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SACE Stage 2 Biology study materials
SACE Stage 2 Biology study resources

SACE Stage 2 Biology practice questions

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Question 1Science understanding

Which molecule stores the genetic instructions that are passed from a parent cell to its daughter cells during mitosis?

  1. ATP
  2. Phospholipid
  3. DNA Correct answer.
  4. Glucose

Explanation

DNA is the hereditary molecule; its base sequence encodes the instructions for building proteins and is copied before mitosis so each daughter cell receives a complete set. ATP is an energy carrier and glucose is a fuel, neither stores heritable information. Phospholipids are structural components of membranes.

Question 2Science understanding

During which phase of the cell cycle does DNA replication occur?

  1. G1 phase
  2. M phase
  3. G2 phase
  4. S phase Correct answer.

Explanation

Replication takes place during S phase (synthesis), when each chromosome is copied to form two sister chromatids. G1 is a growth phase before replication and G2 follows it, while M phase is when the copied chromosomes are separated and the cell divides.

Question 3Science understanding

A cross between two heterozygous pea plants (Tt x Tt), where T is tall and t is short, produces offspring. What proportion of the offspring is expected to show the recessive short phenotype?

  1. 3/4
  2. 1/2
  3. 1/4 Correct answer.
  4. 0

Explanation

Each heterozygous parent produces T and t gametes in equal numbers, giving genotype ratios of 1 TT : 2 Tt : 1 tt. Only the tt genotype expresses the recessive short phenotype, so one quarter of the offspring are expected to be short.

Question 4Science understanding

Which process produces gametes with half the chromosome number of the parent cell?

  1. Meiosis Correct answer.
  2. Binary fission
  3. Mitosis
  4. Cytokinesis

Explanation

Meiosis involves two successive divisions after a single round of replication, so the four resulting gametes each carry half the original chromosome number (haploid). Mitosis and binary fission maintain chromosome number, and cytokinesis is only the physical splitting of the cytoplasm.

Question 5Science understanding

Which structure controls the movement of substances into and out of a cell?

  1. Plasma membrane Correct answer.
  2. Ribosome
  3. Cell wall
  4. Nucleolus

Explanation

The plasma membrane is a phospholipid bilayer with embedded proteins that selectively regulates what enters and leaves the cell. The cell wall is freely permeable and mainly structural, while the nucleolus and ribosomes are involved in making ribosomal components and proteins.

Question 6Science understanding

Which of the following best describes an enzyme's effect on a biochemical reaction?

  1. It is permanently consumed during the reaction.
  2. It lowers the activation energy required. Correct answer.
  3. It changes the equilibrium constant of the reaction.
  4. It increases the activation energy required.

Explanation

Enzymes are biological catalysts that provide an alternative reaction pathway with a lower activation energy, increasing the rate at which substrate is converted to product. They are not consumed and do not alter the overall energy difference between reactants and products, so equilibrium position is unchanged.

Question 7Science understanding

In a food chain, what is the main reason that only about ten percent of energy passes from one trophic level to the next?

  1. Photosynthesis converts energy into new matter.
  2. Energy is lost as heat and through respiration and excretion. Correct answer.
  3. Most energy is destroyed when organisms are eaten.
  4. Predators absorb all the energy stored in their prey.

Explanation

At each trophic level energy is used for respiration and released as heat, lost in excreted and undigested material, and some remains in uneaten parts. Only a small fraction is incorporated into new biomass available to the next level, giving the familiar ten percent transfer.

Question 8Science understanding

Which of the following is a correct statement about the role of decomposers in an ecosystem?

  1. They convert atmospheric nitrogen into nitrates.
  2. They fix carbon dioxide into glucose using sunlight.
  3. They break down dead organic matter and release inorganic nutrients. Correct answer.
  4. They transfer energy directly to top predators.

Explanation

Decomposers such as bacteria and fungi break down dead organisms and wastes, releasing inorganic nutrients like nitrates and phosphates back into the soil for producers to reuse. Nitrogen fixation and photosynthesis are carried out by different organisms and are separate processes.

Question 9Science understanding

A mutation changes a single DNA base, but the amino acid sequence of the resulting protein is unchanged. What is this type of mutation called?

  1. Nonsense mutation
  2. Silent mutation Correct answer.
  3. Missense mutation
  4. Frameshift mutation

Explanation

Because the genetic code is degenerate, several codons can specify the same amino acid, so a base substitution may not alter the protein sequence; this is a silent mutation. Frameshift mutations shift the reading frame, nonsense mutations create a stop codon, and missense mutations change one amino acid.

Question 10Science understanding

In a population of 500 individuals, 20 are homozygous recessive for a particular trait. Assuming Hardy-Weinberg equilibrium, what is the approximate frequency of the recessive allele?

  1. 0.80
  2. 0.20 Correct answer.
  3. 0.32
  4. 0.04

Explanation

The homozygous recessive frequency q squared is 20/500 = 0.04, so q, the recessive allele frequency, is the square root of 0.04, which is 0.20. The dominant allele frequency p is therefore 0.80, but that is not what the question asks.

Question 11Science understanding

A cell is placed in a hypertonic solution. What is the most likely immediate effect on an animal cell?

  1. It actively pumps solute in to equalise concentration.
  2. It shrinks as water leaves by osmosis. Correct answer.
  3. It swells and may burst.
  4. It remains unchanged because the membrane is impermeable.

Explanation

In a hypertonic solution the external solute concentration is higher than inside the cell, so the water potential outside is lower and water moves out by osmosis. An animal cell, lacking a rigid wall, loses volume and shrinks.

Question 12Science understanding

Which statement best explains why the rate of an enzyme-controlled reaction plateaus at high substrate concentration?

  1. The substrate molecules begin to repel each other.
  2. All active sites become saturated with substrate. Correct answer.
  3. The enzyme is destroyed by high substrate levels.
  4. The temperature rises and denatures the enzyme.

Explanation

When substrate concentration is high, essentially every enzyme active site is occupied, so the enzyme works at its maximum rate (Vmax). Adding more substrate cannot increase the rate further unless more enzyme is present.

Question 13Science understanding

In a dihybrid cross between two individuals heterozygous for both genes (AaBb x AaBb), assuming independent assortment, what fraction of offspring is expected to show both recessive phenotypes?

  1. 9/16
  2. 1/4
  3. 1/16 Correct answer.
  4. 1/8

Explanation

Each gene behaves independently, so the probability of the recessive phenotype for gene A is 1/4 and for gene B is also 1/4. Multiplying independent probabilities gives 1/16 for offspring showing both recessive traits.

Question 14Science understanding

A woman who is a carrier of an X-linked recessive disorder has children with an unaffected man. What proportion of their sons is expected to be affected?

  1. 0
  2. 1/2 Correct answer.
  3. All sons
  4. 1/4

Explanation

Sons receive their single X chromosome from the mother, and a carrier mother passes the affected allele to half of her sons. Because males have only one X, each son receiving the affected allele will express the disorder, so half of the sons are expected to be affected.

Question 15Science understanding

Which of the following provides evidence for evolution by comparing the same structure in different species, such as the forelimbs of mammals?

  1. Homologous structures Correct answer.
  2. Fossil dating
  3. Vestigial structures
  4. Analogous structures

Explanation

Homologous structures share a common underlying anatomy and developmental origin, such as the forelimbs of whales, bats and humans, indicating descent from a common ancestor. Analogous structures perform similar functions without shared ancestry, and vestigial structures are reduced remnants rather than comparisons of the same structure.

Question 16Science understanding

A population of insects develops resistance to an insecticide over several generations. Which mechanism best explains this?

  1. Pre-existing resistant variants survive and reproduce more successfully. Correct answer.
  2. The insecticide is inherited by offspring and protects them.
  3. Insects learn to tolerate the insecticide during their lifetime.
  4. The insecticide causes mutations that make insects resistant.

Explanation

Natural selection acts on variation already present in the population; individuals with alleles conferring resistance survive and reproduce, increasing the frequency of those alleles in later generations. Mutations arise spontaneously and are not directed by the insecticide, and acquired tolerance is not inherited.

Question 17Science understanding

Which part of the human nervous system is primarily responsible for maintaining homeostasis by regulating involuntary functions such as heart rate and body temperature?

  1. Cerebrum
  2. Medulla oblongata only
  3. Hypothalamus Correct answer.
  4. Cerebellum

Explanation

The hypothalamus monitors internal conditions and coordinates homeostatic responses, including temperature regulation and control of the autonomic nervous system and endocrine glands. The cerebrum handles conscious thought, the cerebellum coordinates movement and balance, and the medulla controls reflexes such as breathing but is not the master regulator.

Question 18Science understanding

In a human karyotype, an individual has three copies of chromosome 21. Which condition does this indicate?

  1. Klinefelter syndrome
  2. Down syndrome Correct answer.
  3. Turner syndrome
  4. Sickle cell anaemia

Explanation

Trisomy 21, the presence of an extra chromosome 21, causes Down syndrome. It usually results from non-disjunction during meiosis. Turner syndrome involves a single X chromosome, Klinefelter syndrome involves an extra X in males, and sickle cell anaemia is caused by a gene mutation.

Question 19Science understanding

Which process in gene expression is directly inhibited by a molecule that blocks RNA polymerase from binding to the promoter?

  1. Transcription Correct answer.
  2. Post-translational modification
  3. DNA replication
  4. Translation

Explanation

RNA polymerase binds to the promoter to synthesise mRNA during transcription, so blocking this binding prevents transcription. Translation occurs later at ribosomes, DNA replication uses DNA polymerase, and post-translational modification affects an already-made protein.

Question 20Science understanding

A student measures the rate of photosynthesis in a water plant at increasing light intensities while keeping carbon dioxide constant. The rate rises then levels off. Which factor is most likely limiting the rate once it plateaus?

  1. Light intensity
  2. Water temperature
  3. Oxygen concentration
  4. Carbon dioxide concentration Correct answer.

Explanation

Once light is no longer limiting, some other factor must restrict the rate; because carbon dioxide was held constant at a fixed level, it becomes the limiting factor at the plateau. Increasing carbon dioxide would be expected to raise the rate again.

Question 21Science understanding

A deletion of one nucleotide occurs in the middle of a coding sequence. What is the most likely consequence for the resulting protein?

  1. No change, because one base is insignificant.
  2. The protein is always longer by one amino acid.
  3. A single amino acid substitution only.
  4. A frameshift that alters all subsequent amino acids. Correct answer.

Explanation

Codons are read in non-overlapping triplets, so removing one base shifts the reading frame from that point onward. This typically changes every downstream codon and often introduces a premature stop codon, drastically altering the protein.

Question 22Science understanding

In a pedigree, an unaffected couple has an affected daughter and unaffected sons. Which mode of inheritance is most consistent with this pattern?

  1. Autosomal recessive Correct answer.
  2. X-linked dominant
  3. X-linked recessive
  4. Y-linked

Explanation

Two unaffected parents producing an affected child indicates a recessive trait, and an affected daughter rules out X-linked recessive inheritance because her father would then have to be affected. Autosomal recessive inheritance fits, since both parents can be unaffected carriers.

Question 23Science understanding

Two species of finch live on the same island and eat seeds of different sizes, with their beak depths differing accordingly. This is best described as an example of which phenomenon?

  1. Competitive exclusion leading to extinction
  2. Resource partitioning reducing competition Correct answer.
  3. Convergent evolution of beak shape
  4. Genetic drift in small populations

Explanation

When species share a habitat but use different resources, they partition those resources and reduce direct competition, allowing coexistence. Competitive exclusion would involve one species eliminating the other, and convergent evolution would produce similar traits in unrelated lineages for similar environments.

Question 24Science understanding

A reaction has a positive change in free energy. Which statement about this reaction in a cell is most accurate?

  1. It will always proceed spontaneously without energy input.
  2. It releases heat equal to its activation energy.
  3. It cannot occur in living organisms under any circumstances.
  4. It can proceed only if coupled to an energy-releasing reaction. Correct answer.

Explanation

An endergonic reaction, with positive free-energy change, is non-spontaneous on its own and requires energy input. Cells drive such reactions by coupling them to exergonic reactions, commonly the hydrolysis of ATP, so the overall coupled process is favourable.

Question 25Science understanding

In an experiment, a plant is grown in a sealed chamber with carbon dioxide containing radioactive carbon-14. After several hours, which compound is expected to be labelled first in the light-independent reactions?

  1. RuBP
  2. 3-phosphoglycerate Correct answer.
  3. Glucose
  4. Oxygen

Explanation

Carbon dioxide is fixed onto RuBP by rubisco, and the first stable product formed is 3-phosphoglycerate, which becomes labelled immediately. Glucose forms later from the products of the Calvin cycle, RuBP is regenerated rather than being the first product, and oxygen is released from water in the light reactions.

Question 26Science understanding

Which structural feature of the plasma membrane is best described as a phospholipid bilayer with proteins embedded within it?

  1. The induced fit model
  2. The lock-and-key model
  3. The fluid mosaic model Correct answer.
  4. The operon model

Explanation

The fluid mosaic model describes membranes as a phospholipid bilayer in which proteins float and move, giving a mosaic appearance. The other named models describe enzyme-substrate interaction or gene regulation, not membrane architecture.

Question 27Science understanding

During which stage of mitosis do sister chromatids separate and move toward opposite poles of the cell?

  1. Anaphase Correct answer.
  2. Metaphase
  3. Prophase
  4. Telophase

Explanation

Anaphase begins when the spindle fibres shorten and pull the separated chromatids, now individual chromosomes, to opposite poles. Prophase involves chromosome condensation and metaphase involves alignment at the equator.

Question 28Science understanding

Which gas is taken up by a leaf and used as a reactant during the light-independent reactions of photosynthesis?

  1. Carbon dioxide Correct answer.
  2. Nitrogen
  3. Hydrogen
  4. Oxygen

Explanation

Carbon dioxide enters the leaf through stomata and is fixed in the Calvin cycle, where it is combined with a five-carbon compound to build carbohydrate. Oxygen is a product rather than a reactant of these reactions.

Question 29Science understanding

Which type of organism converts atmospheric nitrogen into ammonia during the nitrogen cycle?

  1. Nitrogen-fixing bacteria Correct answer.
  2. Denitrifying bacteria
  3. Decomposing fungi
  4. Nitrifying bacteria

Explanation

Nitrogen-fixing bacteria, often living in root nodules, reduce atmospheric nitrogen gas to ammonia that plants can use. Nitrifying bacteria convert ammonia to nitrite and nitrate, while denitrifying bacteria return nitrogen to the atmosphere.

Question 30Science understanding

In a DNA molecule, adenine pairs with which base?

  1. Thymine Correct answer.
  2. Uracil
  3. Cytosine
  4. Guanine

Explanation

In DNA, adenine forms two hydrogen bonds with thymine, while guanine pairs with cytosine. Uracil replaces thymine only in RNA and does not normally occur in DNA.

Question 31Science understanding

Which type of cell division produces gametes that contain half the chromosome number of the parent cell?

  1. Binary fission
  2. Budding
  3. Meiosis Correct answer.
  4. Mitosis

Explanation

Meiosis involves two successive divisions that halve the chromosome number, producing haploid gametes. Mitosis and the asexual processes listed maintain the parental chromosome number.

Question 32Science understanding

Which blood vessel carries oxygenated blood away from the heart to the body tissues?

  1. Vein
  2. Lymph vessel
  3. Capillary
  4. Artery Correct answer.

Explanation

Arteries carry blood away from the heart under high pressure, and most arteries in the systemic circuit carry oxygenated blood. Veins return blood toward the heart at lower pressure.

Question 33Science understanding

A scientist measures the rate of an enzyme-catalysed reaction at a range of temperatures and finds a peak at 37 degrees Celsius, after which the rate falls sharply. Which explanation accounts for the fall above 37 degrees?

  1. The substrate molecules gain kinetic energy and collide less often with the enzyme
  2. The enzyme is chemically consumed by the reaction and its concentration falls
  3. The enzyme's tertiary structure is disrupted so the active site no longer binds the substrate Correct answer.
  4. The pH of the solution rises and competes with the substrate for the active site

Explanation

Above the optimum, increased thermal energy breaks hydrogen bonds and other interactions holding the enzyme's tertiary shape, so the active site loses its complementary shape and catalysis slows. The change is called denaturation and is usually irreversible.

Question 34Science understanding

A cell with a solute potential of -400 kPa and a pressure potential of 150 kPa is placed in pure water. What is its water potential?

  1. 250 kPa
  2. -550 kPa
  3. -250 kPa Correct answer.
  4. -400 kPa

Explanation

Water potential equals solute potential plus pressure potential, so -400 plus 150 gives -250 kPa. Pure water has a water potential of zero, so water will tend to move into the cell.

Question 35Science understanding

In a monohybrid cross, a heterozygous tall pea plant is crossed with a homozygous short pea plant, with tall dominant. What proportion of offspring is expected to be short?

  1. One half Correct answer.
  2. Three quarters
  3. One quarter
  4. Zero

Explanation

The cross Tt x tt gives offspring of genotypes Tt and tt in equal numbers, so half are tall and half are short. Only the tt genotype expresses the recessive short phenotype.

Question 36Science understanding

A section of mRNA has the sequence AUG GCU UAA. Assuming standard genetic code, what is produced when this is translated?

  1. A dipeptide, because UAA is a stop codon Correct answer.
  2. A tripeptide, because all three codons specify amino acids
  3. No peptide, because the sequence begins with a start codon
  4. A single amino acid, because AUG is a start codon

Explanation

AUG codes for methionine and GCU codes for alanine, so two amino acids are joined. UAA is a stop codon that terminates translation without adding an amino acid.

Question 37Science understanding

A student counts 60 individuals with the dominant phenotype and 40 with the recessive phenotype in a population of 100. Assuming Hardy-Weinberg equilibrium, what is the approximate frequency of the recessive allele?

  1. 0.63 Correct answer.
  2. 0.40
  3. 0.60
  4. 0.37

Explanation

The recessive phenotype frequency q squared is 0.40, so q is the square root of 0.40, about 0.63. The dominant allele frequency p is then about 0.37.

Question 38Science understanding

A cross is made between a heterozygous individual with genotype AaBb and a homozygous recessive individual aabb. Assuming independent assortment, what proportion of offspring is expected to show both recessive traits?

  1. One half
  2. One quarter Correct answer.
  3. Zero
  4. One eighth

Explanation

This is a test cross. The AaBb parent produces four gamete types in equal proportions, and only the ab gamete combined with the ab gamete gives aabb, so the expected proportion is one quarter.

Question 39Science understanding

Which observation from comparative anatomy is best explained by homologous structures shared through common ancestry?

  1. The wings of insects and birds both function in flight
  2. The forelimbs of whales, bats and humans share the same basic bone arrangement Correct answer.
  3. The streamlined bodies of dolphins and sharks have a similar shape
  4. The eyes of octopuses and mammals both contain a lens

Explanation

Homologous structures have the same underlying anatomical plan inherited from a common ancestor, even when their functions differ. The other examples describe analogous structures that evolved independently in unrelated lineages.

Question 40Science understanding

In a population of birds, individuals with intermediate beak size survive best because the available seeds are medium-sized. Which type of selection is occurring?

  1. Stabilising selection Correct answer.
  2. Disruptive selection
  3. Artificial selection
  4. Directional selection

Explanation

Stabilising selection favours the intermediate phenotype and reduces variation at the extremes of the range. Directional selection shifts the population toward one extreme, and disruptive selection favours both extremes.

Question 41Science understanding

A molecule binds to a receptor on the surface of a liver cell and triggers the breakdown of glycogen. Which feature of this signalling molecule is most important for it to act on the cell surface rather than inside the cell?

  1. It is large and positively charged
  2. It is water-soluble and cannot cross the lipid bilayer Correct answer.
  3. It is hydrophobic and freely enters the cytosol
  4. It is a steroid with a four-ring structure

Explanation

Water-soluble signalling molecules such as peptide hormones cannot pass through the hydrophobic core of the plasma membrane, so they bind surface receptors and relay the signal through second messengers. Steroid hormones are lipid-soluble and instead act on intracellular receptors.

Question 42Science understanding

During aerobic respiration, most ATP is produced by oxidative phosphorylation. Where does this process occur in a eukaryotic cell?

  1. On the rough endoplasmic reticulum
  2. In the mitochondrial matrix
  3. On the inner mitochondrial membrane Correct answer.
  4. In the cytoplasm

Explanation

The electron transport chain and ATP synthase are embedded in the inner mitochondrial membrane, where the proton gradient drives ATP synthesis. Glycolysis occurs in the cytoplasm and the Krebs cycle in the matrix.

Question 43Science understanding

A cell in G1 phase has 20 chromosomes. After completing the S phase, how many chromatids does the cell contain?

  1. 80
  2. 10
  3. 40 Correct answer.
  4. 20

Explanation

During S phase each chromosome is replicated into two sister chromatids held at a centromere, so 20 chromosomes become 40 chromatids while the chromosome count stays at 20. The number of chromosomes is only halved later, during meiosis.

Question 44Science understanding

A student finds that a plant wilts when its roots are placed in a concentrated salt solution. Which explanation best accounts for this observation?

  1. The salt destroys the cell walls so the cells can no longer support the plant
  2. Water leaves the root cells by osmosis because the external solution has a lower water potential Correct answer.
  3. Salt is actively pumped into the root cells, causing them to burst
  4. Water enters the root cells because the salt solution has a higher water potential

Explanation

A concentrated salt solution has a lower water potential than the root cell contents, so water moves out of the cells by osmosis. Loss of turgor pressure causes the tissue to wilt.

Question 45Science understanding

A woman and a man, both with normal colour vision, have a son who is red-green colour blind. Neither parent has a family history of the disorder. Which explanation is most likely?

  1. The son developed the disorder because both parents are homozygous recessive
  2. The son inherited the recessive allele on the X chromosome from his carrier mother Correct answer.
  3. The son inherited two recessive alleles, one from each parent
  4. The son inherited the recessive allele on the Y chromosome from his father

Explanation

Red-green colour blindness is X-linked recessive. A son inherits his X chromosome from his mother, so an unaffected carrier mother can pass the allele to a son who then expresses the trait. The father contributes a Y chromosome, which does not carry this gene.

Question 46Science understanding

A single base substitution changes a codon from GAG to GUG. What is the most likely effect on the protein if GAG codes for glutamic acid and GUG codes for valine?

  1. A nonsense mutation that shortens the protein
  2. A missense mutation that replaces one amino acid with another Correct answer.
  3. A silent mutation with no change to the protein
  4. A frameshift mutation that alters all later codons

Explanation

A substitution that changes one codon to a different amino acid codon is a missense mutation, so the protein has a single altered residue. Silent mutations leave the amino acid unchanged, and nonsense mutations create a stop codon.

Question 47Science understanding

A reaction in a cell has a positive change in free energy. Which statement about this reaction is most accurate?

  1. It will occur spontaneously without any energy input
  2. It cannot occur in a living cell under any circumstances
  3. It is endergonic and can proceed only if coupled to an energy-releasing reaction Correct answer.
  4. It is exergonic and releases energy to the surroundings

Explanation

A positive change in free energy means the reaction is endergonic and non-spontaneous on its own. Cells drive such reactions by coupling them to exergonic reactions, commonly the hydrolysis of ATP.

Question 48Science understanding

In a species of flowering plant, red flower colour is controlled by a gene with incomplete dominance, so RR plants are red, rr plants are white and Rr plants are pink. A pink plant is crossed with a white plant. What phenotypic ratio is expected among the offspring?

  1. 1 red to 2 pink to 1 white
  2. 1 pink to 1 white Correct answer.
  3. All pink
  4. 3 pink to 1 white

Explanation

The cross Rr x rr produces Rr and rr offspring in equal proportions, giving pink and white plants in a 1 to 1 ratio. Incomplete dominance means the heterozygote shows an intermediate phenotype rather than the dominant one.

Question 49Science understanding

Two enzymes act in the same metabolic pathway. Enzyme 1 converts substrate X to intermediate Y, and Enzyme 2 converts Y to product Z. A competitive inhibitor of Enzyme 2 is added. Which prediction about the pathway is most likely?

  1. Both X and Z accumulate equally because the pathway is unaffected
  2. Z accumulates because the inhibitor increases Enzyme 2 activity
  3. Y accumulates because its conversion to Z is slowed Correct answer.
  4. X accumulates because Enzyme 1 is directly inhibited

Explanation

A competitive inhibitor competes with the substrate for Enzyme 2's active site, reducing the rate at which Y is converted to Z, so Y tends to build up. Enzyme 1 is not directly affected and continues converting X to Y.

Question 50Science understanding

A population of lizards lives on an island with two distinct food sources: soft leaves and hard seeds. Birds with very deep beaks crack the seeds efficiently, birds with shallow beaks eat the leaves efficiently, and birds with intermediate beaks do neither well. Over many generations, the population splits into two distinct beak-size groups. Which process best explains this pattern?

  1. Genetic drift removing all variation from the population
  2. Directional selection favouring a single extreme phenotype
  3. Stabilising selection acting against extreme phenotypes
  4. Disruptive selection acting against intermediate phenotypes Correct answer.

Explanation

Disruptive selection favours individuals at both ends of the phenotypic range and acts against intermediates, which can split a population into two groups. Stabilising selection removes extremes, and directional selection shifts the mean toward one extreme.