Human Variation and Adaptation

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• Human Variation and Adaptation

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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
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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
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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.

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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.

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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.

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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

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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

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Influence of Environment
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.
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Figure 16.19
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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.
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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.
• "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

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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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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.
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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.)
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Figure 16.22
(a) Evolutionary trees for human populations
based on morphological similarity look very
different from (b) trees based on genetic
similarity.
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Figure 16.11
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.
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Figure 16.12
(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.
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Figure 16.14
(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.
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Figure 16.13
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.
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Distribution of Three Genetic Diseases
The distribution of three genetic diseases (PKU,
cystic fibrosis, and Tay-Sachs) illustrates the
existence of variation among human groups.
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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,
Mendels 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.
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Groupings Used by Lewontin in Population Genetics
Study (1972)
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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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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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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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Skin Pigmentation
This map shows contours in skin color. Notice
that there are smooth gradients away from the
equator.
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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).
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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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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.

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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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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.
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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)

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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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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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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

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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.
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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. A balanced
polymorphism occurs when the average fitness of
the two alleles is equal.
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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.

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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.

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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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Figure 16.09
The LACP 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).
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Dimensions of Stratification

• Powercontrol resources in ones own interest.
• Wealthaccumulation of material resources or
  access to the means of producing these resources.
• Prestigesocial honor or respect.

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Ascribed Vs. Achieved Status

• Ascribed StatusSocial position into which a
  person is born. (sex, race, kinship group)
• Achieved StatusSocial position that a person
  chooses or achieves. (professor, criminal, artist)

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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.

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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.

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Hindu Caste System

• Four caste categories
• Brahmins - priests and scholars
• Kshatriyas - ruling and warrior caste
• Vaisyas - the merchants
• Shudras - menial workers and artisans
• Harijans untouchables

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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.

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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.

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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 similaritythere 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.
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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.

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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
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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.

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European Colonial Laws

• Defined acceptable behavior.
• Established written penal codes, constitutions,
  and western-style courts.
• Many colonial laws involved restraints on
  sexuality.

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U.S. Cultural Diversity

• Native Americans
• European immigrants - Dutch, Spanish, French, and
  English.
• Africans - brought to the new colony as slaves.

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Ethnic Identify

• Changes as social and economic circumstances
  change.
• Provides a basis for group solidarity despite
  differences within the group.

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Incorporating Immigrants Into Society

• Assimilation model
• Melting pot model
• Mosaic Model

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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.

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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.

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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.

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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.

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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.