Transcript Chapter_12_ol1

Chapter 12
Human Variation and Adaptation
• How do you define “race” and do you think it’s 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?
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.
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
Intelligence
• Genetic and environmental factors contribute to
intelligence.
• IQ scores change during a person’s lifetime
• Cognitive abilities are polygenic -- measured by
many genes
• Nature and Nurture!
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.
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.”
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…
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.
Frequencies of Lactose Intolerance
Population Group
Percent
U.S. whites
2–19
Finnish
48
Swiss
12
Swedish
4
Frequencies of
Lactose Intolerance
Population Group
U.S. blacks
Ibos
Bantu
Fulani
Thais
Asian Americans
Native Americans
Percent
70–77
99
90
22
99
95–100
85
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.
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.
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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).
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:
1. Long-term evolutionary changes
characterize all individuals within a
population or species.
2. Short-term, temporary physiological
response is called acclimatization.
Skin Color
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.
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 didn’t wear clothing that
would have protected them from the sun.
• Since this is how we evolved, when don’t get
enough sun, we may develop…
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.
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)
Human Response to Heat
• Long-term adaptations to heat evolved in our
ancestors:
▫ Sweat Glands
▫ Vasodilation – capillaries near skin’s surface widen
Human Response to Cold
• Short-term responses to cold:
▫ Metabolic rate and shivering
▫ Vasoconstriction – to reduce blood flow to
the skin
High Altitude
• Multiple factors produce stress on the human
body at higher altitudes:
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Hypoxia (reduced available oxygen)
Intense solar radiation
Cold
Low humidity
Wind (which amplifies cold stress)
▫ Bergmann’s and Allen’s rules...
Bergmann’s 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.
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.