Human variation
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Transcript of Human variation
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SOC111INTRODUCTION TO ANTHROPOLOGY
Human Variation and Adaptation
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Human Variation and Adaptation
• What is the race concept, and why
have anthropologists rejected it?
• How does natural selection
work on contemporary and
recent human populations?
• Does biological adaptation occur
during an individual’s lifetime?
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Race: A Discredited
Concept in Biology
• Historically, scientists approached the
study of human biological diversity in two
ways:
– Racial classification, now largely rejected
– Explanatory approach that focuses on
understanding specific differences
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Race: A Discredited
Concept in Biology
Racial classification is the attempt toassign humans to discrete categories
(purportedly) based on commonancestory.
Biological differences are real, importantand apparent. But not a source to
categorize people into race groups.
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Race: A Discredited
Concept in Biology
• Race refers to a geographically
isolated subdivision of a species
–Human populations have not
been isolated enough from one
another to develop into discrete
groups
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Race: A Discredited
Concept in Biology
• Phenotypic traits (skin color) have been used for racial classification
White
Black
Yellow
– This overly simplistic classification was compatible with the political use of race during the colonial period.
– Race kept white Europeans separate from African, Asian, and Native American subjects.
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Races Are Not Biologically Distinct
• Problems with using a tripartite scheme
– “Color based” racial labels are not accurate.
• Caucasoid, Negroid, Mongoloid
– Many populations don’t fit neatly into any one
of the three “great races.”
– No single trait can be used as a basis for
racial classification.
– Phenotypic similarities and differences do not
necessarily have a genetic basis.
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Genetic Markers Don’t Correlate with Phenotype
– The analysis of human DNA indicates that 94
% of human genetic variation occurs within
“races”.
– There is only 6 % variation between
conventional geographic “racial” groupings
(Africans, Asians and Europeans).
– There is much greater variation within each of
traditional “races” than between them.
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Genetic Markers Don’t Correlate with Phenotype
– Phenotypical similarities and differences
are not precisely or necessarily
correlated with genetic relationships.
– Because of environment that affect
individuals during growth and
development, the range of phenotypes
characteristic of a population maychange without any genetic change
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• Traditional racial classification assumes
biological characteristics are determined
by heredity and were stable over many
generations.
• Role of natural selection in
producing variation in skin
color illustrates an explanatory
approach to human biological diversity.
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Explaining Skin Color
• Skin color biological trait is influenced by
several genes.
– Melanin: a natural sun screen produced
by skin cells responsible for pigmentation
– By screening out ultraviolet (UV) radiation
from sun, melanin offers protection
against a variety of maladies, including
sunburn and skin cancer.
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How can we explain the geographic distribution of skin color?
• Prior to the16th century, very dark skinned
populations lived in the tropics: a belt
extending about 23 degrees north
and south of the equator.
– Outside the tropics, skin
color tends to be lighter.
– Melanin confers a selective
advantage on darker-skinned
people living in the tropics.
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Explaining Skin Color
• Geographic distribution of skin color
involved effects of UV on folate.
• Folate is needed for cell division and
the production of new DNA.
• Folate deficiency can cause male
sterility.
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• The discussion of skin color shows that, common ancestry is not the only reason for biological similarities.
• Natural selection makes a major contribution to variations in human skin color, as well as to many other human biological differences and similarities.
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Human Biological Adaptation
• Abundant evidence exists for human
genetic adaptation and evolution
through selection working in specific
environments
• With thousands of human genes
known, new genetic traits are being
discovered every day.
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Genes and Disease
• According to the World Health Report,
tropical diseases affect more than 10
percent of the world’s population.
– Malaria: 350 million to 500 million
people
– Schistosomiasis (snail fever): more than
200 million
– Filariasis: 120 million
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Genes and Disease
• Microbes were the major selective agent
for humans, particularly before
the arrival of modern medicine.
– After food production emerged
10,000 years ago, infectious diseases posed
a mounting risk and became the foremost
cause of human mortality.
– ABO blood groups vary in their resistance to
disease.
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Genes and Disease
• Smallpox had been a major threat to humans and a determinant of blood frequencies until 1977.
• The A B 0 blood groups have figured in human resistance to smallpox.
• People with A or AB blood type are more susceptible to smallpox than are people with type B or type 0.
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Genes and Disease
• Associations between A B 0 blood types and noninfectious disorders also have been noted.
• Type 0 – duodenal and gastric ulcers.
• Type A – stomach and cervical cancer and ovarian tumors.
• However, since these noninfectious disorders tend to occur after reproduction has ended, their relevance to adaptation and evolution through natural selection is doubtful.
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Facial Features
• Natural selection also
affects facial features.
– Long noses seem to be adaptive
in arid areas and cold environments.
– Thomson’s Nose Rule: There is an
association between nose form and
temperature for those who have lived for
many generations in the areas they now
inhabit.
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Size and Body Build
• Bergmann’s rule: The smaller of two
bodies similar in shape has more surface
area per unit of weight.
Within the same species of warm-blooded
animals, populations having smaller
individuals are found more in warm climates.
• Allen’s rule: Relative sizes of protruding
body parts increase with temperature.
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Phenotype Differences
• Tropical climates- slender bodies with long limbs are advantageous.
• North polars – Eskimos with short limbs and stocky bodies.
• Altitudes also affects phenotypes.
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Lactose Tolerance
• Another difference between human groups.
• Lactose tolerence is important factor for survival when other foods are scarce and milk is available.
• Lactose tolerance appears to be one of many aspects of human biology governed both by genes and by phenotypical adaptation to environmental conditions.