Secondary 4 Combined Science Biology Evolution Diversity Quiz

Secondary 4 Combined Science Biology Quiz - Evolution Diversity

Answer Key

Section A: Multiple Choice & Short Answer (Questions 1–10)

1. (b) A change in the genetic composition of a population over successive generations [1]
Note: Evolution occurs at the population level, not the individual level. Option (a) describes phenotypic plasticity, not evolution. Option (c) is an oversimplification (survival of the "fittest," not necessarily the "strongest"). Option (d) confuses adaptation within a lifetime with evolutionary change across generations.

2. (b) Mutation [1]
Note: Mutation is the ultimate source of new alleles. Natural selection, genetic drift, and selective breeding act on existing variation but do not create new genetic material.

3. (b) Bacteria developing resistance to an antibiotic over many generations [1]
Note: This is natural selection acting on pre-existing genetic variation in a bacterial population. Option (a) is artificial selection. Options (c) and (d) are not heritable changes.

4. (b) Ability to survive and reproduce in its environment [1]
Note: In evolutionary biology, "fitness" is defined reproductively, not physically.

5. (c) Molecular evidence [1]
Note: Molecular evidence compares DNA, RNA, or protein sequences to determine evolutionary relationships. Fossil evidence examines preserved remains; anatomical evidence compares body structures; embryological evidence compares developmental stages.

6. Definition of gene pool: [2]
The complete set of all alleles (or genes) present in a population of organisms at a given time. [2]
Marking: Award 1 mark for "all alleles/genes" and 1 mark for "in a population." Answers that say "all the genes in an organism" are incorrect — the gene pool refers to a population, not an individual.

7. Two conditions for Hardy-Weinberg equilibrium: [2]
(a) No mutation (or random mating / no natural selection / large population size / no migration — any valid condition) [1]
(b) A second valid condition different from (a) [1]
Acceptable answers include: no mutation, random mating, no natural selection, infinitely large population (no genetic drift), no gene flow (migration/emigration). Award 1 mark per valid condition, max 2.

8. Directional vs. stabilising selection: [3]
Directional selection: Selection that favours one extreme phenotype over the mean and other extreme, causing the population mean to shift in one direction over time. [1]
Example: Antibiotic resistance in bacteria — bacteria with greater resistance survive and reproduce, shifting the population toward higher resistance. (Accept any valid example.) [0.5]

Stabilising selection: Selection that favours the intermediate phenotype and acts against both extremes, reducing variation and maintaining the population mean. [1]
Example: Birth weight in humans — babies of intermediate weight have higher survival rates; very small or very large babies face higher risks. (Accept any valid example.) [0.5]
Marking: Award marks for correct definitions and valid examples. Examples need not be identical to those given above.

9. Finch beak depth after drought — natural selection explanation: [3]

• Before the drought, finches had a range of beak depths (variation existed in the population). [1]
• The drought reduced the availability of small, soft seeds, leaving mostly large, hard seeds. [1]
• Finches with deeper (stronger) beaks were better able to crack the hard seeds, so they survived and reproduced more successfully. They passed the genes for deeper beaks to offspring. Over generations, the average beak depth increased. [1]
  Marking: Award 1 mark each for (i) recognition of pre-existing variation, (ii) identification of the selective pressure, and (iii) explanation of differential survival/reproduction leading to change in the population.

10. Speciation: [2]
Speciation: The evolutionary process by which populations evolve to become distinct (separate) species. [1]
Factor: Geographic isolation / reproductive isolation / genetic mutation / natural selection in different environments (accept any valid factor). [1]
Marking: Award 1 mark for a correct definition (must include the idea of new species forming from existing ones). Award 1 mark for any valid factor.

Section B: Structured Response (Questions 11–17)

11. Peppered moth scenario:
(a) [4]

• Before industrialisation, light-coloured moths were better camouflaged against light-coloured (lichen-covered) tree trunks, so they were less likely to be eaten by predators. [1]
• After industrialisation, soot darkened the tree trunks. Dark-coloured moths were now better camouflaged and less likely to be preyed upon. [1]
• Dark-coloured moths therefore had a higher chance of surviving and reproducing. [1]
• They passed the allele for dark coloration to their offspring. Over many generations, the frequency of the dark allele increased, and the dark morph became more common in the population. [1]
  Marking: Award 1 mark for each of the four points above. Answers must reference natural selection (differential survival/reproduction due to camouflage/predation).

(b) [2]
The proportion of dark moths would likely decrease. [1]
As tree trunks become lighter again, light-coloured moths would regain their camouflage advantage and be less preyed upon, so they would survive and reproduce more successfully, increasing in frequency over generations. [1]
Marking: Award 1 mark for the correct prediction and 1 mark for a valid explanation linked to natural selection.

12. Flower genotype frequency calculation:
(a) Allele frequency calculation: [3]
Total individuals = 36 + 48 + 16 = 100
Total alleles = 100 × 2 = 200
Number of r alleles = (48 × 1) + (16 × 2) = 48 + 32 = 80
Frequency of r = 80 / 200 = 0.4 (or 40%) [3]
Marking: Award 1 mark for correct total number of alleles, 1 mark for correct count of r alleles, 1 mark for correct final answer. Accept 0.4 or 40% or 2/5.

(b) Hardy-Weinberg equilibrium check: [2]
Expected genotype frequencies if in H-W equilibrium:
p² = (frequency of R)² = (0.6)² = 0.36 → Expected RR = 36
2pq = 2(0.6)(0.4) = 0.48 → Expected Rr = 48
q² = (0.4)² = 0.16 → Expected rr = 16

The observed values (36, 48, 16) match the expected values exactly. [1]
Therefore, this population is in Hardy-Weinberg equilibrium. [1]
Marking: Award 1 mark for showing the calculation or stating expected values, 1 mark for the correct conclusion. If the student calculates correctly and concludes "yes," award full marks.

13. Fossil evidence for evolution: [4]

• Fossils are the preserved remains or traces of organisms that lived in the past, found in sedimentary rock layers. [1]
• The fossil record shows that organisms have changed over time — older rock layers contain simpler organisms, while younger layers contain more complex organisms. Transitional fossils (e.g., Archaeopteryx) show intermediate forms between major groups. [1]
• The sequence of fossils in rock layers matches the order in which groups are thought to have evolved. [1]
• Limitation: The fossil record is incomplete because fossilisation is a rare event — most organisms decompose without leaving fossils. Soft-bodied organisms are especially underrepresented. Many fossils have also been destroyed by geological processes. [1]
  Marking: Award 1 mark each for: (i) definition of fossils, (ii) change over time shown in the record, (iii) a valid limitation. The fourth mark is for a coherent, well-structured response covering all required points.

14. Homologous structures:
(a) Homologous structures [1]

(b) The similarity in bone structure suggests that the human, bat, and whale share a common ancestor that had the same basic forelimb bone pattern. [1] Over time, the structures were modified by natural selection for different functions in different environments (descent with modification). [1]

(c) These structures arise through descent with modification from a common ancestor. [1] Natural selection acted on variations in the ancestral population, and over many generations, the forelimb was adapted for different functions (grasping, flying, swimming) in different lineages, while retaining the underlying bone structure. [1]
Marking: Award marks as indicated. For (c), the answer must reference common ancestry and modification by natural selection.

15. Geographic isolation leading to speciation: [5]

1. A population is split into two or more subpopulations by a physical barrier (e.g., a mountain range, river, canyon, or ocean). [1]
2. Gene flow between the subpopulations is prevented — they can no longer interbreed. [1]
3. The two subpopulations experience different environmental conditions and therefore different selection pressures. Mutations arise independently in each group. [1]
4. Natural selection, genetic drift, and mutation cause the genetic composition of each subpopulation to diverge over many generations. [1]
5. Eventually, the populations become so genetically different that they can no longer interbreed successfully even if they come back into contact. At this point, they are considered separate species (reproductive isolation has been achieved). [1]
   Marking: Award 1 mark for each of the five logical steps. The sequence must be logical and complete. Accept equivalent phrasing.

16. Antibiotic resistance:
(a) Development of antibiotic resistance: [4]

• Within a bacterial population, there is genetic variation due to random mutations. Some bacteria may carry a mutation that makes them resistant to a particular antibiotic. [1]
• When the antibiotic is applied, non-resistant bacteria are killed. [1]
• Resistant bacteria survive and reproduce, passing the resistance gene to their offspring. [1]
• Over many generations, the frequency of the resistance allele increases in the population, and the population becomes predominantly resistant. [1]
  Marking: Award 1 mark each for: (i) pre-existing variation/mutation, (ii) antibiotic kills non-resistant bacteria, (iii) resistant bacteria survive and reproduce, (iv) increase in resistance allele frequency over generations.

(b) Two practices to slow antibiotic resistance: [2]
(i) Complete the full course of antibiotics as prescribed (do not stop early when symptoms improve). [1]
(ii) Do not use antibiotics for viral infections (e.g., common cold, flu), as antibiotics only work against bacteria. [1]
Acceptable alternatives: Use antibiotics only when prescribed by a doctor; avoid using antibiotics in livestock as growth promoters; develop new antibiotics; use combination therapy; practise good hygiene to reduce infection spread. Award 1 mark per valid practice, max 2.

17. Phylogenetic tree interpretation:
(a) Species A and B are most closely related. [1] This is determined by the fact that they share the most recent common ancestor (their branching point/node is the most recent compared to other pairs). [1]

(b) A node (branching point) represents the most recent common ancestor of the species that descend from that point. [1]

(c) DNA sequences / amino acid (protein) sequences / anatomical (morphological) data / fossil data / embryological data (accept any one valid type). [1]
Marking: Award marks as indicated.

Section C: Data-Based & Extended Response (Questions 18–20)

18. Deer mouse body mass and natural selection:
(a) Trend: [2]
The average body mass of the deer mouse population increased steadily over the 20 generations, from approximately 18 g to approximately 24 g. [2]
Marking: Award 2 marks for a clear description that includes both the direction of change and approximate values. Award 1 mark if only the direction is stated without values.

(b) Natural selection explanation: [4]

• The environment became increasingly cold over the 20 generations. [1]
• Mice with larger body mass have a smaller surface area-to-volume ratio, which reduces heat loss and helps them maintain body temperature more efficiently in cold conditions. [1]
• Larger mice therefore had a survival and reproductive advantage in the cold environment. [1]
• These mice were more likely to survive, reproduce, and pass on the alleles associated for larger body mass to their offspring. Over many generations, the average body mass of the population increased. [1]
  Marking: Award 1 mark each for the four points. The explanation must link the environmental change to the selective advantage of larger body mass.

(c) Prediction for warmer environment: [2]
The average body mass would likely decrease. [1]
In a warmer environment, smaller mice (with a larger surface area-to-volume ratio) would lose heat more efficiently and avoid overheating, giving them a survival advantage. Natural selection would favour smaller body mass, and the population mean would decrease over generations. [1]
Marking: Award 1 mark for the correct prediction and 1 mark for a valid explanation.

19. Grasshopper speciation scenario:
(a) Behavioural (prezygotic) isolation [1]
Accept "temporal isolation" if the student focuses on the difference in breeding seasons, or "behavioural isolation" if focusing on mating calls. Both are valid prezygotic barriers.

(b) The canyon acts as a physical barrier that prevents the grasshoppers from moving between the two populations. [1] This prevents gene flow (the exchange of alleles through interbreeding) between the two populations, allowing them to evolve independently. [1]

(c) Sequence to speciation: [4]

1. The canyon physically separates the two populations, preventing gene flow. [1]
2. The two populations experience different environmental conditions and selection pressures. Random mutations arise independently in each population. [1]
3. Over many generations, natural selection, genetic drift, and mutation cause the genetic makeup of each population to diverge. Differences accumulate in traits such as mating calls and breeding seasons. [1]
4. Eventually, the populations become so genetically and behaviourally different that even if they came into contact again, they would not recognise each other as mates or would not be able to breed at the same time. Reproductive isolation is complete, and they are now separate species. [1]
   Marking: Award 1 mark for each logical step. The answer must describe a clear sequence from isolation to reproductive isolation.

(d) Reproductive isolation (or speciation) [1]
Marking: Award 1 mark for the correct term.

20. Darwin's finches passage:
(a) Evidence for evolution by natural selection: [4]

• The different finch species all descended from a common ancestor, showing that new species can arise from existing ones. [1]
• The variation in beak size and shape among the finches corresponds to different food sources on different islands. [1]
• On each island, finches with beak shapes best suited to the available food source had a survival and reproductive advantage. [1]
• Over many generations, natural selection caused the beak characteristics to diverge in different populations, leading to the formation of distinct species — this is adaptive radiation. [1]
  Marking: Award 1 mark for each of the four points. The answer must link beak variation to natural selection and speciation.

(b) Different beak shapes on different islands: [3]

• Different islands have different available food sources (e.g., seeds of different sizes, insects, cactus flowers). [1]
• The ancestral finch population colonised different islands and became geographically isolated. [1]
• On each island, natural selection favoured the beak shape best suited to the local food source. Over time, the populations diverged and evolved into distinct species with different beak morphologies. [1]
  Marking: Award 1 mark for each of the three points.

(c) Adaptive radiation [1]
Marking: Award 1 mark for the correct term.