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.
1.
2.
3.
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:
▫
▫
▫
▫
▫
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.