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Chapter 16
Modern Human Biology:
Patterns of Adaptation
Chapter Outline
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The Adaptive Significance of Human
Variation
Infectious Disease
The Continuing Impact of Infectious
Disease
New Frontiers in Research: Molecular
Applications in Modern Human Biology
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 environmental change.
Because it is under genetic influence,
acclimatization is subject to evolutionary
factors such as natural selection or genetic
drift.
Geographical Distribution of Skin
Color in Indigenous Populations
Ultraviolet Light and Skin Cells
Skin Color
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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.
Three Points: Skin Color and
Evolution
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2.
3.
Early hominids 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 hominids didn’t wear clothing that would
have protected them from the sun.
Folate and UV Radiation
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Folate is a B vitamin that isn’t stored in
the body and must be replenished
through dietary sources.
In pregnant women, insufficient levels of
folate are associated with numerous fetal
developmental disorders, including neural
tube defects such as spina bifida.
Folate and UV Radiation
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Studies have shown that UV radiation depletes
folate serum levels both in laboratory
experiments and in light-skinned individuals.
These findings have implications for pregnant
women, for children, and for the evolution of
dark skin in early hominids.
Question
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Which of the following has not been
linked with light skin color in humans?
a) cloudy skies
b) folate
c) all of these choices
d) clothing
Answer: c
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None of the following has been linked with
light skin color in humans:
cloudy skies
folate
clothing
Vitamin D Synthesis
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
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.
Examples of Bergmann’s and
Allen’s Rules
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(a) This African
woman has the linear
proportions
characteristic of
many inhabitants of
sub-Saharan Africa.
(b) By comparison,
the Inuit woman is
short and stocky.
Human Response to Cold
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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.
Vasodilation
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Expansion of blood vessels, permitting
increased blood flow to the skin.
Vasodilation permits warming of the skin
and facilitates radiation of warmth as a
means of cooling.
Vasodilation is an involuntary response to
warm temperatures, various drugs, and
even emotional states (blushing).
Vasoconstriction
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Narrowing of blood vessels to reduce
blood flow to the skin.
Vasoconstriction is an involuntary
response to cold and reduces heat loss at
the skin’s surface.
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.
Question
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The number one cause of death among
humans until the 20th Century was:
a) AIDS.
b) TB.
c) malaria.
d) infectious diseases.
Answer: d
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The number one cause of death among
humans until the 20th Century was
infectious diseases.
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.
Zoonotic
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Pertaining to a zoonosis, a disease that’s
transmitted to humans through contact
with nonhuman animals.
Antibodies
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Proteins that are produced by immune cells and
serve as major components of the immune
system.
Antibodies recognize and attach to foreign
antigens on bacteria, viruses, and other
pathogens.
Pathogens are substances or
microorganisms, such as bacteria, fungi, or
viruses, that cause disease.
Environmental Factors
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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.