Human Variation and
Adaptation
Ancient Racial Classifications
Homer (fl. 1200 - 850 B.C.)
Iliad and Odyssey acknowledge variability
Aethiopians: People at the eastern and western edges of the known world
Cubit-men: African (?) pygmies
Herodotus (484?-425? B.C.)
Historiae argues for an environmental cause of variability between human groups
Egyptians have strong skulls due to exposure
Persian skulls are brittle due to the use of felt hats
Hippocrates (460 - 377 B.C.)
Environmental influences on human variability are noted in Volume I of Corpus Hippocraticum
Body build and temperment of different peoples are said to be related to their
climate and life style
Aristotle (384-322 B.C.)
Claims environmental causes of physical variation in humans
Wooly hair of Aethiopians due to arid climate
Straight hair of Scythians due to moist air
St. Augustine (354-430)
In De Civitate Dei Contra Paganos he says all men born everywhere, no matter
how
strange they appear to us, are descended from Adam, i.e., are descended from a single ancestral stock
Early Racial Classifications
Leonardo da Vinci (1452-1519)
Questioned the environmental hypothesis in accounting for human variation,
suggesting an early hereditarian argument based on the power of the mother's seed.
Andreas Vesalius (1514-1564)
Noted a relationship between race and the shape of the skull
Historical Views of Human
Variation
• Two schools of thought developed to
explain diversity:
– Monogenists believed that all humans were
descended from a original pair of humans.
– Polygenists believed that all humans were
descended from a number of pairs of humans.
Racism
• Based on false belief that intellect and
cultural factors are inherited with physical
characteristics.
• Uses culturally defined variables to typify
all members of particular populations.
• Assumes that one's own group is superior.
• A cultural phenomenon found worldwide.
Intelligence
• Genetic and environmental factors contribute to
intelligence.
• Many psychologists say IQ scores measure life
experience.
• Innate differences in abilities reflect variation
within populations, not differences between
groups.
• There is no convincing evidence that populations
vary in regard to intelligence.
Types of Human Variation
• Genetic variation
– Variation due to genes inherited from the
parents
• Environmental variation
– Variation caused by environmental factors such
as culture, climate, habitat, or competition from
other species
The Big Problem with
Understanding Human Variation
• It is very difficult to separate the influences
of genes, environment, and culture in the
variation of actual human individuals
• Variation WITHIN groups can have very
different causes than variation BETWEEN
groups
• The definition of a biological population in
modern humans is problematic
The differences
between two separate
lawns planted from the
same bag of seed must
be environmental.
However, if the seed
used was genetically
variable, the
differences within each
lawn could be genetic.
Influence of Environment
Figure 16.19
Height in British Soldiers
The height of men joining the British army in 1939 varied considerably, illustrating the
range of variation in morphological characters within populations. The tallest men joining
the army were more than 2 m (84 in., about 7 ft) tall, while others were less than 1.5 m (60
in., or 5 ft) tall.
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.
• "Racial traits" are not the only phenotypic
expressions that contribute to social identity: sex
and age are also critically important.
• In the 1950's the use of the term "race" was
replaced with "ethnicity “
Allele Frequencies
Within 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.
Researchers often use short pieces of DNA called Alu polymorphisms to determine whether various
populations are related to one another. Alus have no known function, yet they copy and insert
themselves at random throughout a person's genome. Because previously inserted Alus do not excise
themselves, Alu patterns can be used as yardsticks to estimate how close two people--and, on average,
two populations--are genetically.
Counting the number of DNA units called short tandem repeats on chromosomes
can allow scientists to group individuals according to probable ancestry. One such
repeat, AAAG, occurs between two and seven times in people with African heritage
but between five and eight times in those whose ancestors came from Europe or
the Middle East. (Every person inherits one set of repeats from their mother and
one from their father.)
(a) Evolutionary trees
for human populations
based on
morphological
similarity look very
different from (b) trees
based on genetic
similarity.
Figure 16.22
This tree, based on the frequencies of 120 genes in 42 populations from every part of the
globe, is consistent with the hypothesis that humans originated in Africa and spread from
there to the rest of the globe.
Figure 16.11
(a) This model of
the expansion of
early anatomically
modern human
populations
generates the
phylogenetic tree
shown in (b) if
geographically
separate
populations remain
genetically
isolated.
Figure 16.12
(a) This model of the
expansion of early
anatomically modern
human populations
generates the
phylogenetic tree shown
in (b). Because European
populations are assumed
to be descended from a
relatively recent mixture
of African and Asian
populations, the genetic
distances between
Europeans and Asians
and between Europeans
and Africans is smaller
than the distance
between Asians and
Africans.
Figure 16.14
This tree, based on genes from over 100 loci, indicates that European populations have
undergone less genetic change than Asian or African populations. The length of the path
between any two contemporary populations represents the genetic distance between them.
The fact that the genetic distance between African and Asian populations is much greater
than the distances between African and European populations or between Asian and
European populations indicates that European populations have experienced less genetic
change than other populations.
Figure 16.13
Distribution of Three Genetic Diseases
The distribution of three
genetic diseases (PKU,
cystic fibrosis, and TaySachs) illustrates the
existence of variation
among human groups.
Mendelian Genetics in Humans: SLI
The pattern of SLI in the KE family tree suggests that some cases of specific language
impairment (SLI) are caused by a single dominant gene. Circles represent women,
triangles represent men, and blue symbols represent people with SLI. If SLI is caused by
a dominant gene, then, since SLI is rare in the population as a whole, we know from the
Hardy-Weinberg equations that almost all people with SLI will be heterozygotes. Thus,
Mendel’s principles tell us that, on average, half of the offspring of a mating between a
person with SLI and a person without it will have SLI, and half the offspring will have
normal linguistic skills. Notice how well the family shown in this tree fits this prediction.
Groupings Used by Lewontin in Population Genetics
Study (1972)
Geographical Group
Caucasians
Black Africans
Examples of Populations Included
Arabs, Armenians, Tristan da
Cunhans
Bantu, San, U.S. blacks
Asians
Ainu, Chinese, Turks
South Asians
Andamanese, Tamils
Amerinds
Aleuts, Navaho, Yanomama
Oceanians
Easter Islanders, Micronesians
Australians
All treated as a single group
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 (genetic) evolutionary changes
characterize all individuals within a population
or species.
2. Short-term, temporary physiological response
is called acclimatization.
Skin Color
Influenced by three substances:
1. Hemoglobin, when it is
carrying oxygen, gives a
reddish tinge to the skin.
2. Carotene, a plant
pigment which the body
synthesizes into vitamin
A, provides a yellowish
cast.
3. Melanin, has the ability
to absorb ultraviolet
radiation preventing
damage to DNA.
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.
Skin Pigmentation
This map shows contours in skin color. Notice that there are smooth gradients away from
the equator.
Skin Color
Skin color varies
continuously when a
representative sample of the
world's populations is
analyzed. In other words, it
is impossible to establish
neat categories or "pigeon
holes" for populations or
individuals based on this
trait (continued below
image).
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.
Human Response to Heat
• Long-term adaptations to heat evolved in
our ancestors:
– Sweat Glands
– Vasodilation
• Bergmann's rule - body size tends to be
greater in populations that live in cold
environments.
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.
Climate and Body Size
People living in colder
climates have larger bodies.
The vertical axis plots mean
chest girth for numerous
human groups, and the
horizontal axis plots the mean
yearly temperature in the
regions in which each group
lives. Because chest girth is a
measure of overall size, these
data show that people living
in colder climates have larger
bodies.
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.
Malaria
• Falciparum malaria has historically been the
leading killer of children and older adults in
the southern hemisphere
• Malaria originated as an animal parisite
• Malarial strains have afflicted human
populations for many thousands of years
• Agriculture has significantly spread the
disease due to sedentism and irrigation
Distribution of Hemoglobin S
Hemoglobin S is only
common in areas of the
world in which falciparum
malaria is prevalent. (a) The
colors show the frequency of
hemoglobin S throughout the
world. (b) The regions of the
Old World in which
falciparum malaria is
prevalent are in red.
Balanced Polymorphism in hemoglobin S
The average
fitness of the
S allele of
hemoglobin S
declines as
the
frequency of
S increases
because more
and more S
alleles are
found in SS
homozygotes.
Similarly, the
average
fitness of the
A allele of
hemoglobin
A increases
as the
frequency of
S increases
because more
and more A
alleles are
found in AS
heterozygotes
Small Pox
• The only disease considered to be eliminated as a
result of medical technology (Polio is close,
except in Nigeria)
• 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.
Impact of Infectious Disease in
the West
• 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.
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.
The LAC*P gene could have spread in the 7000 years (300 to 350 generations) since the origin of
dairying if the ability to digest lactose as an adult leads to even as little as a 3% increase in fitness (s).
Figure 16.09
Dimensions of Stratification
• Power—control resources in one’s own
interest.
• Wealth—accumulation of material
resources or access to the means of
producing these resources.
• Prestige—social honor or respect.
Ascribed Vs. Achieved Status
• Ascribed Status
Social position into which a person is
born. (sex, race, kinship group)
• Achieved Status
Social position that a person chooses or
achieves. (professor, criminal, artist)
Social Class in the United States
• Status depends on occupation, education,
and lifestyle.
• “The American Dream,” is based on the
democratic principle of equality and
opportunity for all.
• Social class in the United States correlates
with attitudinal, behavioral, and lifestyle
differences.
Caste System
• System of stratification based on birth.
• Movement from one caste to another is not
possible.
• Castes are hereditary, endogamous, ranked
in relation to one another and usually
associated with a traditional occupation.
Hindu Caste System
Four caste categories
1. Brahmins - priests and scholars
2. Kshatriyas - ruling and warrior caste
3. Vaisyas - the merchants
4. Shudras - menial workers and artisans
5. Harijans – “untouchables”
U.S. Racial Stratification
Systems
• Race is constructed on the basis of skin
color and presumed ancestry.
• Divides people into “blacks” and “whites”
ignoring the reality of the skin color
spectrum.
• By the 20th century, the system of race in
the American south was very similar to the
caste system in India.
Race Stratification in the U.S.
and Brazil
• Two largest multiracial societies in the Americas.
• In both societies the legacy of slavery continues in
the form of racial inequality.
• Brazil: 45% of nonwhite families and 25% of
white families live below the poverty line.
• U.S.: 30% of nonwhite families and 8% of white
families live below the poverty line.
Figure 16.20
This contour map of overall genetic similarity is based on a sample of 120 genes from 42 populations
assembled by Cavalli-Sforza and his colleagues. The fact that the contours of equal genetic similarity
are roughly evenly spaced indicates that there is a smooth east-to-west gradient of overall genetic
similarity—there are no sharp boundaries between groups. Sharp boundaries would produce a map in
which many contour lines would be positioned closely together. This map is drawn from the same data
used to construct the tree shown in Figure 16.11.
The Genetics of Race ~ Summary
• The outward signs on which most definitions of race are
based--such as skin color and hair texture--are dictated by
a handful of genes. But the other genes of two people of
the same "race" can be very different. Conversely, two
people of different "races" can share more genetic
similarity than two individuals of the same race.
• Nevertheless, scientists can use genetics to sort most large
populations according to their ancestral geographic origin.
This approach does not work as well for populations
resulting from recent mixing with other groups, however.
• The medical implications of racial genetic differences are
still under debate.
The Social Reality of Race
“But genetics cannot prove that race doesn't
exist, [Troy] Duster explains. No amount of
logic will erase the concept or destroy the
disparities that arise from it, because people
use race to sort their social groupings and to
define their social and economic interactions.
Moreover, they do so in ways that have
significant biological consequences.”
Sally Lehrman, Scientific American Feb. 2003
Perspectives in Ethnicity
• Essentialist - ethnicity comes from
historical, demographic, and economic
conditions.
• Constructionalist - ethnicity comes
from responses to changing realities
within the group and in the society of
which it is a part.
European Colonial Laws
• Defined acceptable behavior.
• Established written penal codes,
constitutions, and western-style courts.
• Many colonial laws involved restraints
on sexuality.
U.S. Cultural Diversity
• Native Americans
• European immigrants - Dutch, Spanish,
French, and English.
• Africans - brought to the new colony
as slaves.
Ethnic Identify
• Changes as social and economic
circumstances change.
• Provides a basis for group solidarity
despite differences within the group.
Incorporating Immigrants Into
Society
• Assimilation model
• Melting pot model
• Mosaic Model
Assimilationist Model
• Immigrants should abandon traditions
and become absorbed in American
culture.
• Resulted in the building of urban
Settlement Houses, designed to teach
immigrants “American” ways.
Assimilationist and Immigration
• Supported minimal immigration to the U.S.
• Nations seen as similar to the U.S., Such as
England, were allowed almost unrestricted
immigration.
• Nations seen as different to the U.S., Such as
Greece and Poland, were allowed minimal
immigration.
• Immigration of Asians was all but completely
halted.
Assimilation and Native
Americans
• In the mid-19th century, forced onto
reservations, Indians became a captive
audience for the teaching of American
values.
• By early 1870s, it was clear that the
reservation policy had not transformed
Indians into mainstream Americans.
Melting Pot Model
• Immigrants will melt together into a new
American culture.
• By the late 1950s, it was clear that the
melting pot theory had only limited
application.
• Excluded Asians, Native Americans,
Mexican Americans, and African
Americans.
Mosaic Model
• Cultural diversity is a positive aspect of
American national identity.
• Arose in response to the swell of
immigration in the past 25 years.