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EVOLUTION AND GENETICS IN PSYCHOLOGY El El El El El El Evolutionary Theory Genetics Hereditary Transmission Chromosomes and Chromosomal Abnormalities Behavior Genetics Nature and Nurture 4.1. Evolutionary Theory A. What is evolutionary theory? 1. Evolutionary theory describes the way our bodies and behaviors change across many generations of individuals. This theory is important because it helps us understand how humans have come to be the way they are over the course of time since life first appeared on our planet. 2. Charles Darwin proposed in his 1859 book, The Origin of Species, that biological organisms have devel oped and changed through a mechanism of natural selection, known commonly as “survival of the fittest.” On this view, species show a great deal of biological variation. At a given time, particular members of a species will prove to be better able to cope with environmental conditions than will others, and so they will have an advantage for survival. As a result, they will multiply, and eventually, their progeny will become more prevalent. They have been selected by nature for survival—hence, the term, “natural selection,” 3. Scientists often refer to the ability of an organism to cope with the environment as its adaptability. This term is also sometimes used to refer to the ability of an organism to reproduce its own genes. 4. Those species whose members are not able to cope with the environment—to adapt—will die off and perhaps eventually become extinct, meaning that the species will cease to exist. 5. Darwin believed that the evolution of organisms was gradual. However, recent theorists have suggested an altemative viewpoint, referred to as punctuated equilibrium. \ccording to this viewpoint, kinds of organisms remain relatively stable for long periods of time, with such penods of stability punctuated by relatively brief periods (evolutionarily speaking) of rapid change in kinds of organisms. B, How does evolutionary theory work? Natural selection works when environments start to favor certain adaptations to those environments over others. For example, during the Industrial Revolution in late nineteenth-century England. a particular lightcolored moth was replaced by a dark-colored related moth as the prevalent variety. Why? Industrial pollu tion had blackened the forests, improving the darker moth’s camouflage against predators such as birds. The light-colored moth became too visible to predators to survive. Recently. however, with restrictions on air pollution, the light moth is making a comeback. Thus, natural selection is a constantly shifting process, intluenLed h\ in or.zanism s hioloy in Intenction stth thc dern inds ot the Ln\lronment 36 College Outlinefor Psychology Ca The evolution of humankind 1. Our own species. IIonzo sapiens. is a member of the biological family of hominids. In general. this species is distinguished from earlier human-like ones by a number of factors. including physical appearance and greater development of the brain. In particular, a greater proportion of our behavior is under the voluntary control of the brain—and thus is selfdirected—than was the case for earlier human-like species. 2. The earliest known horninids. called Horno habilis, h\e(l about 2 million years ago. The brain volume of Homo habilis was about 450 cubic centimeters (cm ; roughly a pint), in contrast with the volume of the 3 modem human brain. which is roughly I .400 cm (about 1 .5 quarts). 3. For unknon reasons, another species. Horno erectus, appeared about 1 to 2 million years ago. This had a much larger brain capacity than did Horno habilis, with a volume of about 950 cm 3 (roughly a quart). Eventually, this species replaced Horno habilic. The species spread out more widely than did the earlier species: Whereas traces of Homo hahilis are limited to Africa. traces of Hoino erectus can be found on three continents, Eventually, Homo erectus replaced by Horno 3apien3, our n species. Later. another protohuman species. Horno neanderthalis, appeared. hut it is not clear that humans evolved from them. species new was o D. Evolution and brain capacity 1. Scientists have been interested in the question of what makes one organism evolutionarih more “advanced” than another, and some scientists have focused on aspects of brain capacity as a basis for understanding ad ancement of organisms. 2. Brain size or weight itself does not provide a good index of intelligence or of any kind of evolutionary superiority. For example. the Indian elephant has a brain weight almost four times that of humans, it is not seen as more advanced intellectually. yet 3. A better indicator of intelligence appears to be the relationship between brain weight and total body weight. As hominids have evolved, their brain weights relative to their total body weights have increased significantly. But this measure is still imperfect. as shown by the fact that the rodent shrew has a higher ratio of brain-to-body weight than do humans. It appears that what is most important is the development of a particular part of the brain, the cerebral cortex, discussed in the next chapter. 4. Comparisons of brain size or weight within species have led to conflicting points of view. Stephen Jay Gould reported on a study conducted in the nineteenth century that showed that the average weight of the brains of 119 assassins, murderers, and thieves was ell above the average for the time. He also reported that the largest female brain e er weighed (1,565 g belonged to a woman who had killed her husband. At the same other investigators, such as Richard Lynn, have reported relations between aspects of brain size and intelligence. The issue thus remains unresolved. time, 4.2, Genetics A. Genetics is the stuth of the hereditary characteristics ia genes. the physiological building blocks of hereditary transmission. The study important because it helps us understand one way in which people develop the psychological, and of course. physical attributes that the express throughout their lives, transmission of of genetics ‘ is B. Genetics provides the key to how the evolution of kinds of organisms actually occurs. The mechanism change which when there is a sudden structural change in a hereditar characteristic that is not predictable from the of the parent. Mutations can be caused bx a ariet things, including chemical substances and s arious types of electromagnetic for genetic is mutation, occurs genes of radiation, C. Genetics also provides part of the key to understanding how we come to be who we are. Genetic mission leads to the formation of biological traits. ‘a hich are distinctix e characteristics or heha ior patterns that are genetically determined. All organisms receive their genes, and hence the capacity to develop certain biological traits, at the time of conception. trans D. Modem genetic theory dates hack to the research of an Austrian monk and botanist. Gregor Menclel (l822—lS84. who perlormed breeding experiments on common sarieties of the garden pea. Mendel surmised that certain inherited attributes are than others. The stronger attributes are referred to as dommant traits and the ‘a eaker ones as recessi e traits. stronger Evolution and Genetics in Psychology 37 4.3. Hereditary Transmission A. Mende1s ohervation of stronger and \\eaker attributes resulted from his ork it1i pea plants. 1 ). Nlendel observed that if true-breeding tall pea plants ones that al a s produce tall offspring ) are crossed ith true—breeding dwarf pea plants ones that al a s produce small oltspring the offspring of the tall and the dwarf plants ill al\\ a\ s he tail. Thus. tallness is a dominant trait and dwarfism a recessi e trait. . If one interhreeds all of the tall members of the first generation of offspring. the second generation of offspring ill not all be tall. Rather. there i11 be both fall and short plants. in a ratio Of about tlwee tall plants to e er’v one short plant. What mechanism could account for the difference in the offspring of all tall parents in the two generations? 2). B. The mechanism is controlled by the laws of hereditary transmission. To make the example simple. suppose that the height of a plant is controlled by exactly two genes (building blocks of heredity, mentioned above), one of which comes from each parent and forms each half of a gene pair. Both inherited genes may be for tallness, both may be for dwarfism, or one may be for tallness and one for dwarfism, If we represent tallness by T and dwarfism by d, then the possible inherited gene combinations, \\hich are each referred to as a genotype, are TI; Td, dT and dd, for the height of plants. C. The law of dominance states that whenever a dominant gene is paired with a recessive gene. even though both genes are present. the observable result in an organism-—the phenotype—u ill he that of the dominant trait. Thus, any plant that has as its genotype TI; Ti, or dT will show up as taIl. shereas only a plant ‘.s ith a dd genotype will he phenotpicall\ short. D. We can nos understand hy. in the tsso generations. Mendel’s plants had different distributions of phenotypes. In the parent generation, plants were either TTor dd. They were thus pure-breeding. When these two types of plants were crossed, the offspring were all hybrids, meaning that they all had mixed genotypes: Td and dT All would show up as tall because the dominant genotype prevailed in the phenotype. In the second generation of offspring, the genotypes were equally matched among Td, TI; dd, and dT (the four possible permutations of Td with dl). The Td, TI; and dT plants showed up as tall, the dd ones as short— hence, the 3:1 ratio of tall to short plants. E. In humans, the expression of a single genotype can give rise to a range of phenotypes, because heredi tary transmission is almost never as simple as in the aboe example. Consider the example of height in humans. A person’s height is largely’ genetically controlled. But other factors, such as nutrition, hormones. and immune-system deficiency. can affect the height a person achieves. Moreover, genetic transmission itself is complicated by many factors. Thus, even traits that are highly heritable (genetically based and passed from generation to generation) are not completely controlled by simple laws of genetics. 4.4. Chromosomes and Chromosomal Abnormalities A. The genes that are responsible fur hereditary transmission are themsels Cs parts of chromosomes, u hich are rod-shaped bodies containing very large numbers of the genes. Humans has e 23 pairs of chromosomes. for a total of 46 in all, although the number of chromosomes differs from one species to another. One of each pair of chromosomes was received at conception from the mother, and the other from the father, so that half of each individual’s heredity can be traced to each parent. B. The genetic material of which chromosomes are composed is deoxyribonucleic acid, also known as DNA. C. Chromosomes gosern many aspects of a person. including eye color and blood type. as well as sex. In psychology. we distinguish between a person’s sex, or ph\ siological characteristic of being male or female. and the person’s gender, or psscholorical identification as being male or female. For most people. although not es cry one. sex and gender identification correspond. D. Two specific chromosomes are crucial in determining sex: the X and Y chromosomes, Females receis e an X chromosome from both parents. and thus has e XX pairin of their sex-determinatis e chromosomes. Males receis e an X chromosome from their moiheN and a ‘1 chromosome from their fathers, resulting in their having an XV pairing of the sex-determinatis e chromosomes. Although the overwhelming majority of people have either the XX or XY pairing on the last chromosome, occasional abnormalities occur, 38 College Outlinefor Psychology E. Some people receive just a single X chromosome. and nothing more. They have Turner’s syndrome, which occurs in roughly 1 of every 3,000 live female births. People with this syndrome are female but are sexually underdeveloped. They are typically short, and their necks have a webbed appearance. Some psychological research has shown that Turner’s syndrome patients typically show decreased ability to perceive spatial organization. F. Some males receive an X chromosome, and thus their chromosome code is XXY. These individuals exhibit Klinefelter’s syndrome, which occurs in roughly 1 or 2 per 1,000 live births, The individuals are male, but with underdeveloped testes and secondary male sexual characteristics, such as facial hair. They are also infertile (unable to produce offspring). About half of the individuals are mentally retarded. extra G. Yet another syndrome is XYY syndrome, sometimes called the “Supermale syndrome,” in which a male is born with an Y chromosome. This syndrome occurs in roughly I of every 1,000 live births. XYY males are likely to be taller than average and typically test lower than normal on intelligence tests. There is also some evidence of increased aggressiveness. Some psychological investigators have suggested an association between XYY syndrome and criminality, but this association remains unproved. extra 4.5. Behavior Genetics A. Behavior genetics is the study of how psychological attributes, including behavior, are passed genetically. on B. The contribution of heredity to various psychological attributes is often expressed in terms of a heritability coefficient, a number on a scale from 0 to 1 that expresses the proportion of the variation among individuals that is alleged to be due to heredity. It is important to remember that the coefficient indicates vari ation in an attribute. In other words, the coefficient is affected by the amount of variation in the attribute in the population being studied. Were there no variation, there could be no meaningful estimation of heritability. C. Several different methods are used to estimate heritability. The three most widely used methods are based on separated identical twins reared apart, identical versus fraternal twins, and adoption. 1). In the method of identical twins reared apart, use is made of the fact that identical twins have identical genes. If two identical twins are separated at (or. at least, near) birth, then any differences between them must be due to environment. By studying a number of such twin pairs. it is possible to estimate heri tability of various attributes, In using this method, one must ensure that the twins were truly separated early, and that their environments were truly independent (not, for example, that one twin was raised by the par ents and the other by a family member such as a sister). 2). In the method of identical versus fraternal twins, use is made of the fact that whereas identical twins share all their genes, fraternal twins share only half. By comparing the similarities of identical versus fraternal attributes, it is possible to estimate heritability. Note that this method assumes that iden tical and fraternal twins share environments to an equal degree. an assumption that may be questionable if identical twins are brought up in more similar environments than fraternal ones. solely by virtue of their being identical twins, twins on 3). In the method of adoption, one compares the similarity of biological children and adoptive chil dren to their parents. Such studies require use of families in which there are both natural-horn and adoptive children. In such a study, one starts with the fact that the natural-born children will have received all their genes from their parents, whereas the adoptive children will have received none of their the par ents with whom they are living. Note that this method that natural-born and adoptive children share comparable environments, an assumption that may not always be met. genes from assumes 4.6. Nature and Nurture A. Just as peas follow a certain pattern of genetic transmission, so do all living organisms. Some people have taken advantage of this fact in attempts to introduce selective breeding, or the purposeful mating of organisms with each other in order to encourage certain biological traits or to discourage others. For example, selective breeding is used in the mating of many race horses in an attempt to produce racewinning horses in subsequent generations. certain Evolution and Genetics in Psychology 39 Robert Tryon. in an experiment done in I Q4O investigated the results of sehctive breeding in a strain of rats. \arious rats cre tested for their ability to run a maze and then were bred in successive generations to he brighter or less bright, as assessed by their ability to run the maze. Results were mixed. On the one hand. it was possible to breed maze-bright and maie—dull rats. on the other hand. the rats abilities turned out to be extremely specific. Even small changes in the nature of the maze eliminated the significant difference in maze-running ability betsseen the groups. B c* In the nineteenth centur. a mosement arose that argued for selective breeding in humans. This rnove ment. called the eugenics movement, was motix ated h the belief that the world ssould be a better place if people who were more genetically fit were encouraged to reproduce. whereas those who were genetically less fit were discouraged from reproduction. Because there is no clear definition of what makes one person more genetically fit than another, nor any clear ethical basis for making such decisions for other people. the movement has never gained uni\ersal popularlt\. A school of thought related to the eugenics movement was social Darwinism. championed by Herbert Spencer. Spencer and others proposed that the concept of survival of the fittest could be applied to social status, such that people who achiesed more in their lives (e.g., greater incomes or social stand ing) could be iewed as sociall\ the most fit. This mo ement floundered in large part because it could he used to justify any existing socioeconomic hierarchy at all. no matter how unfair or even unjust it happened to he. D. ‘ E. In modem times, a movement of sociohiology has sprung up. led by Edward Wilson, among others. This movement seeks to understand human behavior in terms of evolutionary principles. For example, observed differences between male and female behavior might be attributed to different selective pressures on males and females as they evolved. It is sometimes hard to disconfirm sociobiological explanations. as they are all necessarily after the fact rather than predictive. identities is sometimes called human adaptability through to increase the euthenics movement, Followers of this movement have sought still try to improve people today, environmental change. Although we rarely use the term “euthenics” potential. environments for the young in order to let them develop to their full F. An alternative movement that has continued for many years under various - ,— 1. Darwin’s view of natural selection holds that organisms tend to survive and reproduce as a func tion of their ability to adapt to the environment, with less adaptable kinds of organisms diminishing and, often, eventually becoming extinct. 2. A mutation occurs when the genetic message that would normally he passed on from parents to offspring is altered. resulting in a new organism with a genetic code not predictable from the genetic material of the parents. This mechanism for natural selection was not understood before Darwin proposed his re oluti onarv theory. 3. Species that we vie as more intelligent are generally characterized by greater brain-to-body weight. Within a species. however, there is no solid evidence that brain weight corresponds to intelligence. 3. Punctuated equilibrium refers to the view that evolutionary change proceeds in bursts rather than in smooth, gradual transitions, 5. Genes are the biological units that contribute to the hereditary transmission of biological traits, that is. inherited characteristics, Genes are located on chromosomes, which come in pairs. Humans have 23 such pairs. the23rd of which is responsible for determining sex. 6. Females have an XX chromosome pairing. males, an XY pairing. 7. Various sex—linked chromosomal abnormalities have been identified, Some of the main ones are Turner’s syndrome, associated with a single X chromosome; Klinefelter’s syndrome, associated with an XXY chromosome combination; and XYY syndrome. S. A genotype is the genetic code for a trait. A phenotype is the actual ‘.isible expression of the trait in offspring. A gi’en genotype can produce a variety of phenotpes. 40 college Outline for Psychology 9. Behavior genetics is the study of the transmission of behavior (and other psychological character istics) through the genes. 10. The coefficient of heritability is a number on a 0-to-I scale that indicates the proportion of variation in a population attribute that is due to hereditary influence. 11. Three of the main methods used to estimate coefficients of heritability are the methods of identical twins raised apart, identical versus fraternal twins, and adoption. 12. Two movements that have attempted to apply principles of natural selection to behavior are social Darwinism and sociobiology. The former movement is now discredited, because it could be and was used to justify virtually any existing social order, The latter movement is active today in the attempt to understand how people’s current behavior can be understood as constituting an evolutionary adaptation. 13. Selective breeding has been performed Lo produce animals with particular characteristics, such as in the breeding of race horses. The eugenics movement attempted to apply the philosophy of selective breeding to humans. The euthenics movement seeks to improve humankind through envi ronmental manipulations. adaptability behavior genetics biological trait chromosome deoxyribonucleic acid DNA dominant trait eugenics movement euthenics movement evolutionary theory extinct gender gene genetics genotype heritability coefficient heritable hominid Jlonio erectus Horno habilis Homo neanderthalis Horno sapiens hybrid Klinefelter’s syndrome method of adoption method of identical twins reared apart method of identical versus fraternal twins mutation natural selection phenotype punctuated equilibrium recessive trait selective breeding sex social Darwinism sociobiology Supermale syndrome Turner’s syndrome XX XY XYY syndrome A. Select the best response option from among the four that are given. 1. Of the following, is the best indicator of evolutionary advancement. A. ratio of brain-to-body weight B. brain capacity C. amount of cerebral cortex D. chronological age 2. In the method of a researcher compares natural-born children to nonhiological children under the same environmental conditions. A. identical twins reared apart B. identical versus fraternal twins C. heritability D. adoption , 3. A. B. C. D. is the study of the hereditary transmission of attributes via Heritability, biological traits Genetics, genes Adaptability, dominant and recessive traits Mutation, differentiation ________ _______ ________ Evolution and Genetics in Psychology 41 4. A male born with an extra Y chromosome. XYY, sometimes referred to as shows sonic evidence of increased aggressiveness and typically scores lower than normal on intelligence tests. A. Turner’s syndrome B. Supermale syndrome C. XXX syndrome D. Klinefelter’s syndrome 5. The A. B. C. D. B. material of which chromosomes are composed is called deoxyribonucleic acid genes atoms mvelin Answer each of the following questions with the appropriate word or phrase. 6. is a way of understanding how species change through time. 7. An unpredictable change or characteristic. occurs when there is a structural change in a hereditary 8. Behavior genetics attempts to explain how behavior is passed along through generations via 9. Genes are located on which are composed of 10. The total number of pairs of chromosomes in a normal human is 11. The constitutes the observable characteristics of an organism, while the consti tutes the characteristics that an individual has inherited and will transmit to his or her descendants. 12. An individual whose sex chromosome pairing is XX is biologically 13. C. is the theory that evolution is not a gradual process, but rather proceeds in small bursts. Answer T (true) or F (false) to each of the following statements. 14. An organism with a higher brain weight is always more evolutionarily advanced than is one with a lower brain weight. 15. Homo habilis had a greater brain capacity than did flomo erectus. 16. A heritability coefficient indicates the proportion of variability of a characteristic that is due to heredity. 17. Recessive traits are traits that are never manifested in an organism. 18. Euthenics is concerned with the improvement of society through improved adaptability and envi ronmental change. 19. Eugenicists believe that societal improvement is best sought through selective genetic engineering. 20. The method of identical versus fraternal twins estimates heritability by measurinT the extent to which environmental variables influence the behavior of genetically identical individuals. 21. A heritabilit coefficient of 1.5 indicates a high degree of heritability. 22. Modern genetic theory started with the work of Gregor Mendel, who studied the heritability of certain attributes of pea plants. 11