymorphic traits.b) Comparing allele frequencies between

populations can tell us nothing about evolutionary events.

c) Distributions of alleles for a single genetic trait do not conclusively demonstrate genetic relationships between populations.

d) The best way to understand patterns of population relationships is to follow a single polymorphic trait.

Answer: c The following is a true statement:

Distributions of alleles for a single genetic trait do not conclusively demonstrate genetic relationships between populations.

2. One of the results of mitochondrial DNA (mtDNA) research has been the discovery that:

a) the variation of mtDNA within Homo sapiens is much less than found in other species.

b) the variation of mtDNA within Homo sapiens is much more than found in other species.

c) chimpanzees have much less variation in their mtDNA than humans do.

d) the length of the mtDNA is as long as nuclear DNA, about 3 billion nucleotides.

Answer: a One of the results of mitochondrial DNA

(mtDNA) research has been the discovery that the variation of mtDNA within Homo sapiens is much less than found in other species.

3. The total complement of genes shared by reproductive members of a population, is that population's

a) gene flow.

b) gene drift.

c) gene pool.

d) bottleneck effect.

Answer: c The total complement of genes shared

by reproductive members of a population, is that population's gene pool.

4. The pigment which helps protect against ultraviolet radiation by absorbing it is

a) carotene.

b) lactose.

c) hemoglobin.

d) melanin.

Answer: d The pigment which helps protect against

ultraviolet radiation by absorbing it is melanin.

5. Inuits have a large "globular" body, while the body structure of the Kalahari !Kung is thin and linear. This is explained by

a) Bergmann's rule.

b) Allen's rule.

c) Gloger's rule.

d) Kleiber's rule.

Answer: a Inuits have a large "globular" body, while

the body structure of the Kalahari !Kung is thin and linear. This is explained by Bergmann's rule.