Human Variation and Adaptation

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Chapter 12

• Human Variation and Adaptation

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• How do you define race and do you think its a
  useful concept in understanding variation in our
  species?
• Are observable characteristics (i.e. skin color
  and color of eyes) as important as the
  fundamental differences among humans that are
  defined by evolutionary processes?

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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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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.
• In any racial group, there will be individuals
  who fall into the normal range of variation for
  another group for one or several characteristics.
• Polytypic species

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Intelligence

• Genetic and environmental factors contribute to
  intelligence.
• IQ scores change during a persons lifetime
• Cognitive abilities are polygenic -- measured by
  many genes
• Nature and Nurture!

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Human Polymorphisms

• Genetic traits with different phenotypic
  expressions
• A cline is a gradual change in the frequency of a
  trait or allele in populations dispersed over
  geographical space.
• Clinal distributions are thought to reflect
  natural selection and/or gene flow.

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Polymorphisms at the DNA Level

• Scattered through the human genome are
  microsatellites, sites where DNA segments are
  repeated.
• Each person has a unique arrangement that defines
  their distinctive DNA fingerprint.

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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.
• Lactose intolerance

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Lactose Intolerance

• 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.

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Frequencies of Lactose Intolerance
Population Group Percent
U.S. whites 219
Finnish 48
Swiss 12
Swedish 4
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Frequencies ofLactose Intolerance
Population Group Percent
U.S. blacks 7077
Ibos 99
Bantu 90
Fulani 22
Thais 99
Asian Americans 95100
Native Americans 85
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Population Genetics

• The study of the frequency of alleles, genotypes,
  and phenotypes in populations from a
  microevolutionary 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.

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Hardy-Weinberg Equilibrium

• The mathematical relationship expressing the
  predicted distribution of alleles in populations
  the central theorem of population genetics.
• Provides a tool to establish whether allele
  frequencies in a human population are changing.
• New variation (i.e., mutation)
• Redistributed variation (i.e., gene flow or
  genetic drift)
• Selection of advantageous allele combinations
  that promote reproductive success (i.e., natural
  selection).

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Adaptive Significance of Human Variation

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

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Skin Color
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Ultraviolet Rays

• 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.

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UV Radiation

• 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 didnt wear clothing that would
  have protected them from the sun.
• Since this is how we evolved, when dont get
  enough sun, we may develop

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Rickets

• Insufficient amounts of vitamin D during
  childhood result in rickets, a condition that
  often leads to bowing of the long bones of the
  legs and deformation of the pelvis.

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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.
• This is due to both Culture.and Biology!
• Biology part (next slide)

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Human Response to Heat

• Long-term adaptations to heat evolved in our
  ancestors
• Sweat Glands
• Vasodilation capillaries near skins surface
  widen

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Human Response to Cold

• Short-term responses to cold
• Metabolic rate and shivering
• Vasoconstriction to reduce blood flow to the
  skin

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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)
• Bergmanns and Allens rules...

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Bergmanns Rule

• Body size tends to be greater in populations that
  live in cold environments.
• 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.

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Allens 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.