WEEK 7 Discussion Forum--Anthropology 1( need high quality assignment!! And due on time !!!)
CHAPTER 12 Human Variation and Adaptation
Historical Views of Human Variation
• Biological determinism - cultural and biological variations are
inherited in the same way as genetic variation.
• Eugenics - "race improvement" through forced sterilization of
members of some groups and encouraged reproduction among
others.
• Eugenic science was taken to a new level during the holocaust, but the
root of modern eugenics science began in the U.S., especially here in
California
"It is better for all the world, if instead of waiting to execute degenerate
offspring for crime, or to let them starve for their imbecility, society can
prevent those who are manifestly unfit from continuing their kind…. Three
generations of imbeciles are enough.“ ~Supreme Court Justice Oliver
Wendell Holmes
Traditional Concept of Race
• Since the 1600s, race has been used to refer to culturally
defined groups.
• Groups that are now often confused with nationality or ethnicity
• Race is generally understood to be a “real,” concrete, and definable
aspect of human biology
• Race is used as a biological term, but is a social construct
which has enormous social significance.
• Connections of aspects such as morality or intelligence to race
Modern Racial Theory
• In any racial group, there will be individuals who fall into the
normal range of variation for another group for one or several
characteristics.
• Human phenotypical expression is so varied and overlapping it
is impossible to draw any clear lines that would definitively
categorize all people
• For example light skinned Africans are often more fair than some Indians
and Asians, and there are dark skinned Indian populations. Many
Mediterranean and Middle Eastern populations are darker skinned than
Non-white populations, yet they are racially “white”
• Modern science has discarded the out of date concept of “race”
all together
Intelligence
• 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.
Polytypic
• 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.
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.
Population Genetics
The study of the frequency of alleles, genotypes, and phenotypes in populations from a micro- evolutionary 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
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.
Acclimatization
• 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.
• Developmental Acclimatization happens during growth and leads to
permanent changes such as heart and lung capacity
• Because it is under genetic influence, acclimatization is
subject to natural selection and genetic drift.
• Tibetans carry several mutations that are not found in other populations
which make them fit for their environment, this is adaptation
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.
• For example Slash and burn agriculture and sickle cell
Sickle Cell Allele
• 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
• 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.
Lactose Intolerance/Persistence
• 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 Percent
U.S. blacks 70–77
Ibos 99
Bantu 90
Fulani 22
Thais 99
Asian Americans 95–100
Native Americans 85
Distribution of Skin Color in Indigenous
Populations
Selective Advantages of Dark Skin
• Ultraviolet Rays penetrate the skin and can eventually damage
DNA within skin cells.
• The three major types of cells that can be affected are squamous cells,
basal cells, and melanocytes.
• 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
• Protects from folate degradation
• Especially important for pregnant mothers in preventing
significant neural tube (the anatomical structure that
develops to form the brain and spinal cord) birth defects
such as spina bifida
Selective Advantages of Light Skin
• Vitamin D absorption is hindered by darker sink and
greater amounts of clothing necessary in northern climes
• Vitamin D is necessary for bone growth and deficiency
can lead to Ricketts
• Vitamin D is also linked to the production of 125D which
regulates many body functions including immune defense
• How/why were Inuit populations not affected by this
selective pressure?
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.
• This has led to a huge amount of adaptive
variation
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.
Bergmann and Allen’s Rules
(a) The Samburu are characteristic of many inhabitants of sub- Saharan Africa.
(b) The Inuit are shorter and stockier.
These individuals serve as examples of Bergmann’s and Allen’s rules.
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.
Human Response to Heat
• Long-term adaptations to heat evolved in our
ancestors:
• Sweat glands and lack of hair: evaporation cools our
bodies
• Capillaries near the skin’s surface widen to permit increased blood flow to the skin, vasodilation
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.
• 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.
Infectious Disease
• Vectors
• 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
• 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
Infectious Disease
• Pathogens
• Any agents, especially microorganisms such as viruses,
bacteria, or fungi, that infect a host and cause disease.
• Pandemic
• An extensive outbreak of disease affecting large
numbers of individuals over a wide area; potentially a
worldwide phenomenon.
Smallpox
The only disease to
date that science has
completely eradicated.
The Continuing Impact of Infectious Disease
• 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
Race-based Medicine.
• 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.
The Problems With This • 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.
• Categorizing drugs along racial lines is likely to lead to the same problems that resulted from categorizing people into racial groups.
• This means that research should focus on gene and gene complexes that underly genetic factors involved in disease, rather than race.