AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011.
Chapter Twelve: Human Variation and Adaption
Pages # 314 - 333
- Physical anthropologists use the approach of population genetics to interpret
microevolutionary patterns of human variation.
- Population genetics is the study of the frequency of alleles, genotypes, and phenotypes in
populations from a microevolutionary perspective.
- Attempts to identify the various factors that cause allele frequencies to change over
- Gene pool is the total complement of genes shared by the reproductive members of a
- As a rule, a population is the group within which individuals are most likely to ﬁnd
- In every generation, the genes in a gene pool are mixed by recombination and then
reunited with their counterparts through mating.
- What emerges in the next generation is a direct product of the genes going into
the pool, which in turn is a product of who is mating with whom.
- Factors that determine mate choice are geographical, ecological, and social.
- Breeding isolates are populations that are clearly isolated geographically and/or socially from
other breeding groups.
- Geography plays a dominant role in producing these isolates by strictly determining the
range of available mates.
- Even within these limits, cultural rules can play a deciding role by prescribing who is
most appropriate among those who are potentially available.
- Inhabitants of large cities may appear to be members of a single population, but within the city,
socioeconomic, ethnic, and religious boundaries crosscut in complex ways to form smaller
- In addition to being members of these smaller local populations, we’re also members of
overlapping gradations of larger populations: the immediate geographical region (a
metropolitan area or a state), a section of a country, a nation, and ultimately the entire
- To determine whether evolution is taking place at a given locus, population geneticists
measure allele frequencies for speciﬁc traits and compare these observed frequencies with a
set predicted by a mathematical model called the Hardy-Wienberg equilibrium equation.
- Hardy-Weinberg Equilibrium is the mathematical relationships expressing, under
conditions in which no evolution is occurring, the predicted distribution of alleles in
populations; the central theorem of population genetics.
- Provides a tool to establish whether allele frequencies in a population are
- Factors that act to change allele frequencies:
- 1. New variation (that is, new alleles produced by mutation)
- 2. Redistributed variation (that is, gene ﬂow or genetic drift)
- 3. Selection of “advantageous” allele combinations that promote reproductive success
(that is, natural selection)
The Adaptive Signiﬁcance of Human Variation
- To survive, all organisms must maintain the normal functions of organs, tissues, and cells
within the context of an ever-changing environment.
- Physical activity also places stress on physiological mechanisms.
1 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011.
- Stress in a physiological context, any factor that acts to disrupt homeostasis; more
precisely, the body’s response to any factor that threatens its ability to maintain
homeostasis. (examples: temperature, wind, solar radiation, humidity, etc)
- The body must accommodate these changes by compensating in some manner to
maintain internal constancy, or homeostasis, and all life-forms have evolved
physiological mechanisms that, within limits, achieve this goal.
- Homeostasis is a condition balance, or stability, within a biological system,
maintained by the interaction of physiological mechanisms that compensate for
changes (both external and internal).
- Physiological response to environmental change is inﬂuenced by genetic factors.
- Examples of long-term adaptations in humans include physiological responses to heat
(sweating) and excessive levels of ultraviolet light (deeply pigmented skin in tropical
- Acclimatization is physiological responses to changes in the environment that occur during
an individual’s lifetime. Such responses may be temporary or permanent, depending on the
duration of the environmental change and when in the individual’s life it occurs. The capacity
for acclimatization may typify an entire species or population, and because it’s under genetic
inﬂuence, it’s subject to evolutionary factors such as natural selection and genetic drift.
- These responses to environmental factors are partially inﬂuenced by genes, but some
can also be affected by the duration and severity of the exposure, technological buffers
(such as shelter or clothing), and individual behaviour, weight, and overall body size.
- The simplest type of acclimatization is a temporary and rapid adjustment to an
environmental change (e.g. tanning).
- Developmental acclimatization is irreversible and results from exposure to an
environmental challenge during growth and development.
Solar Radiation & Skin Colour
- Skin colour is a commonly cited example of adaptation through natural selection in humans.
- Populations with the greatest amount of pigmentation are found in the tropics, while light skin
colour is associated with more northern latitudes, particularly the inhabitants of northwestern
- Skin colour is mostly inﬂuenced by the pigment melanin, a granular substance produced by
specialized cells (melanocytes) found in the epidermis.
- All humans have approximately the same number of melanocytes.
- It’s the amount of melanin and the size of the melanin granules that vary.
- Melanin is important because it acts as a built-in sunscreen by absorbing potentially
dangerous ultraviolet rays present in sunlight.
- Melanin protects us from overexposure to UV radiation, which can cause
genetic mutations in skin cells.
- These mutations may lead to skin cancer.
- Exposure to sunlight triggers a protective mechanisms in the form of tanning, the result of
temporarily increased melanin production (acclimatization).
- This is response occurs in all humans except albinos (genetically mutated).
▯ Dark Skin
- Natural selection has favored dark skin in areas nearest the equator, where the sun’s
rays are the most direct / UV is intense.
- Cancer-causing effects of UV radiation from an evolutionary perspective, 3 points to
keep in mind:
2 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011.
- 1. Early hominins lived in the tropics, where solar radiation is more intense than
in temperature areas to the north and south.
- 2. Unlike modern city dwellers, early hominins spent their days outdoors.
- 3. Early hominins didn’t wear clothing that would have protected them from the
- Given these conditions, UV radiation was probably a powerful agent
selecting for high levels of melanin production in early humans.
- Folate isn’t stored in the body and therefore must be replenished through dietary
sources. Folate deﬁciencies in pregnant women are associated with numerous
complications, including maternal death, and neural tube defects.
- Neural tube is in early embryonic development, the anatomical structure that develops
to form the brain and spinal cord.
- Consequences can include pain, infection, paralysis, and even death.
- Some studies have shown that UV radiation rapidly depletes folate serum levels in fair-
- Implications for pregnant women and children and also for evolution of
dark skin in hominins.
- Jablonski ad Chaplin suggest that the earliest hominins may
have had light body skin covered with dark hair, but a loss of
body hair in hominins occurred, dark skin evolved rather quickly
as a protective response to the damaging effects of the sun.
- Skin cancer and the maintenance of sufﬁcient levels of folate have no doubt been
selective agents favoring dark skin in humans living where UV radiation is most
▯ Lighter Skin
- The populations as hominins migrated out of Africa and Asia into Europe, encountered
cold temperatures and cloudy skies, frequently during the summer as well as in the
winter. Winter also meant fewer hours of daylight, solar radiation is very indirect.
- People in these areas wore animal skins and other types of clothing, which blocked the
- Relaxed selection for dark skin due to reduced exposure to sunlight.
- The need for adequate amounts of vitamin D.
- The theory called the vitamin D hypothesis.
- Vitamin D is produced in the body partly as a result of the interaction between
UV radiation & a substance similar to cholesterol. Also found in some foods.
- VItamin D is necessary for normal bone growth and mineralization, and some
exposure to UV radiation is therefore essential.
- An insufﬁcient amount of vitamin D during childhood results in rickets, a
condition that often leads to bowing of the long bones of the legs and
deformation of the pelvis.
- In addition to its role in bone mineralization, vitamin D is critical to numerous other
biological processes. Through its conversion to 1,25D it can attach directly to DNA,
- Jablonski and Chapin have also concluded the support that vitamin D hypothesis to the
point of stating that the requirement for vitamin D synthesis in northern latitudes was as
important to natural selection as the need for protection from UV radiation in the
3 AR103 - Readings pgs. 314-333 - Week 7 Notes - Oct. 30, 2011.
- The fact that lighter skin developed in two hominin species, but through different
mutations in the same gene, strongly reinforces the theory that there is a signiﬁcant
selective advantage to lighter skin in higher latitudes.
- Except for a person’s sex, more social importance has been attached to variation in
skin colour than to any other single human biological trait.
The Thermal Environment
- Mammals and birds have evolved complex mechanisms to maintain a constant internal body
- While reptiles rely on exposure to external heat sources to raise body temperature and
energy levels, mammals and birds have sources to raise body temperature and energy
levels, mammals and birds have physiological mechanisms that, within certain limits,