Download Chapter 3

Genes, Evolution,
and Environment
Chapter 3
Copyright © 2010 Pearson Education Canada
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Chapter Outline
• Unlocking the Secrets of Genes
• The Genetics of Similarity
• Our Human Heritage: Language
• Our Human Heritage: Courtship & Mating
• The Genetics of Difference
• Our Human Diversity: Intelligence
• Beyond Nature versus Nurture
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Where Do Differences
Come From?
• Previously two perspectives:
– Nativists: emphasized genes & inborn
characteristics (nature)
– Empiricists: focused on learning & experience
(nurture)
• Now studied broadly, including evolutionary
psychology and behavioural genetics
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Unlocking the Secrets of Genes
• Genes are the basic units of heredity that
are composed of DNA and located on
chromosomes
– Chromosomes: rod-shaped structures found in the
nucleus of every cell (23 pairs)
– DNA (deoxyribonucleic acid): chromosomal molecule
that transfers genetic characteristics by way of coded
instructions for the structure of proteins
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Unlocking the Secrets of Genes
• Within genes, four chemical elements of DNA
“code” for protein synthesis
–
–
–
–
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
• Genome – full set of genes in each cell of an
organism (except sperm & egg cells)
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Studying Genetic Material
• Linkage studies
– Studies that look for patterns of inheritance of
genetic markers in large families in which a
particular condition is common
• Genetic marker
– Segment of DNA that varies among individuals,
has a known location on a chromosome, and
can function as a genetic landmark for a gene
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Linking Genes & Behaviour
• Even when researchers locate a gene on a
chromosome, they do not automatically
know its role in physical or psychological
functioning
• Most human traits are influenced by more
than one gene pair (polygenic); from simple
to complex traits
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The Genetics of Similarity
• Evolution is a change in gene frequencies
within a population over many generations
– Mechanism for change in the genetically
influenced characteristics of a population
– Changes may result from mutations (errors in
copying of DNA sequences during division of
cells that produce sperm & eggs)
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Natural Selection
• Evolutionary process formulated by Darwin
• Individuals with genetically influenced traits
that are adaptive in particular environments
tend to survive & reproduce in greater
numbers (“survival of the fittest”)
• As a result, traits become more common in
the population
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Sexual Selection
• Darwin also proposed that genes were
determined by sexual selection
– Intersexual selection: a member of one sex
chooses a mate from the other sex on the
basis of certain characteristics
– Intrasexual selection: members of the same
sex compete for a partner of the other sex
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Traits & Preferences
• Evolutionary biologists start with an
observation about a characteristic and try to
account for it in evolutionary terms
• Evolutionary psychologists ask what sorts of
challenges humans might have faced and
infer what behavioural tendencies may have
been selected to overcome these challenges
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Mental Modules
• One perspective is that mind reflects mental
modules shaped through evolution
– Mental modules: a collection of specialized and
independent sections of the brain, developed to handle
specific survival problems (e.g., location of food, finding
a mate)
• Critics point out that not all traits are adaptive but
may be by-products of other traits (not isolated)
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Innate Human Characteristics
• Evolution has shaped human development
through innate characteristics such as:
1.
2.
3.
4.
5.
Infant reflexes
An interest in novelty
A desire to explore & manipulate objects
An impulse to play & fool around
Basic cognitive skills
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Phenotype Matching
• According to evolution, we should favour
family members to promote survival of
genetic material
– Phenotype matching: an individual’s ability to
assess relatedness to another individual based
on phenotype (expression of gene-environment
interaction)
• E.g., DeBruine’s (2002) trust study
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Our Human Heritage: Language
• Language
– A system that combines meaningless elements
such as sounds or gestures to form structured
utterances that convey meaning
– Surface structure: the way a sentence is spoken
– Deep structure: how a sentence is to be understood
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Innate Capacity for Language
• Chomsky argued that we are born with a language
acquisition device that guides the development of
language with proper exposure
– To transform surface into deep structures, children must
use syntax (which is not taught)
– Reflects a universal grammar where brains are sensitive
to core features common to all languages (e.g., nouns &
verbs)
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Evidence Supporting Chomsky
1. Children in different cultures go through similar stages of
linguistic development
2. Children combine words in ways that adults never would
3. Adults do not consistently correct their children’s syntax,
yet children learn to speak correctly anyway
4. Children not exposed to adult language might invent a
language of their own
5. Infants as young as 7 months can derive simple linguistic
rules from a string of sounds
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Learning & Language
• Other scholars argue environment plays
larger role in language (not all innate)
– Computer neural networks: mathematical models
of the brain that can “learn” some aspects of
language
– Other arguments: major differences in acquisition,
parents recast sentences rather than corrections,
children imitate recasts & expansions
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Our Human Heritage:
Courtship & Mating
• Sociobiology
– Interdisciplinary field that emphasizes
evolutionary explanations of social behaviour in
animals & humans
– We have a tendency to act in ways that
maximize chances of passing on genes as well
as helping close biological relatives do the same
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Evolution & Sexual Strategies
• Differences in survival & mating problems
have led to differences in aggression,
dominance, & sexual strategies between sexes
– Males compete with other males to access females,
inseminate as many as possible
– Females have larger biological investment in pregnancy
so choose dominant males with resources & status
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Differences in Sexual Strategies
Males
• Want sex more often
• Are fickle and
promiscuous
• Drawn to sexual novelty
and even rape
• Are undiscriminating in
partner choice
• Concerned with
competition and
dominance
Females
• Want sex less often
• Are devoted and faithful
• Drawn to stability and
security
• Are cautious and choosy
in partners
• Less concerned with
competition and
dominance
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Evolution & Preferences
• Cross-cultural studies have found consistent
differences between males and females
• Mating preferences
such as age predict
status & dominance
in men and fertility
in women
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Culture & the “Genetic Leash”
• Criticisms of evolutionary explanations of sex
differences:
– Explanations of infidelity & monogamy based on gender
stereotypes
– Sexual behaviour is varied and not solely for reproductive
purposes
– Ignores cultural influences on sexual behaviour
– Available mates were much more limited than currently
– Similarity & proximity among strongest predictors of mate
choice
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The Genetics of Difference
• The meaning of heritability:
– A statistical estimate of the proportion of the total
variance in some trait that is attributable to genetic
differences among individuals within a group
– Expressed as proportion (e.g., .60 or 60/100), with
maximum value of 1.0
– High heritability means greater genetic contribution
to trait
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Facts about Heritability
1. Estimates of heritability
apply only to a particular
group living in a particular
environment
2. Heritability estimates do not
apply to individuals, only to
variations within a group
3. Even highly heritable traits
can be modified by the
environment
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Computing Heritability
• Infer heritability by studying people whose
genetic similarity is known
• Separate role
of genetics &
environment
by studying
adopted
children
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Computing Heritability
• Another approach compares identical
(monozygotic) with fraternal (dizygotic) twins
• If identical more
similar than
fraternal twins,
greater the genetic
influence
• Also study twins
separated early in
life & raised apart
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Our Human Diversity: Intelligence
• Intellectual functioning usually measured by
an intelligence quotient (IQ) score
– Originally computed by dividing a person’s mental
age by chronological age and multiplying by 100;
now derived from norms on standardized IQ tests
– Kind of intelligence that produces high IQ scores
highly heritable:
• Children & adolescents (.40-.50); adults (.60-.80)
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Twins, Adoption, & Intelligence
• IQ scores of identical twins more highly
correlated than those of fraternal twins
• Also, scores
of adopted
children are
highly
correlated
with their
biological
parents
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Group Differences
• If genes influence individual differences, can
they account for differences between
groups?
– Differences have been used to justify differential
treatment of groups (e.g., ethnicity, gender)
• Example: Differences in IQ scores between African
Americans and Caucasian Americans
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Explaining Group Differences
• Genetic explanations have a fatal flaw
– Heritability estimates rely on Caucasian samples to
estimate role of heredity between groups
• Studies that overcome methodological flaws fail to
reveal genetic differences in IQ scores as a
function of ethnicity (e.g., Eyferth, 1961; Fagan, 1992)
– Differences within groups may have a genetic basis but
that does not mean differences between groups are
genetic
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Environment & Intelligence
• Environmental influences associated with
reduced mental ability:
– Poor prenatal care
– Malnutrition
– Exposure to toxins
– Stressful family circumstances
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Environment & Intelligence
• Environmental influences associated with
enhanced mental abilities:
– Good health care & nutrition
– Mental enrichment in home, child care, school
– Parental interaction, discussion, &
encouragement of mental processing
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Beyond Nature versus Nurture
• Heredity and environment always interact to
produce the unique mixture of qualities that
make a human
– E.g., variations in gene “expression” according to
internal & external environments
• Psychological diversity is adaptive
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End of Chapter 3
Copyright © 2010 Pearson Education Canada
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