Human Variation and Adaptation

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Human Variation and Adaptation

• The Concept of Race and
• Patterns of Variation

2
Introduction

• This chapter deals with past and current views
  concerning human phenotypic variation.
• The traditional view of human variation focused
  on attempts to classify humans into races.
• Now, we tend to focus on the genetics and
  adaptive significance of human variation.

3
Historical Views of Human Variation

• Because differences in skin color were apparent
  when different human groups came into contact,
  skin color became one of the most common traits
  of racial classification.
• Johann Friedrich Blumenbach (1752-1840)
  classified humans into four races.
• He used skin color (white, yellow, brown, and
  black) as well as other traits, but emphasized
  that humans do not fall neatly into these
  categories.

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• Believing that head shape was a stable racial
  trait, Anders Retzius developed the cephalic
  index in 1842 to describe the shape of the head.
• Dolichocephalic-
• A head with a width that measures less than 75
  of its length.
• Mesocephalic-
• A head with a width that measures between 75 to
  80 of its length.
• Brachycephalic-
• A head with a width that is more than 80 of its
  length.
• The cephalic index ceased to be used as a racial
  trait when it was discovered that allegedly
  superior dolichocephalic northern Europeans
  shared a similar head shape with some so-called
  inferior African populations.

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Historical Views of Human Variation

• 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.

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• After 1850 biological determinism became a
  dominant theme in the European and American
  schools of racial classification.
• Charles Darwins cousin, Francis Galton, wrote
  and lectured on eugenics, the theme of race
  improvement through selective breeding.
• The Eugenics movement became very popular in
  America throughout the 1930s, and the Eugenics
  movement in Germany formed the basis of Nazi
  ideas of racial purity.
• The 1930s synthesis of natural selection with
  Mendelian genetics caused many physical
  anthropologists to apply evolutionary principals
  to the study of human variation.

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Traditional Concept of Race

• 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.
• Racial traits are not the only phenotypic
  expressions that contribute to social identity
  sex and age are also critically important.
• In the 1950s the use of the term race was
  replaced with ethnicity

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Contemporary Interpretations of Human
VariationHuman Polymorphisms

• Contemporary studies of human variation focus on
  polymorphisms
• characteristics that have different phenotypic
  expressions (the genetic locus governing the
  trait has two or more alleles).
• Populations often differ with respect to their
  allele frequencies for polymorphic traits and
  this requires evolutionary explanations.
• A popular alternative to the racial approach is
  the study of the clinal (cline) distributions of
  polymorphic traits.
• Clinal distributions are thought to reflect the
  microevolutionary processes of gene flow and/or
  natural selection.

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Polymorphisms

• Those traits that differ in expression among
  various populations and between individuals.
• Loci with more that one allele.
• They can be expresses in the phenotype as the
  result of gene action.
• As in ABO.

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Cline

• A gradual change in the frequency of genotypes
  and phenotypes from one geographical region to
  another.

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Allele FrequenciesWithin and Between Populations

• After World War II, the study of human variation
  shifted to the study of differences in allele
  frequencies within and between populations.
• The application of evolutionary principles to
  human variation has replaced the view that was
  based solely on observed phenotype.
• Races are no longer viewed as fixed biological
  entities, composed of individuals fitting a
  particular type.

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Groupings Used by Lewontin in Population Genetics
Study (1972)
13
Human Polymorphism

• The ABO system is of anthropological interest
  because its three alleles vary among human
  populations and their frequencies follow a clinal
  distribution.
• The A or B allele rarely reach frequencies of 50
  in human populations.
• South American Indian populations have
  frequencies of O that approach 100.
• High frequencies of O are also found in northern
  Australians.
• These unusually high frequencies may be the
  product of genetic drift.

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Distribution of B allele in Indigenous Populations
15
Genetic PolymorphismsUsed to Study Human
Variation
16
Population Genetics Research
17
Environmental Factors

• 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.

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Adaptive Significance of Human Variation

• Human variation is the result of adaptations to
  environmental conditions.
• Physiological response to the environment
  operates at two levels
• Long-term (genetic) evolutionary changes
  characterize all individuals within a population
  or species.
• Short-term, temporary physiological response is
  called acclimatization.

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Pigmentation and Geographical Divisions

• Before 1500, skin color in populations followed a
  geographical distribution, particularly in the
  Old World.
• Populations with the greatest amount of
  pigmentation are found in the tropics.
• Populations with lighter skin color are
  associated with more northern latitudes.

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Distribution of Skin Color Among Indigenous
Populations
21
Skin Color

• 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.

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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.

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The Relationship Between Solar Radiation and Skin
Color

• An example of adaptation and natural selection.
• Skin color in native populations follows a
  particular geographic pattern
• the darkest skin is found in populations living
  in the tropics
• lighter skin tones are found in more northern
  latitudes.
• Skin color is primarily influenced by the pigment
  melanin, which is produced in the epidermis by
  melanocytes.
• Although all humans possess about the same number
  of melanocytes, human populations differ with
  respect to the amount and size of the melanin
  granules produced by the melanocytes.

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• Because melanin absorbs ultraviolet (UV)
  radiation, it provides protection against the
  damaging effects of ultraviolet radiation such as
  skin cancer.
• Natural selection has favored dark skin near the
  equator because of the high levels of UV
  radiation there.
• An additional benefit of darkly pigmented skin is
  that it prevents degradation of folate, an
  important B vitamin, by UV radiation.
• Heavily pigmented skin would have been adaptive
  to early hominids living in Africa, but as
  hominids migrated out of Africa the environmental
  factors changed.
• Hominids living in northern Eurasia encountered
  lower levels of UV radiation and the selective
  pressure for heavily pigmented skin was relaxed.

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Vitamin D hypothesis.

• The need for vitamin D production in the skin
  (through the interaction of UV radiation and a
  cholesterol-like compound in skin cells) to
  protect individuals from the deleterious effects
  of rickets was probably the most significant
  selective force for depigmentation.

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Production of Vitamin D
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Human Response to Heat

• Long-term adaptations to heat evolved in our
  ancestors
• Sweat Glands
• Vasodilation
• the widening of the capillaries near the surface
  of the skin to permit increased blood flow, is
  another mechanism for radiating body heat
• Bergmann's rule - body size tends to be greater
  in populations that live in cold environments.

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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.

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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)

30
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.

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Small Pox

• The only disease considered to be eliminated as a
  result of medical technology
• Smallpox has a higher incidence in those with
  type A or AB than in those type O blood.
• The immune systems of individuals with type A
  antigen may not recognize the small pox antigen
  as a threat.

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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.

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Impact of Infectious Disease

• 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.

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Zoonoses and Human Infectious Disease