Biological and Evolutionary Basis of Behaviour

This chapter answers Section XH5.3 of the Humanities and Social Sciences (XH) paper, “Biological and Evolutionary Basis of Behaviour”, which is Section XH5.3 of XH5 Psychology, one of the six optional sections chosen at the time of application. It teaches heredity and behaviour, evolution and natural selection; the neuron and the neural impulse, synaptic transmission and neurotransmitters; the organisation of the nervous system, the structures of the brain and their functions, and hemispheric lateralization; the endocrine system and the hormonal regulation of behaviour; the biological basis of motivation, emotion (the limbic system) and memory; the physiological methods of the field, both invasive and non-invasive; and genetics and behaviour, including chromosomal anomalies and the nature–nurture debate studied through twins and adoption. Two numericals appear: a heritability estimate from twin data and a chromosome count.

1. Heredity, evolution and the nature–nurture question

Heredity works through genes, segments of DNA carried on chromosomes; a typical human cell has 46 chromosomes in 23 pairs — 22 autosomes and one pair of sex chromosomes (XX female, XY male). The genotype is the genetic makeup, the phenotype the expressed trait, and most behaviour is polygenic (shaped by many genes together with the environment). Darwin’s theory of evolution by natural selection rests on three facts: there is heritable variation, more offspring are produced than can survive, and individuals whose traits improve survival and reproduction leave more descendants, so those traits spread. The reproductive success of a trait is its fitness, and traits shaped this way are adaptations. Evolutionary psychology applies this logic to mind and behaviour.

Behaviour genetics asks how much of the variation in a trait across a population is genetic. Heritability (h²) is the proportion of phenotypic variance attributable to genetic variance — a population statistic, not a statement about one person. Twin studies compare monozygotic (MZ, identical) twins, who share nearly all their genes, with dizygotic (DZ, fraternal) twins, who share about half; much higher MZ than DZ similarity points to genetic influence, and Falconer’s estimate is h² = 2(r_MZ − r_DZ). Adoption studies separate the two influences by comparing adopted children with their biological and their rearing parents.

  • Chromosomal anomalies alter behaviour and development. Down syndrome is trisomy 21 (an extra chromosome 21, giving 47 in all). Turner syndrome is a single X in females (45,X); Klinefelter syndrome is an extra X in males (47,XXY); Fragile X is a leading inherited cause of intellectual disability.
  • The nature–nurture debate is now answered by interaction: genes and environment work together, a genotype sets a range that experience fills in, and epigenetic changes let the environment switch gene expression on and off.

2. Neurons, the neural impulse and synaptic transmission

A neuron has dendrites that receive input, a cell body (soma) that integrates it, and an axon that carries the output to its terminals, which form synapses with the next cell. Many axons wear a fatty myelin sheath broken by nodes of Ranvier, which speeds conduction. Functionally there are sensory (afferent) neurons, motor (efferent) neurons and interneurons; glial cells support, insulate and feed them.

At rest the inside of the axon is about −70 mV relative to outside (the resting potential), kept up by the sodium–potassium pump. A stimulus that depolarises the membrane past threshold (about −55 mV) triggers an action potential: voltage-gated Na⁺ channels open and Na⁺ rushes in (rising phase), then K⁺ channels open and K⁺ flows out (repolarisation), followed by a brief refractory period. The impulse obeys the all-or-none law — it either fires fully or not at all, and intensity is coded by firing rate. In myelinated axons the impulse leaps node to node, called saltatory conduction, which is faster.

When the action potential reaches the terminal it opens voltage-gated Ca²⁺ channels; calcium influx makes synaptic vesicles release neurotransmitter into the synaptic cleft. The transmitter binds receptors on the next cell, producing an excitatory post-synaptic potential (EPSP) or an inhibitory one (IPSP); the cell fires only if the summed EPSPs reach threshold. The signal is ended by reuptake, enzymatic breakdown or diffusion. Key transmitters: acetylcholine (muscle action and memory; depleted in Alzheimer’s), dopamine (reward and movement; low in Parkinson’s, implicated in schizophrenia), serotonin (mood, sleep, appetite; low in depression), norepinephrine (arousal), GABA (the main inhibitory transmitter) and glutamate (the main excitatory transmitter).

3. The nervous system, brain structures and lateralization

The nervous system divides into the central nervous system (brain and spinal cord) and the peripheral nervous system, whose autonomic branch runs involuntary organs through a sympathetic division (arousal, fight-or-flight) opposed by a parasympathetic division (rest-and-digest). Within the brain, the hindbrain holds the medulla (heartbeat, breathing), the pons and the cerebellum (balance, coordination, motor learning); the reticular formation (arousal and attention) runs up through the brainstem into the midbrain; and the forebrain holds the thalamus (sensory relay for all senses except smell), the hypothalamus (homeostasis, hunger, thirst, temperature and control of the pituitary), the limbic system and the cerebral cortex.

  • The cortex has four lobes: the frontal (motor cortex, executive planning, and Broca’s area for speech production), the parietal (the somatosensory cortex and spatial processing), the temporal (auditory cortex and Wernicke’s area for language comprehension) and the occipital (visual cortex).
  • Each hemisphere controls the opposite side of the body (contralateral control). In most people the left hemisphere specialises in language, logic and sequential analysis, the right in spatial relations, faces and holistic processing; the corpus callosum joins them, and split-brain studies (severing it) revealed this lateralization.

4. The endocrine system and hormonal regulation

The endocrine system sends hormones through the bloodstream; it acts more slowly than the nervous system but its effects last longer. The pituitary, directed by the hypothalamus, is the master gland: it releases growth hormone and the tropic hormones that drive the other glands. The thyroid secretes thyroxine, which sets metabolic rate. The adrenal glands have a cortex (cortisol, the stress hormone) and a medulla (adrenaline/epinephrine and noradrenaline, which drive the fight-or-flight response alongside the sympathetic nervous system). The pancreas balances blood glucose with insulin and glucagon; the gonads secrete sex hormones — testosterone from the testes, oestrogen and progesterone from the ovaries; and the pineal gland secretes melatonin, which regulates the sleep–wake cycle.

🧠 Two messengers, two speeds
Neural signals are fast and brief and travel a fixed wired path; hormonal signals are slow and sustained and broadcast through the blood to any tissue with the right receptor. The adrenal medulla sits at the junction of the two: it is triggered by the sympathetic nervous system yet acts by releasing hormones, which is why a startle both fires instantly and leaves you shaky for minutes.

5. Biological basis of motivation, emotion and memory

  • Hunger is governed by the hypothalamus: the lateral hypothalamus drives eating (its damage causes refusal to eat) and the ventromedial hypothalamus signals satiety (its damage causes overeating), with blood glucose, the hunger hormone ghrelin and the fat-signal leptin feeding in.
  • Thirst arises from cellular dehydration and from a fall in blood volume, again sensed through the hypothalamus. Sleep follows a circadian rhythm set by the hypothalamic suprachiasmatic nucleus and melatonin, cycling through NREM and REM stages, with the reticular formation controlling overall arousal. Sex behaviour is organised by the hypothalamus and the sex hormones.

Emotion is centred on the limbic system — the amygdala (which attaches emotional, especially fearful, significance to events), the hippocampus, the hypothalamus (which drives the bodily expression of emotion) and the cingulate gyrus. Memory draws on several structures: the hippocampus consolidates new declarative (explicit) memories, so damage to it causes anterograde amnesia — an inability to form new long-term memories — while old memories and skills survive; the amygdala contributes the emotional colouring of memories, and the cerebellum and basal ganglia hold procedural (skill) memory. At the cellular level, repeated use strengthens synapses through long-term potentiation (LTP), a basis of learning.

📖 The case of H.M.
The link between the hippocampus and new memory was made vivid by the patient known as H.M., whose medial temporal lobes, including most of the hippocampus, were removed on both sides to treat severe epilepsy. Afterwards he could hold a conversation and recall his distant past, but he could not form new long-term memories of people or events — a profound anterograde amnesia that showed the hippocampus is needed to lay down new declarative memories, not to store the old ones.

6. Physiological methods: invasive and non-invasive

  • Invasive methods enter the body or brain. Anatomical methods stain and section tissue to map structure; degeneration techniques trace pathways by following a cut axon as it degenerates; lesion techniques destroy or remove (ablation) a region and observe the behavioural loss; chemical methods microinject drugs or neurotoxins; and microelectrode studies record the firing of a single neuron.
  • Non-invasive methods work from outside. The EEG records the brain’s electrical rhythms from scalp electrodes and has excellent timing. Scanning methods include CT (an X-ray image of structure), MRI (a magnetic image of structure in fine detail), fMRI (which tracks blood-oxygen change to map function) and PET (which uses a radioactive tracer to image metabolism and function). The muscular and glandular systems are read by EMG (muscle activity) and the galvanic skin response (sweat-gland activity).
⚠️ Structure scans versus function scans
CT and ordinary MRI show what the brain looks like (structure); fMRI and PET show what it is doing (function), by tracking blood oxygen or a metabolic tracer. EEG also shows function and has the sharpest timing but poor localisation, while MRI localises finely but says nothing on its own about activity. Matching the question to the method — structure or function, timing or location — is the usual exam point.

Key takeaways

  • Genes on chromosomes (46 in humans, 23 pairs) carry heredity; natural selection needs heritable variation, over-production and differential reproductive success (fitness). Heritability h² is a population statistic; twin and adoption studies separate genes from environment, and nature and nurture interact.
  • The resting potential (−70 mV) is maintained by the sodium–potassium pump; past threshold, Na⁺ in then K⁺ out makes an all-or-none action potential, with saltatory conduction in myelinated axons. Synaptic transmission is Ca²⁺ influx, vesicle release, receptor binding (EPSP/IPSP), then reuptake or breakdown.
  • The CNS plus PNS (with sympathetic and parasympathetic autonomic branches); hindbrain (medulla, cerebellum), reticular formation (brainstem into midbrain), forebrain (thalamus, hypothalamus, cortex). Lobes: frontal (motor, Broca), parietal (somatosensory), temporal (auditory, Wernicke), occipital (vision); control is contralateral and the left hemisphere is usually the language hemisphere.
  • The pituitary is the master gland under the hypothalamus; thyroid sets metabolism, the adrenal medulla drives fight-or-flight with adrenaline, the pancreas controls glucose, gonads make sex hormones, the pineal makes melatonin. Hormones are slower and longer-lasting than neural signals.
  • The hypothalamus runs hunger (lateral drives eating, ventromedial signals satiety), thirst, sleep (suprachiasmatic nucleus, melatonin) and sex. The limbic system (amygdala for fear) underlies emotion; the hippocampus consolidates declarative memory, so its damage causes anterograde amnesia, and LTP is the cellular basis of learning.
  • Invasive methods — anatomical, degeneration, lesion/ablation, chemical and microelectrode — enter the tissue; non-invasive methods — EEG (timing), CT and MRI (structure), fMRI and PET (function) — work from outside. Match structure-versus-function and timing-versus-location to the right tool.

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Which trio of conditions is together necessary for evolution by natural selection to occur in a population?

    1. Heritable variation among individuals, more offspring than can survive, and differential reproductive success
    2. Identical individuals, unlimited resources for all, and reproduction that is entirely independent of any trait
    3. Traits acquired during an individual’s lifetime that are then directly passed on to that individual’s offspring
    4. A fixed environment, a constant population size, and the complete absence of any genetic mutation over time
    Show answer

    Answer: A — Heritable variation among individuals, more offspring than can survive, and differential reproductive success

    Natural selection requires heritable variation, over-production of offspring, and differential reproductive success tied to that variation. Identical individuals leave nothing to select on; inheritance of acquired characteristics is the discarded Lamarckian idea, not Darwinian selection; and a fixed environment with no mutation describes conditions under which selection would have nothing to act on.

  2. For a given trait, monozygotic twins are far more similar to each other than dizygotic twins are. Assuming the twins share their rearing environment similarly, what does this pattern most directly suggest?

    1. That the trait is purely environmental, since twins are raised in the same home and share their surroundings
    2. That genes contribute to the trait, since the more genetically alike pair is also the more similar pair
    3. That the trait has no genetic basis at all, because both kinds of twins come from the same two parents
    4. That adoption, rather than any twin comparison, is the only method capable of detecting heritability
    Show answer

    Answer: B — That genes contribute to the trait, since the more genetically alike pair is also the more similar pair

    MZ twins share nearly all their genes and DZ twins about half, so when the more genetically similar pair is the more alike pair under matched environments, genes are implicated. The purely-environmental option is contradicted by the MZ–DZ gap; both twin types do not share the same genes (that is the whole point of the comparison); and twin studies, not only adoption studies, estimate heritability.

  3. Arrange the key events of synaptic transmission in their correct temporal order:
    (P) neurotransmitter binds receptors on the post-synaptic membrane;
    (Q) the action potential arrives at the axon terminal;
    (R) voltage-gated calcium channels open and Ca²⁺ enters;
    (S) synaptic vesicles fuse and release neurotransmitter into the cleft.

    1. P, then Q, then S, then R
    2. Q, then P, then R, then S
    3. Q, then R, then S, then P
    4. R, then Q, then P, then S
    Show answer

    Answer: C — Q, then R, then S, then P

    The action potential arrives first (Q), which opens calcium channels so Ca²⁺ enters (R), which makes vesicles release transmitter into the cleft (S), which then binds post-synaptic receptors (P). The orders beginning with binding or with calcium entry reverse this causal chain — nothing is released or binds before the impulse arrives and calcium enters.

  4. During the rising phase of an action potential, the inside of the axon swings from about −70 mV toward a positive value. Which ionic movement is directly responsible for this rapid depolarisation?

    1. Potassium ions (K⁺) flowing out of the axon through voltage-gated potassium channels that have just opened
    2. Chloride ions (Cl⁻) flooding into the axon and making the interior still more negative than at rest
    3. The sodium–potassium pump actively exchanging three sodium ions for every two potassium ions it imports
    4. Sodium ions (Na⁺) rushing into the axon through voltage-gated sodium channels that open at threshold
    Show answer

    Answer: D — Sodium ions (Na⁺) rushing into the axon through voltage-gated sodium channels that open at threshold

    At threshold the voltage-gated sodium channels open and Na⁺ rushes in, driving the rapid rise toward a positive interior. Potassium outflow causes the later repolarisation, not the rise; chloride influx would hyperpolarise; and the sodium–potassium pump restores gradients slowly and does not produce the fast spike.

  5. After a stroke, a patient can understand speech perfectly well but produces only slow, halting, effortful speech. Damage to which area best accounts for this pattern?

    1. Broca’s area in the frontal lobe, which is involved in the production of fluent, articulated speech
    2. Wernicke’s area in the temporal lobe, whose damage instead impairs the comprehension of language
    3. The occipital lobe, whose damage impairs vision rather than either the production or understanding of speech
    4. The cerebellum, whose damage impairs balance and motor coordination rather than the use of language
    Show answer

    Answer: A — Broca’s area in the frontal lobe, which is involved in the production of fluent, articulated speech

    Preserved comprehension with laboured, non-fluent speech is the classic picture of damage to Broca’s area, which supports speech production. Wernicke’s damage would impair comprehension (the opposite of this case); occipital damage affects vision; and cerebellar damage affects balance and coordination, none of which fit a pure speech-output problem.

  6. In a classic split-brain patient (corpus callosum severed), an object is shown only in the left visual field, which projects to the right hemisphere. The patient typically cannot name the object aloud. Why?

    1. Because the right hemisphere is completely blind, so no visual information about the object reaches either side of the brain at all
    2. Because visual information reaches the right hemisphere but, with the callosum cut, cannot cross to the left hemisphere’s language centres
    3. Because naming objects is a function that both hemispheres perform equally well, so severing the callosum removes it from the patient entirely
    4. Because the left visual field projects to the left hemisphere, which in these patients has lost all of its language ability
    Show answer

    Answer: B — Because visual information reaches the right hemisphere but, with the callosum cut, cannot cross to the left hemisphere’s language centres

    The left visual field goes to the right hemisphere; language is in the left hemisphere; with the corpus callosum cut, the information cannot transfer, so the verbal left hemisphere never receives it and cannot name the object. The right hemisphere is not blind, naming is not shared equally across hemispheres, and the left field does not project to the left hemisphere.

  7. Which gland is called the ‘master gland’ because, under the direction of the hypothalamus, it releases hormones that control the activity of several other endocrine glands?

    1. The thyroid gland, which secretes thyroxine and thereby sets the body’s overall metabolic rate
    2. The adrenal medulla, which secretes adrenaline and drives the body’s rapid fight-or-flight response
    3. The pituitary gland, which releases tropic hormones that in turn regulate several other endocrine glands
    4. The pineal gland, which secretes melatonin and in doing so regulates the daily sleep–wake cycle
    Show answer

    Answer: C — The pituitary gland, which releases tropic hormones that in turn regulate several other endocrine glands

    The pituitary is the master gland: directed by the hypothalamus, its tropic hormones control the thyroid, adrenal cortex, gonads and more. The thyroid sets metabolism, the adrenal medulla drives fight-or-flight, and the pineal controls the sleep cycle — each an important gland, but none of them the one that governs the others.

  8. A person is suddenly frightened and their heart races, pupils widen and they feel a surge of energy that lingers for minutes. Which structure releases the hormone most directly producing this sustained bodily arousal?

    1. The pancreas, by releasing insulin, which lowers blood glucose during moments of acute psychological stress
    2. The pineal gland, by releasing melatonin, which raises alertness and heart rate in response to sudden fear
    3. The thyroid gland, by releasing extra thyroxine within seconds to meet the immediate demand of the threat
    4. The adrenal medulla, by releasing adrenaline (epinephrine), which sustains the fight-or-flight arousal
    Show answer

    Answer: D — The adrenal medulla, by releasing adrenaline (epinephrine), which sustains the fight-or-flight arousal

    The adrenal medulla releases adrenaline, which sustains the racing heart and energy surge of the fight-or-flight response for minutes after the sympathetic nerves fire. Insulin lowers glucose and would not arouse; melatonin promotes sleep, not alarm; and thyroxine sets long-term metabolic rate rather than a sudden emergency surge.

  9. Within the limbic system, which structure is most strongly associated with attaching emotional significance to events, especially the detection of threat and fear?

    1. The amygdala, which flags the emotional and especially the threatening significance of a stimulus
    2. The cerebellum, which coordinates balance and the smooth timing of voluntary movements
    3. The medulla oblongata, which regulates automatic vital functions such as heartbeat and breathing
    4. The occipital cortex, which carries out the first stages of the processing of visual information
    Show answer

    Answer: A — The amygdala, which flags the emotional and especially the threatening significance of a stimulus

    The amygdala is the limbic structure central to emotion and to fear and threat detection. The cerebellum handles balance and movement timing, the medulla controls vital autonomic functions, and the occipital cortex handles vision — none of them the emotional-significance structure.

  10. A researcher wants to map which brain regions become more active while a healthy volunteer performs a mental arithmetic task, without entering the skull. Which method is most suitable?

    1. A stereotaxic lesion of the suspected region, followed by observation of the resulting loss of arithmetic ability
    2. Functional MRI (fMRI), which tracks changes in blood oxygenation to show which regions are active during the task
    3. A microelectrode inserted into single neurons of the volunteer’s cortex to record their individual firing rates
    4. A degeneration study that cuts a tract and follows the degenerating axons to trace the arithmetic pathway
    Show answer

    Answer: B — Functional MRI (fMRI), which tracks changes in blood oxygenation to show which regions are active during the task

    fMRI maps task-related activity through blood-oxygen changes and is non-invasive, which fits a healthy volunteer exactly. Stereotaxic lesions, microelectrode insertion and degeneration studies are all invasive procedures that cannot ethically be performed on a healthy person to map normal function.

  11. Which of the following statements about chromosomal anomalies and heritability are correct?

    1. Down syndrome results from an extra copy of chromosome 21 (trisomy 21)
    2. Turner syndrome involves a single X chromosome in females (45,X)
    3. Heritability is a statistic about variation within a population, not a fixed figure for any one individual
    4. A heritability of 0.5 for a trait means exactly half of any single person’s trait is caused by their genes
    Show answer

    Answer: A — Down syndrome results from an extra copy of chromosome 21 (trisomy 21); B — Turner syndrome involves a single X chromosome in females (45,X); C — Heritability is a statistic about variation within a population, not a fixed figure for any one individual

    Down syndrome is trisomy 21, Turner syndrome is 45,X, and heritability describes variation across a population. The claim that a heritability of 0.5 means half of a single person’s trait is genetic is the classic misreading: heritability is a population-level proportion of variance and cannot be split within one individual.

  12. Which of the following statements about neurotransmitters are correct?

    1. GABA is the main inhibitory neurotransmitter in the central nervous system
    2. Dopamine is involved in reward and movement and is depleted in Parkinson’s disease
    3. Acetylcholine plays a role in muscle action and in memory and is reduced in Alzheimer’s disease
    4. Glutamate is the principal inhibitory neurotransmitter and always hyperpolarises the next neuron
    Show answer

    Answer: A — GABA is the main inhibitory neurotransmitter in the central nervous system; B — Dopamine is involved in reward and movement and is depleted in Parkinson’s disease; C — Acetylcholine plays a role in muscle action and in memory and is reduced in Alzheimer’s disease

    GABA is the main inhibitory transmitter, dopamine is tied to reward and movement and falls in Parkinson’s, and acetylcholine serves muscle action and memory and declines in Alzheimer’s. The glutamate claim is false and reversed: glutamate is the main excitatory transmitter, which depolarises rather than hyperpolarises the next neuron.

  13. Which of the following statements about the biological basis of motivation and memory are correct?

    1. Damage to the hippocampus can cause anterograde amnesia, an inability to form new long-term memories
    2. The lateral hypothalamus is involved in initiating eating, and its damage can lead to a failure to eat
    3. The suprachiasmatic nucleus of the hypothalamus helps regulate the circadian sleep–wake rhythm
    4. The cerebellum is the structure that consolidates new facts and events into declarative long-term memory
    Show answer

    Answer: A — Damage to the hippocampus can cause anterograde amnesia, an inability to form new long-term memories; B — The lateral hypothalamus is involved in initiating eating, and its damage can lead to a failure to eat; C — The suprachiasmatic nucleus of the hypothalamus helps regulate the circadian sleep–wake rhythm

    Hippocampal damage causes anterograde amnesia, the lateral hypothalamus initiates eating, and the suprachiasmatic nucleus regulates the circadian rhythm. The cerebellum claim is false: the cerebellum supports procedural (skill) memory, while the hippocampus is what consolidates declarative memory.

  14. Which of the following correctly describe physiological methods used to study the brain and behaviour?

    1. The EEG records the brain’s electrical activity from scalp electrodes and has very good temporal resolution
    2. A lesion technique that destroys a region to observe the resulting behavioural change is an invasive method
    3. PET imaging uses a radioactive tracer to show metabolic activity and is classed as a functional method
    4. A plain structural MRI, on its own, directly measures the moment-to-moment activity of neurons during a task
    Show answer

    Answer: A — The EEG records the brain’s electrical activity from scalp electrodes and has very good temporal resolution; B — A lesion technique that destroys a region to observe the resulting behavioural change is an invasive method; C — PET imaging uses a radioactive tracer to show metabolic activity and is classed as a functional method

    EEG records electrical activity with excellent timing, a lesion is an invasive technique, and PET is a functional method using a radioactive tracer. The MRI claim is false: a plain structural MRI shows anatomy, not moment-to-moment neural activity — that is what fMRI, not ordinary MRI, provides.

  15. For a certain trait the correlation between monozygotic (identical) twins is r_MZ = 0.80 and between dizygotic (fraternal) twins is r_DZ = 0.50. Using Falconer’s formula, heritability h² = 2(r_MZ − r_DZ), compute h². Give the answer to one decimal place.

    Numerical answer — type the value.

    Show answer

    Answer: 0.6

    h² = 2(r_MZ − r_DZ) = 2(0.80 − 0.50) = 2(0.30) = 0.60. This estimates that 60% of the variance in the trait across this population is attributable to genetic variance.

  16. A typical human body cell contains 46 chromosomes. A person with Down syndrome has an extra copy of chromosome 21 (trisomy 21). How many chromosomes does such a person have in each body cell?

    Numerical answer — type the value.

    Show answer

    Answer: 47

    Trisomy 21 adds one extra chromosome to the usual 46, giving 46 + 1 = 47 chromosomes per body cell.