- Define Darwin's Theory of Evolution
- Define evolutionary psychology
- Understand how hereditary traits are passed from parents to children
- Understand how psychologists use kinship studies
[SLIDE 1]
Charles Darwin proposed the idea that humans were genetically related to monkeys and other animals in his book, The Descent of Man, published in 1871. He made the case that humans, like other species, were a product of evolution. Darwin believed that the great apes, such as chimpanzees and gorillas, shared a common primate ancestor.
The concept of a struggle for existence is at the core of Darwin’s theory of evolution. Which species survive and which become extinct is determined by natural selection. In other words, species that are better adapted to their environment are more likely to survive and reproduce.
Biology gives rise to specific behavioral tendencies in some organisms. But the behavior of many species, especially higher species such as humans, is flexible and affected by experience and choice, as well as by heredity.
According to the theory of evolution, species and individuals compete for the same resources. Natural variations from individual to individual, along with sudden changes in genes called mutations lead to differences among individuals -- differences which affect the ability to adapt to change.
Those individuals whose traits are better adapted are more likely to survive. In other words, they are naturally selected. Survival permits them to reach sexual maturity, to reproduce, and to transmit their features or traits to the next generation. What began as chance variation is then embedded over the generations if they’re successful. Chance variations that hinder survival, however, are likely to disappear from the gene pool.
[SLIDE 2]
Evolutionary psychology studies the ways in which adaptation and natural selection are connected with mental processes and behavior. Evolution has provided organisms with advantages such as stronger fins and wings, sharper claws, and camouflage. Human evolution has given rise to various physical traits and also to such diverse activities as language, art, committed relationships, and warfare.
One of the concepts of evolutionary psychology is that not only physical traits but also many patterns of behavior, including social behavior, evolve and can be transmitted genetically from generation to generation. Behavior patterns that help an organism to survive and reproduce may be transmitted to the next generation. Such behaviors are believed to include aggression, strategies of mate selection, even altruism. Such behavior patterns are termed instinctive or species-specific because they evolved within certain species.
[SLIDE 3]
An instinct is a stereotyped pattern of behavior that is triggered in a specific situation. Instinctive behavior is nearly identical among the members of the species in which it appears and tends to resist modification, even when it serves no purpose. Instinctive behavior also appears when the individual is reared in isolation from others of its kind and thus cannot learn the behavior from experience. As one example, if you rear a white crowned sparrow in isolation from other sparrows, it will still sing a recognizable species-specific song when it matures.
Many psychologists consider language and even consciousness to be instinctive among humans. Psychologists are trying to determine what other kinds of human behavior may be instinctive. However, even instinctive behavior can be modified to some degree by learning, and most psychologists agree that the richness and complexity of human behavior would not be possible in the absence of learning.
[SLIDE 4]
Heredity defines one’s nature, which is based on biological structures and processes. Heredity refers to the biological transmission of traits that have evolved from generation to generation. The subfield of biology that studies heredity is called genetics. The field of genetics looks at both species-specific behavior patterns, or instincts, and individual differences among the members of a species.
Behavioral genetics focuses on the contribution of genes to behavior and bridges the sciences of psychology and biology. It is concerned with the genetic transmission of traits that give rise to patterns of behavior. Psychologists are thinking in terms of behavioral genetics when they ask about the inborn reasons why individuals may differ in their behavior and mental processes.
[SLIDE 5]
Genes are basic units of heredity which are found at a specific point on a chromosome. Genes regulate the development of specific traits. Some traits, such as blood type, are controlled by a single pair of genes, one each from each of our two parents. Other traits are determined by combinations of genes.
Relevant to this course, the inherited component of complex psychological traits, such as intelligence, is believed to be determined by combinations of genes. It is estimated that the cells within your body contain 20,000 to 25,000 genes.
Genes are segments of chromosomes. That is, chromosomes are made up of strings of genes. Each cell in the body contains 46 chromosomes arranged in 23 pairs. Chromosomes are large complex molecules of DNA, which is an acronym for deoxyribonucleic acid, the substance that forms the basic material of chromosomes. It takes the form of a double helix and contains the genetic code.
The tightly wound structure of DNA was first demonstrated in the 1950s by James Watson and Francis Crick. The rungs of the ladder-like structure are made up of nucleotide chemicals whose names are abbreviated as A, T, C, and G. A always links up with T to complete a rung, and C always combines with G. Therefore, you can describe the genetic code in terms of the nucleotides you find along just one of the rungs: for example, CTGAGTCAC and so on. Genes, chromosomes, and DNA structure are illustrated in Figure 2.15 above.
[SLIDE 6]
The genetic code -- that is, the order of A, G, T, and C -- determines your species and all those traits that can be inherited. A group of scientists working together around the globe, referred to as the Human Genome Project, has learned that the sequencing of your DNA consists of about three billion DNA sequences spread throughout your chromosomes.
Psychologists debate the extent to which genes influence complex psychological traits such as intelligence, aggressiveness, and happiness, and the appearance of psychological disorders such as schizophrenia. Some traits are determined by a single pair of genes. Other traits, especially complex psychological traits, are thought to be polygenic, or influenced by combinations of genes.
Your genetic code provides your genotype, your full genetic makeup, as determined by the sequencing of the chemicals in your DNA. but the person you see in the mirror was also influenced by your early experiences in the home, injuries, adequacy of nourishment, educational experiences, and numerous other environmental influences.
Therefore, you see the outer appearance of your phenotype, or the manner in which your genetic code manifests itself. Your genotype provides what psychologists refer to as your "nature"-- the inborn, innate character of an organism. On the other hand, your phenotype represents the interaction of your nature (heredity) and your nurture (environmental influences). Psychologists are especially interested in the roles of nature and nurture in intelligence and psychological disorders.
[SLIDE 7]
We normally receive 23 chromosomes from our father’s sperm cell and 23 chromosomes from our mother’s egg cell, or ovum. When a sperm cell fertilizes an ovum, the chromosomes form 23 pairs.
The 23rd pair consists of sex chromosomes, whose genetic material determines the sex of the individual. We all receive an X sex chromosome, so called because of the X shape, from our mother. If we also receive an X sex chromosome from our father, we develop into a female. If we receive a Y sex chromosome, again named for its Y shape, from our father, we develop into a male.
When people do not have the normal number of 46 chromosomes, or 23 pairs, physical and behavioral abnormalities may result. Most persons with Down syndrome, for example, have an extra, or third, chromosome on the 21st pair. Persons with Down syndrome have a downward-sloping fold of skin at the inner corners of the eyes, a round face, a protruding tongue, and a broad, flat nose. They are intellectually disabled and usually have physical problems that cause death by middle age.
[SLIDE 8]
Kinship studies are ways in which psychologists compare the presence of traits and behavior patterns in people who are biologically related or unrelated to help determine the role of genetic factors in their occurrence. The more closely people are related, the more genes they have in common.
Identical twins share 100% of their genes. Parents and children have 50% of their genes in common, as do siblings - brothers and sisters. Aunts and uncles related by blood have a 25% overlap with nieces and nephews. First cousins share 12.5% of their genes, and so on.
If genes are involved in a trait or behavior pattern, people who are more closely related should be more likely to show similar traits or behavior. Psychologists and behavioral geneticists are especially interested in running kinship studies with twins and adopted individuals.
[SLIDE 9]
The fertilized egg cell that carries genetic messages from both parents is called a zygote. Now and then, a zygote divides into two cells that separate, so that instead of developing into a single person, it develops into two people with the same genetic makeup. Such people are identical, or monozygotic (MZ) twins, also called identical twins.
If the woman releases two ova in the same month and they are both fertilized, they develop into fraternal, or dizygotic (DZ) twins, also called fraternal twins. Dizygotic twins, like other siblings, share 50% of their genes. Monozygotic twins are important in the study of the relative influences of nature and nurture because differences between MZ twins are the result of nurture.
Of course, twin studies are not perfect. MZ twins may resemble each other more closely than DZ twins partly because they are treated more similarly. The results of kinship studies can be confused when relatives share similar environments as well as genes. Adoption studies overcome some of this problem by comparing children who have been separated from their parents at an early age or in which identical twins are separated at an early age and reared in different environments.