Transcript Chapter 12 - HCC Learning Web

CHAPTER 12
Human Variation and Adaptation
Chapter Outline
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Historical Views of Human Variation
The Concept of Race
Intelligence
Contemporary Interpretations of Human Variation
Chapter Outline
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Human Biocultural Evolution
Population Genetics
The Adaptive Significance of Human Variation
The Continuing Impact of Infectious Disease
Focus Question
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How do you define “race”?
What “races” do you define?
What traits do you use to assign “race”?
Historical Views of Human Variation
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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
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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.
Polytypic
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A polytypic species is composed of local populations
that differ in the expression of one or more traits.
Even within local populations, there’s a great deal of
genotypic and phenotypic variation between
individuals.
All contemporary humans are members of the same
polytypic species, Homo sapiens.
Shifting Focus for Physical
Anthropology
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From visible phenotypic variation to the explanation
of differences in allele frequencies
What is the adaptive significance of such
differences?
Forensic anthropologists use expertise to identify
victims of war and crime
Intelligence
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Genetic and environmental factors contribute to intelligence.
IQ scores change during a person’s lifetime; average IQ
scores of different populations overlap.
Complex cognitive abilities, however measured, are influenced
by many genes and are thus polygenic .
Individual abilities result from complex interactions between
genetic and environmental factors.
No convincing evidence exists that populations vary in their
cognitive abilities, regardless of what some popular books
suggest.
Human Polymorphisms
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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
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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 natural
selection and/or gene flow.
Consequently, clinal distributions are explained in
evolutionary terms.
Polymorphisms at the DNA Level
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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
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Humans live in cultural environments that are
continually modified by their activities.
Evolutionary processes can be understood only
within this cultural context.
Slash-and-burn Agriculture
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A traditional land-clearing practice whereby trees
and vegetation are cut and burned.
In many areas, fields are abandoned after a few
years and clearing occurs elsewhere.
Sickle Cell Allele
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With the spread in Africa of slash-and-burn
agriculture, in the last 2,000 years, penetration and
clearing of tropical forests occurred.
As a result, rain water was left to stand in stagnant
pools that provided mosquito breeding areas close to
human settlements.
DNA analyses have confirmed such a recent origin
and spread of the sickle-cell allele in a population
from Senegal, in West Africa.
Sickle Cell Allele
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A recent study estimates the origin of the HbS
mutation in this group at 1,250 to 2,100 ya.
At least in some areas, malaria began to have an
impact on human populations only recently.
But once it did, it became a powerful selective force.
The increase in the frequency of the sickle-cell allele
is a biological adaptation to an environmental
change.
Evolutionary Interactions Affecting the Sickle-cell
Allele
Thinking About Race
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Sickle-cell anemia has often been called a “racial”
disease as it is more common in the United States
among African-Americans.
From what you’ve just learned, how is that
generalization correct and incorrect?
Lactose Intolerance
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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
U.S. blacks
Ibos
Bantu
Fulani
Thais
Asian Americans
Native Americans
Percent
70–77
99
90
22
99
95–100
85
Population Genetics
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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.
Human populations complicated by cultural and social
structures
Hardy-Weinberg Equilibrium
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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.
2.
3.
New variation (i.e., mutation)
Redistributed variation (i.e., gene flow or genetic
drift)
Selection of “advantageous” allele combinations
that promote reproductive success (i.e., natural
selection).
Adaptive Significance of Human Variation
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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
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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.
Acclimatization
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Physiological responses to changes in the environment.
 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 natural selection and genetic drift.
Distribution of Skin Color in Indigenous
Populations
Ultraviolet Rays
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Ultraviolet Rays penetrate the skin and can
eventually damage DNA within skin cells.
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three major types of cells that can be affected are
squamous cells, basal cells, and melanocytes.
Ultraviolet Rays
UV Radiation
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Considering the cancer-causing effects of UV
radiation from an evolutionary perspective:
 Early hominins lived in the tropics, where solar
radiation is more intense than in temperate areas
to the north and south.
 Unlike modern city dwellers, early hominids spent
their days outdoors.
 Early hominins didn’t wear clothing that would have
protected them from the sun.
Selective Advantages of Dark Skin
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Protects from folate degradation
Especially important for pregnant mothers
Rickets
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Insufficient amounts of
vitamin D during
childhood result in
rickets, a condition that
often leads to bowing
of the long bones of the
legs and deformation of
the pelvis.
Neural Tube
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In early embryonic development, the anatomical
structure that develops to form the brain and spinal
cord.
Spina Bifida
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A condition in which the arch of one or more
vertebrae fails to fuse and form a protective
barrier around the spinal cord.
Thermal Environment
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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
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Long-term adaptations to heat evolved in our
ancestors:
 Sweat
Glands
 Vasodilation
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Bergmann's rule - body size tends to be greater in
populations that live in cold environments.
Bergmann’s Rule
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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
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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.
Bergmann and Allen’s Rules
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(a) These Samburu
women have
characteristic of many
inhabitants of subSaharan Africa.
(b) The Inuit women
are shorter and
stockier.
These individuals serve
as examples of
Bergmann’s and Allen’s
rules.
Human Response to Cold
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Short-term responses to cold:
 Metabolic rate and shivering
 Capillaries near the skin’s surface widen to permit
increased blood flow to the skin, vasodilation
 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
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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
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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
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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
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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.
Vectors
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Agents that serve to transmit disease from one
carrier to another.
Mosquitoes are vectors for malaria, just as fleas
are vectors for bubonic plague.
Endemic
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Continuously present in a population.
Sufficient numbers of people must be present
Small bands of hunter-gatherers were not faced
with continuous exposure to endemic disease
Pathogens
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Any agents, especially microorganisms such as
viruses, bacteria, or fungi, that infect a host and
cause disease.
Smallpox
Pandemic
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An extensive outbreak of disease affecting large
numbers of individuals over a wide area;
potentially a worldwide phenomenon.
The Continuing Impact of Infectious
Disease
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Humans and pathogens exert selective pressures on
each other
Microorganisms evolve
A pathogen must not be so virulent as to kill its host
too quickly
Selection acts to produce resistance in host
populations and/or to reduce the virulence of disease
organisms
HIV
Why It Matters
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One goal of the human genome project is to find
DNA variants associated with disease and to design
treatments that target those genes.
Because some of these variants cluster in certain
populations, there have been efforts to identify
ancestry to predict risks.
This has been referred to as race-based medicine.
Why It Matters
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The result has been the use of the imprecise term race to
design treatment protocols.
There are several problems with this effort:
 Clinically observed or self-identification of race or ethnicity
is often not congruent with genetic profiles.
 If treatment is assigned for a person based on self-reported
race rather than a genetic test, serious illnesses may be
missed.
 The use of race as a basis for treatment ignores real
differences that lead to ill health.
Why It Matters
Example:
 A drug is designed to treat hypertension in African
Americans.
 If the drug is automatically prescribed and other
drugs are not considered, the patient may not benefit
at all.
 If the drug is the best choice for a white person, but is
viewed as an “African American drug,” it may not be
prescribed as needed.
Why It Matters
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Categorizing drugs along racial lines is likely to lead
to the same problems that resulted from categorizing
people into racial groups.
This doesn’t mean that the quest for underlying
genetic factors involved in disease should be halted;
it just means that the search should focus on gene and
gene complexes rather than race.
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
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The following is a true statement:
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Distributions of alleles for a single genetic trait do
not conclusively demonstrate genetic relationships
between populations.
2. One of the results of DNA research has been the
discovery that:
a) Many variations exist in the DNA of the human
genome.
b) Everyone has their own unique arrangement that
defines their distinctive “DNA fingerprint”.
c) Human variation is being researched at a
microevolutionary level.
d) All of the above.
Answer: d
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All of the above are result of DNA research.
3. Human intelligence is a result of genetic and
environmental factors.
a)
b)
True.
False.
Answer: a
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True. Human intelligence is a result of genetic and
environmental factors.
4. The pigment which helps protect against ultraviolet
radiation by absorbing it is
a)
b)
c)
d)
carotene.
lactose.
hemoglobin.
melanin.
Answer: d
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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)
b)
c)
d)
Bergmann's rule.
Allen's rule.
Gloger's rule.
Kleiber's rule.
Answer: a
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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.