Evolution, Genes, and Behaviour
Chapter 3
Evolution, Genes, and Behaviour
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Unlocking the secrets of genes
The genetics of similarity
Our human heritage: courtship and mating
Our human heritage: language
The genetics of difference
Our human diversity: the case of
intelligence
Unlocking the Secrets of Genes
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Genes and how they operate
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Genes
Chromosomes
DNA
Genome
Studying genetic material
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Linkage studies
Genetic markers
The link between genetics and behaviour
Genes and How They Operate
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Genes:
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Chromosomes:
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functional units of heredity which are composed of
DNA and specify the structure of proteins.
rod-shaped structures within cells that carry genes.
DNA (dioxyribonucleic acid):
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transfers genetic characteristics by way of coded
instructions for the structure of proteins.
Studying Genetic Material
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Linkage studies
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Because genes lying close together on a chromosome
may be inherited together across generations,
researchers can look for genetic markers in families.
Genetic markers
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A segment of DNA that varies among individuals, has a
known location on a chromosome, and can function as a
genetic landmark for a gene.
The Link Between Genes and Behaviour
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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.
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Examples include height and eye color.
The Genetics of Similarity
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Evolution
Natural selection
Evolutionary biologists and psychologists
Innate human characteristics
Evolution
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A change in gene frequencies within a population
over many generations.
A mechanism by which genetically influenced
characteristics of a population may change.
Changes may occur due to mutations or errors
occurring during copying of original DNA
sequence.
Changes may occur due to natural selection.
Natural Selection
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The evolutionary process in which individuals
with genetically influenced traits that are adaptive
in a particular environment:
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tend to survive; and
to reproduce in greater numbers.
As a result, their traits become more common in
the population.
Evolutionary Biologists and Psychologists
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Evolutionary biologists start with an observation
about a characteristic and try to account for it in
evolutionary terms.
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Plumage differences in male and female peacocks
Evolutionary psychologists start by asking what sorts
of challenges human beings might have faced and
then draw inferences about which behavioural
tendencies might have been selected to overcome
these challenges.
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Avoiding poisonous food and an innate dislike for bitter
tastes.
Innate Human Characteristics
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Infant reflexes
An attraction to novelty
A desire to explore and manipulate objects
An impulse to play and fool around
Basic arithmetic skills
Our Human Heritage: Courtship and
Mating
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Sociobiology
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Interdisciplinary field that emphasizes evolutionary
explanations of social behaviour in animals,
including human beings.
We behave in ways that maximize our chances of
passing on our genes, and to help our close biological
relatives, with whom we share genes, to do the same.
Gender, Evolution, and Sexual Strategies
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Because of different kinds of survival and mating
problems, the sexes have evolved differently in
the areas of aggressiveness, physical dominance,
and sexual strategies.
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Males compete with other males for access to
females, and try to inseminate as many females as
possible.
Females conceive and carry only a limited number of
pregnancies so they choose fewer more dominant
males with good resources and high status.
Gender Differences in Sexual Strategies
Males
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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
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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.
Evolutionary Psychologists and the
Question of Gender Differences
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While sociobiologists study nonhuman species’
behaviours and make analogies to human
behaviours, evolutionary psychologists consider
such analogies simplistic and misleading.
Focus more on commonalities of human mating
and dating around the world.
Culture and the “Genetic Leash”
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Criticisms of sociobiological and evolutionary
explanations for sexual behaviour include:
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Evolutionary explanations of infidelity and
monogamy are based on stereotypes.
Actual behaviour of humans and other animals fails
to conform to images of promiscuous males and coy
females.
Human sexual behaviour is too varied to be
explained solely from an evolutionary perspective.
Historically, available mates for selection were much
fewer than they are today.
Our Human Heritage:
The Nature of Language
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Language
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A system that combines meaningless
elements such as sounds or gestures to form
structured utterances that convey meaning.
The Innate Capacity for Language
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Language too complex to be learned bit by bit
(Chomsky, 1957, 1980). For example, sentences
can have surface and deep structures.
 Surface structure
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Deep structure
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the way a sentence is spoken
how a sentence is to be understood
To transform surface sentence structures into
deep ones, children must apply rules of grammar
(syntax).
Examples of Surface and Deep Structures
The Language Acquisition Device
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Chomsky suggests that since we don’t teach syntax
to toddlers, human brains must contain a language
acquisition device.
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An innate module that allows young children to develop
language if they are exposed to an adequate sampling of
conversation.
Therefore, children are born with universal
grammar or a sensitivity to the core features
common to all languages.
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Examples include nouns and verbs, subjects and
objects, and negatives.
Evidence Supporting the LAD
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Children in different cultures go through similar
stages of linguistic development.
Children combine words in a way that adults never
would.
Adults do not consistently correct their children’s
syntax, yet children learn to speak or sign correctly
anyway.
Children not exposed to adult language may invent
a language of their own.
Infants as young as 7 months can derive simple
linguistic rules from a string of sounds.
Evidence for Learning and Language
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The design of computerized neural networks to
“learn” aspects of language such as regular and
irregular verbs.
Children learn the probability that any given word
or syllable will follow another.
Although there are commonalities in language
acquisition, there are also many differences.
Parents respond to children’s sentence errors by
restating or elaborating on the phrase.
Children imitate these adult recasts and expansions.
The Genetics of Difference
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The meaning of heritability.
Facts about heritability.
Computing heritability.
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The study of adopted children.
The study of monozygotic and dizygotic twins.
The study of twins separated at birth.
The Meaning of Heritability
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A statistical estimate of the proportion of the total
variance in some trait that is attributable to
genetic differences among individuals within a
group.
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Expressed as proportion (.60 or 60/100).
Maximum value is 1.0.
Some variables such as height are highly
heritable, other variables such as musical ability
are moderately heritable.
Facts About Heritability
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An estimate of heritability
applies only to a particular
group living in a particular
environment.
Heritability estimates do not
apply to individuals, only to
variations within a group.
Even highly heritable traits
can be modified by the
environment.
Computing Heritability
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Studying adopted children allows researchers to
compare correlations between the traits of
adopted children and those of their biological and
adoptive relatives.
These results are used to compute a estimate of
heritability.
Computing Heritability
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Behavioural geneticists
can estimate the
heritability of a trait by
comparing identical or
monozygotic twins to
fraternal or dizygotic
twins.
If identical twins are more
alike than fraternal twins,
then the increased
similarity must be due to
genetic influences.
Computing Heritability
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Investigators have also studied identical twins
who were separated early in life and reared apart.
Any similarities in traits between them should be
primarily genetic and should permit a direct
estimate of heritability.
Our Human Diversity: The Case of
Intelligence.
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Genes and individual differences.
The question of group differences.
The environment and intelligence.
Intelligence: Genes and Individual
Differences
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Intelligence Quotient (IQ)
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A measure of intelligences originally computed by
dividing a person’s mental age by his or her
chronological age and multiplying the result by 100.
It is now derived from norms provided for standard
intelligence tests.
The kind of intelligence that produces high IQ
scores is highly heritable.
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.50 for children and adolescents
.60 -.80 for adults.
Twins and Intelligence
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Intelligence scores of identical twins are always
more highly correlated than those of fraternal
twins.
Adopted Children and Intelligence
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The scores of adopted children are highly
correlated with their biological parents.
The Question of Group Differences
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Genetics are used to explain differences between
groups.
Often these differences are used to justify
differential treatment for these groups.
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Example: Differences between average IQ scores for
African Americans and Caucasian Americans
Genetic explanations have a flaw.
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They use heritability estimates based on white samples
to estimate the role of heredity in group differences.
The Question of Group Differences
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The handful of studies that have overcome past
methodological problems fail to reveal any genetic
differences between blacks and whites on IQ.
Examples:
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Children fathered by black and white American
soldiers in Germany after WWII and reared in similar
German communities did not differ significantly in IQ.
Black and White infants perform equally well on tests
for novelty.
The Environment and Intelligence
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Experiences that hinder intellectual performance:
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Poor prenatal care.
Malnutrition.
Exposure to toxins.
Stressful family circumstances.
Experiences that help intellectual performance:
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Good health care and nutrition.
Mental enrichment in home and child care or school.