Evolution, Genes, and Behaviour

1. Evolution, Genes, and Behaviour Chapter 3

2. Evolution, Genes, and Behaviour • 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

3. Unlocking the Secrets of Genes • Genes and how they operate • Genes • Chromosomes • DNA • Genome • Studying genetic material • Linkage studies • Genetic markers • The link between genetics and behaviour

4. Genes and How They Operate • Genes: • functional units of heredity which are composed of DNA and specify the structure of proteins. • Chromosomes: • rod-shaped structures within cells that carry genes. • DNA (dioxyribonucleic acid): • transfers genetic characteristics by way of coded instructions for the structure of proteins.

5. Studying Genetic Material • Linkage studies • Because genes lying close together on a chromosome may be inherited together across generations, researchers can look for genetic markers in families. • Genetic markers • 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.

6. The Link Between Genes and 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. • Examples include height and eye color.

7. The Genetics of Similarity • Evolution • Natural selection • Evolutionary biologists and psychologists • Innate human characteristics

8. Evolution • 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.

9. Natural Selection • The evolutionary process in which individuals with genetically influenced traits that are adaptive in a particular environment: • tend to survive; and • to reproduce in greater numbers. • As a result, their traits become more common in the population.

10. Evolutionary Biologists and Psychologists • Evolutionary biologists start with an observation about a characteristic and try to account for it in evolutionary terms. • 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. • Avoiding poisonous food and an innate dislike for bitter tastes.

11. Innate Human Characteristics • Infant reflexes • An attraction to novelty • A desire to explore and manipulate objects • An impulse to play and fool around • Basic arithmetic skills

12. Our Human Heritage: Courtship and Mating • Sociobiology • 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.

13. Gender, Evolution, and Sexual Strategies • Because of different kinds of survival and mating problems, the sexes have evolved differently in the areas of aggressiveness, physical dominance, and sexual strategies. • 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.

14. 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. Gender Differences in Sexual Strategies

15. Evolutionary Psychologists and the Question of Gender Differences • 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.

16. Culture and the “Genetic Leash” • Criticisms of sociobiological and evolutionary explanations for sexual behaviour include: • 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.

17. Our Human Heritage:The Nature of Language • Language • A system that combines meaningless elements such as sounds or gestures to form structured utterances that convey meaning.

18. The Innate Capacity for Language • Language too complex to be learned bit by bit (Chomsky, 1957, 1980). For example, sentences can have surface and deep structures. • Surface structure • the way a sentence is spoken • Deep structure • how a sentence is to be understood • To transform surface sentence structures into deep ones, children must apply rules of grammar (syntax).

19. Examples of Surface and Deep Structures

20. The Language Acquisition Device • Chomsky suggests that since we don’t teach syntax to toddlers, human brains must contain a language acquisition device. • 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. • Examples include nouns and verbs, subjects and objects, and negatives.

21. Evidence Supporting the LAD • 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.

22. Evidence for Learning and Language • 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.

23. The Genetics of Difference • The meaning of heritability. • Facts about heritability. • Computing heritability. • The study of adopted children. • The study of monozygotic and dizygotic twins. • The study of twins separated at birth.

24. 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 (.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.

25. Facts About Heritability • 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.

26. Computing Heritability • 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.

27. Computing Heritability • 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.

28. Computing Heritability • 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.

29. Our Human Diversity: The Case of Intelligence. • Genes and individual differences. • The question of group differences. • The environment and intelligence.

30. Intelligence: Genes and Individual Differences • Intelligence Quotient (IQ) • 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. • .50 for children and adolescents • .60 -.80 for adults.

31. Twins and Intelligence • Intelligence scores of identical twins are always more highly correlated than those of fraternal twins.

32. Adopted Children and Intelligence • The scores of adopted children are highly correlated with their biological parents.

33. The Question of Group Differences • Genetics are used to explain differences between groups. • Often these differences are used to justify differential treatment for these groups. • Example: Differences between average IQ scores for African Americans and Caucasian Americans • Genetic explanations have a flaw. • They use heritability estimates based on white samples to estimate the role of heredity in group differences.

34. The Question of Group Differences • The handful of studies that have overcome past methodological problems fail to reveal any genetic differences between blacks and whites on IQ. • Examples: • 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.

35. The Environment and Intelligence • Experiences that hinder intellectual performance: • Poor prenatal care. • Malnutrition. • Exposure to toxins. • Stressful family circumstances. • Experiences that help intellectual performance: • Good health care and nutrition. • Mental enrichment in home and child care or school.