Biological Psychology: Genetics, Evolution, Development, & Plasticity

© Cengage Learning 2016 © Cengage Learning 2016

Chapter 4

Genetics, Evolution, Development

and Plasticity

© Cengage Learning 2016

4.1 Genetics and Evolution of Behavior

• Does human behavior depend on genetics, environmental influences, or both?

• A review of genetics provides a springboard for evaluating this controversial question


Genetics and Behavior

• Both genes and environment interact to shape human behavior

• The fundamental issue is how much a role each factor plays in shaping human behaviors– Examples: psychological disorders, weight

gain, personality, and sexual orientation


A Genetics Contribution to Facial Expression


Mendelian Genetics

• 19th century monk Gregor Mendel demonstrated that inheritance occurs through discrete units of heredity, called genes

• Genes come in pairs, called alleles, and are aligned along chromosomes


Mendelian Genetics – DNA and RNA

• A gene is defined as a portion of a chromosome and is composed of deoxyribonucleic acid (DNA)

• DNA serves as a model for the synthesis of ribonucleic acid (RNA)

• RNA is a single strand chemical that can serve as a template/model for the synthesis of proteins


Mendelian Genetics –Proteins

• Proteins determine the development of the body by:– Forming part of the structure of the body

– Serving as enzymes, biological catalysts that regulate chemical reactions in the body


How DNA Controls the Development of an Organism


Mendelian Genetics – Heterozygous and Homozygous Genes

• Being homozygous for a gene means that a person has an identical pair of genes on the two chromosomes

• Being heterozygous for a gene means that a person has an unmatched pair of genes on the two chromosomes


Mendelian Genetics – Dominant and Recessive Genes

• Genes are either dominant, recessive, or intermediate– Examples: eye color, ability to taste PTC

• A dominant gene shows a strong effect in either the homozygous or heterozygous condition

• A recessive gene shows its effect only in the homozygous condition


Four Equally Likely Outcomes of a Mating Between Heterozygous Tasters


Mendelian Genetics – Gene Expression

• Examples such as PTC and hair color can be misleading– Implies that a single gene combination

completely controls a characteristic, but this is not always true

• Some genes are only expressed partly: in some cells and not others, or only under certain circumstances


Types of Genes

• Autosomal genes: all other genes except for sex-linked genes

• Sex-linked genes: genes located on the sex chromosomes

• In mammals, the sex chromosomes are designated X & Y – Females have two X chromosomes (XX)

– Males have an X and a Y chromosome (XY)


Mendelian Genetics – X and Y

• During reproduction:– Females contribute an X chromosome

– Males contribute either an X or a Y chromosome that determines the sex of the child

• If an X chromosome is contributed by the male, the offspring is female

• If a Y chromosome is contributed by the male, the offspring will be male


Mendelian Genetics – Sex-Linked and Sex-Limited Genes

• The human Y chromosome has genes for far fewer proteins than the X chromosome

• Thus, sex-linked genes usually refer to X-linked genes: e.g., red-green color deficiency

• Sex-limited genes are genes that are present in both sexes but mainly have an effect on one sex (chest hair, breast size, etc.)


Genetic Changes

• Genes change in several ways– Mutation: a heritable change in a DNA

molecule

– Microduplication/microdeletion: part of a chromosome that might appear once might appear twice or not at all

– Example: some researchers believe schizophrenia might be a result of microduplications and microdeletions of brain-relevant genes


Epigenetics

• A field that is concerned with changes in gene expression without the modification of the DNA sequence– Some genes are active only at a certain point

in one’s life, a certain time of day, etc.

– Changes in gene expression are central to learning and memory

– Epigenetic differences are a likely explanation for differences between monozygotic “identical” twins


Activation of Certain Genes


Epigenetic Effects

• What you do at any moment not only affects you now, but produces epigenetic effects that alter gene expressions for a longer period of time

• Experiences alter the activity of genes


Heritability

• Refers to how much characteristics depend on genetic differences– Researchers have found evidence for

heritability in almost every behavior they have tested

– Heritability of a certain trait is specific to a given population

– Strong environmental influences may cause genetic influences to have less of an effect


Heredity and Environment

• Almost all behaviors have both a genetic and an environmental component

• Researchers: – Study monozygotic and dizygotic twins to infer

contributions of heredity and environment

– Study adopted children and their resemblance to their biological parents to infer hereditary influences

– Identify specific genes linked to some behavior


Environmental Modification

• Traits with a strong hereditary influence can by modified by environmental intervention– e.g., PKU: a genetic inability to metabolize the

amino acid phenylketonuria

– Environmental interventions can modify PKU


How Genes Affect Behavior

• Genes do not directly produce behaviors

• Genes produce proteins that increase the probability that a behavior will develop under certain circumstances

• Genes can also have an indirect affect– Genes can alter your environment by

producing behaviors or traits that alter how people in your environment react to you


The Evolution of Behavior

• Evolution refers to a change in the frequency of various genes in a population over generations– Regardless of whether the change is helpful or

harmful to the species

• Evolution attempts to answer two questions:– How did some species evolve?

– How do species evolve?


How Did Some Species Evolve?

• How species evolved is based on inferences from fossils/comparisons of living species


How Do Species Evolve?

• How species evolve rests upon some assumptions:– Offspring generally resemble their parents for

genetic reasons

– Mutations, recombination, and microduplications of genes introduce new heritable variations

– Certain individuals successfully reproduce more than others


Artificial Selection

• Refers to choosing individuals with desired traits and making them parents of the next generation

• According to Darwin, nature also selects, and successful individuals’ genes will be prevalent in later generations


Common Misconceptions about Evolution

• Lamarckian evolution: “The use or disuse of some structure or behavior causes an increase or decrease in that behavior”

• “Humans have stopped evolving”

• “Evolution means improvement”

• “Evolution acts to benefit the individual or the species”


Brain Evolution

• One explanation is that our ancestors managed to get enough nutrition to provide a big brain with all the fuel it needs– Cooking food made it easier to digest

– Group hunting was more efficient

– Humans have better capacity for glucose transport


Evolutionary Psychology

• Focuses upon functional and evolutionary explanations of how behaviors evolved– Assumes that behaviors characteristic of a

species have arisen through natural selection and provide a survival advantage

• Examples: differences in peripheral/color vision, sleep mechanisms in the brain, eating habits, temperature regulation


Behavior and Natural Selection

• Some behaviors are more debatable with regard to the influence of natural selection

• Examples– Life span length

– Gender differences in sexual promiscuity

– Altruistic behavior: a behavior that benefits someone other than the actor

• Altruism is hard to find outside of humans


Group and Kin Selection

• A gene:– Only spreads if individuals with it reproduce

more than individuals without it

– That benefits the species but not the individual dies out with that individual

• Group selection: controversial hypothesis that states that altruistic groups survive better than less cooperative ones

• Kin selection: more plausible; selection for a gene benefits the individual’s relatives


Reciprocal Altruism

• The idea that individuals help those that will return the favor

• Building a reputation for helpfulness only works if others are willing to cooperate with you


4.2 Development of the Brain

• Brain development depends upon:– Maturation

– Learning

• We can refine this understanding by learning how:– Neurons develop

– Their axons connect

– Experience modifies development.


Maturation of the Vertebrate Brain

• The human central nervous system begins to form when the embryo is approximately two weeks old– The dorsal surface thickens, forming a neural

tube surrounding a fluid filled cavity

– The forward end enlarges and differentiates into the hindbrain, midbrain, and forebrain

– The rest of the neural tube becomes the spinal cord


Early Development of the Human Central Nervous System


Human Brain at Four Stages of Development


Cerebrospinal Fluid

• The fluid-filled cavity becomes the central canal of the spinal cord and the four ventricles of the brain– This fluid is the cerebrospinal fluid


Brain Weight

• At birth, the human brain weighs approximately 350 grams

• By the first year, the brain weighs approximately 1000 grams

• The adult brain weighs 1200-1400 grams


The Development of Neurons

• The development of neurons in the brain involves the following processes:– Proliferation

– Migration

– Differentiation

– Myelination

– Synaptogenesis


Proliferation

• The production of new cells/ neurons in the brain primarily occurring early in life– Early in development, the cells lining the

ventricles divide

– Some cells become stem cells that continue to divide

– Others remain where they are or become neurons or glia that migrate to other locations


Migration

• The movement of the newly formed neurons and glia to their eventual locations– Some do not reach their destinations until

adulthood

• Occurs in a variety of directions throughout the brain– Chemicals known as immunoglobulins and

chemokines guide neuron migration


Differentiation

• The forming of the axon and dendrites that gives the neuron its distinctive shape

• The axon grows first either during migration or once it has reached its target and is followed by the development of the dendrites


Myelination

• The process by which glia produce the fatty sheath that covers the axons of some neurons– Myelin speeds up the transmission of neural

impulses

– First occurs in the spinal cord and then in the hindbrain, midbrain and forebrain

– Occurs gradually for decades


Synaptogenesis

• The final stage of neural development – the formation of the synapses between neurons– Occurs throughout the life as neurons are

constantly forming new connections and discarding old ones

– Slows significantly later in the lifetime


New Neurons Later in Life, Part 1

• Originally believed that no new neurons were formed after early development

• Later research suggests otherwise– Stem cells: undifferentiated cells found in the

interior of the brain that generate “daughter cells” that can transform into glia or neurons

– New olfactory receptors also continually replace dying ones


New Neurons Later in Life, Part 2

• Development of new neurons also occurs in other brain regions– Example: songbirds have a steady

replacement of new neurons in the singing area of the brain

• Stem cells differentiate into new neurons in the adult hippocampus of mammals and facilitate learning


The Life Span of Neurons

• Different cells have different average life spans

• Skin cells are the newest; most are under a year old

• Heart cells, on the other hand, tend to be as old as the person

• Mammalian cerebral cortexes form few or no new neurons after birth


Pathfinding by Axons

• Axons must travel great distances across the brain to form the correct connections

• Sperry’s (1954) research with newts indicated that axons follow a chemical trail to reach their appropriate target

• Growing axons reach their target area by following a gradient of chemicals in which they are attracted by some chemicals and repelled by others


Specificity of Axon Connections


Nerve Connections in Newts


Chemical Gradients


Competition among Axons as a General Principle

• When axons initially reach their targets, they form synapses with several cells

• Postsynaptic cells strengthen connection with some cells and eliminate connections with others

• The formation or elimination of these connections depends on the pattern of input from incoming axons


Neural Darwinism

• Some theorists refer to the idea of the selection process of neural connections as neural Darwinism

• In this competition among synaptic connections, we initially form more connections than we need

• The most successful axon connections and combinations survive while the others fail to sustain active synapses


Determinants of Neuronal Survival

• Levi-Montalcini discovered that muscles do not determine how many axons form; they determine how many survive

• Nerve growth factor (NGF) is a type of protein released by muscles that promotes the survival and growth of axons– The brain’s system of overproducing neurons

and then applying apoptosis enables the exact matching of the number of incoming axons to the number of receiving cells


Neurotropins

• Chemicals that promote the survival and activity of neurons (i.e., NGF)– Axons that are not exposed to neurotropins

after making connections undergo apoptosis – a preprogrammed mechanism of cell death

– Therefore, the healthy adult nervous system contains no neurons that failed to make appropriate connections


Neuronal Death

• The elimination of massive cell death is part of normal development and maturation

• After maturity, the apoptotic mechanisms become dormant

• The visual cortex is actually thicker in blind people due to a lack of visual stimuli– It cannot prune out ineffective neurons


The Vulnerable Developing Brain

• Early stages of brain development are critical for normal development later in life

• A mutation on one gene can lead to many defects

• Chemical distortions in the brain during early development can cause significant impairment and developmental problems


Fetal Alcohol Syndrome, Part 1

• A condition that children are born with if the mother drinks heavily during pregnancy

• Marked by the following:– Hyperactivity and impulsiveness

– Difficulty maintaining attention

– Varying degrees of mental retardation

– Motor problems and heart defects

– Facial abnormalities


Fetal Alcohol Syndrome, Part 2

• The dendrites of children born with fetal alcohol syndrome are short with few branches

• Exposure to alcohol in the fetus brain suppresses glutamate and enhances the release of GABA

• Many neurons consequently receive less excitation and exposure to neurotrophins than usual and undergo apoptosis


Differentiation of the Cortex

• Neurons in different parts of the brain differ from one another in their shape and chemical components– Immature neurons transplanted to a

developing part of the cortex develop the properties of the new location

– Neurons transplanted at a later stage of development develop some new properties but retain some old properties

• Example: ferret experiment


A Ferret with a Rewired Temporal Cortex


Fine-Tuning by Experience

• The brain has some ability to reorganize itself in response to experience– Axons and dendrites continue to modify their

structure and connections throughout the lifetime

– Dendrites continually grow new spines

• The gain and loss of spines indicates new connections, which relates to learning


Changes in Dendritic Trees of Two Mouse Neurons


Effect of a Stimulating Environment


Experience and Dendritic Branching, Part 1

• Rats raised in an enriched environment develop a thicker cortex, increased dendritic branching and improved learning

• Measurable expansion of neurons has also been shown in humans as a function of physical activity

• As old neurons die by apoptosis and new ones form to take their place, there is improved learning and memory


Experience and Dendritic Branching, Part 2

• It was once believed that teaching a child a difficult concept (e.g., Greek, advanced math, etc.) would enhance intelligence in other areas– This concept is known as “far transfer”

• Evidence shows that skills associated with the practiced task transfer, but not other skills– The brain cannot be “exercised” like a muscle


Effects of Special Experiences

• Blind people improve their attention to touch and sound, based on practice – Touch information activated this occipital cortex

area, which is ordinarily devoted to vision alone

• The occipital lobe normally dedicated to processing visual information adapts to also process tactile and verbal information


Brain Adaptations

• People who are deaf from an early age become more responsive to touch and vision.

• Touch and vision come to activate what would be the auditory cortex in deaf people


Music Training

• MRI studies reveal the following:– The temporal lobe of professional musicians in

the right hemisphere is 30% larger than non-musicians

– Thicker gray matter in the part of the brain responsible for hand control and vision of professional keyboard players

– Results suggest that practicing a skill reorganizes the brain to maximize performance of that skill


Brain Correlates of Music Practice


Special Training in Adulthood

• Adult experiences can also modify brain anatomy

• However, research is needed to determine whether the effects are strong enough to be observed with MRI or similar technology


When Brain Reorganization Goes Too Far

• Focal hand dystonia or “musicians cramp” refers to a condition where the reorganization of the brain goes too far– The fingers of musicians who practice

extensively become clumsy, fatigue easily, and make involuntary movements

– This condition is a result of extensive reorganization of the sensory thalamus and cortex so that touch responses to one finger overlap those of another


Brain Development and Behavioral Development

• Adolescents tend to be more impulsive than adults– Impulsivity can be a problem when it leads to

drinking, risky driving, sex, etc.

• Adolescents tend to “discount the future”


Brain Development and Adolescents

• Adolescents are not equally impulsive in all situations– Peers, amount of time to make decisions, etc.,

affect their decisions

• The prefrontal cortex of adolescents is relatively inactive in certain situations, but this may or may not be the cause of impulsivity


Brain Development and Old Age

• Some neurons lose their synapses, and the remaining synapses change more slowly than before in response to experiences

• Brain structures begin to lose volume

• Research underestimates older people:– People vary in respect to intellectual decline

– Older people have a greater base of knowledge and experience, and many find ways to compensate for losses


4.3 Plasticity after Brain Damage

• Almost all survivors of brain damage show behavioral recovery to some degree

• Some recovery relies on the growth of new branches of axons and dendrites

• Understanding the processes of recovery will give us new and improved therapies


Plasticity After Brain Damage – Recovery

• Most survivors of brain damage show some degree of behavioral recovery

• Some of the mechanisms of recovery include those similar to the mechanisms of brain development such as the new branching of axons and dendrites


Brain Damage and Short-Term Recovery, Part 1

• Possible causes of brain damage– Tumors

– Infections

– Exposure to toxic substances or radiation

– Degenerative diseases

– Closed head injuries


Brain Damage and Short-Term Recovery, Part 2

• A closed head injury refers to a sharp blow to the head that does not puncture the brain– One of the main causes of brain injury in young

adults

– After a severe injury, recovery can be slow and incomplete

• A stroke or cerebrovascular accident is temporary loss of blood flow to the brain– Common cause of brain damage in elderly


Three Examples of Damaged Human Brains


Types of Strokes

• Ischemia: the most common type of stroke, resulting from a blood clot or obstruction of an artery – Neurons lose their oxygen and glucose supply

• Hemorrhage: a less frequent type of stroke resulting from a ruptured artery– Neurons are flooded with excess blood,

calcium, oxygen, and other chemicals


Effects of Strokes, Part 1

• Ischemia and hemorrhage also cause:– Edema: the accumulation of fluid in the brain

resulting in increased pressure on the brain and increasing the probability of further strokes

– Disruption of the sodium-potassium pump leading to the accumulation of potassium ions inside neurons


Effects of Strokes, Part 2

• Edema and excess potassium triggers the release of the excitatory neurotransmitter glutamate

• The overstimulation of neurons leads to sodium and other ions entering the neuron in excessive amounts

• Excess positive ions in the neuron block metabolism in the mitochondria and kill the neuron


Immediate Treatments for Stroke, Part 1

• A drug called tissue plasminogen activator (tPA) breaks up blood clots and can reduce the effects of an ischemic strokes

• Research has begun to attempt to save neurons from death by blocking:– Glutamate synapses

– Calcium entry



• One of the most effective laboratory methods used to minimize damage caused by strokes is to cool the brain

• Cooling protects the brain after ischemia by reducing overstimulation, apoptosis, and inflammation



• Cannabanoids have also been shown to potentially minimize cell loss after a brain stroke

• Benefits are most likely due to cannabinoids anti-inflammatory effects– Research shows that they are most effective in

laboratory animals when taken before the stroke


Later Mechanisms of Recovery from Brain Damage

• Following brain damage, surviving brain areas increase or reorganize their activity– Diaschisis: decreased activity of surviving

neurons after damage to other neurons• Because activity in one area stimulates other areas,

damage to the brain disrupts patterns of normal stimulation

• Drugs (stimulants) may stimulate activity in healthy regions of the brain after a stroke


Later Mechanisms of Recovery

• Destroyed cell bodies cannot be replaced, but damaged axons do grow back under certain circumstances– If an axon in the peripheral nervous system is

crushed, it follows its myelin sheath back to the target and grows back toward the periphery at a rate of about 1 mm per day


Regrowth of Axons

• Damaged axons do not readily regenerate in a mature mammalian brain or spinal cord– Scar tissue makes a mechanical barrier to

axon growth

– Neurons on the two sides of the cut pull apart

– Glia cells that react to CNS damage release chemicals that inhibit axon growth

• Research on building protein bridges may help


Axon Sprouting

• Collateral sprouts are new branches formed by other non-damaged axons that attach to vacant receptors

• Cells that have lost their source of innervation release neurotrophins that induce axons to form collateral sprouts

• Over several months, the sprouts fill in most vacated synapses and can be useful, neutral, or harmful


Collateral Sprouting


Denervation Supersensitivity

• Postsynaptic cells deprived of synaptic inputs develop increased sensitivity to the neurotransmitter to compensate for decreased input

• Denervation supersensitivity: the heightened sensitivity to a neurotransmitter after the destruction of an incoming axon– Can cause consequences such as chronic pain


Reorganized Sensory Representationsand the Phantom Limb, Part 1

• Phantom limb: the continuation of sensation of an amputated body part – The cortex reorganizes itself after the

amputation of a body part by becoming responsive to other parts of the body

– Original axons degenerate leaving vacant synapses into which others axons sprout


Reorganized Sensory Representationsand the Phantom Limb, Part 2

• The phantom limb can lead to the feeling of sensations in the amputated part of the body when other parts of the body are stimulated– e.g., a touch on the face can bring about the

experience of a phantom arm

– Use of an artificial limb can reduce the likelihood of experiencing phantom limb


Somatosensory Cortex of a Monkey after a Finger Amputation


Sources of Phantom Sensation


Learned Adjustments in Behavior

• Deafferentated limb: limbs that have lost their afferent sensory input– Can still be used but are often not because use

of other mechanisms to carry out the behavior are easier

– Has led to the development of therapy techniques to improve functioning of brain damaged people

• Focuses on what they are capable of doing


Cross-Section Through the Spinal Cord

Biological Psychology: Genetics, Evolution, Development, & Plasticity

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Transcript of Biological Psychology: Genetics, Evolution, Development, & Plasticity