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Secondary 3 Biology Evolution Diversity Quiz
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Questions
Secondary 3 Biology Quiz - Evolution Diversity
Name: ___________________________
Class: ___________________________
Date: ___________________________
Score: ________ / 50
Duration: 55 minutes
Total Marks: 50
Instructions:
- Answer all questions in the spaces provided.
- The number of marks for each question is shown in brackets [ ].
- Where a question requires explanation or reasoning, write in complete biological sentences.
- Diagrams may be drawn in pencil; label clearly.
- This quiz is based on syllabus content for Evolution and Diversity. It is not derived from past-year papers but is designed to reflect the style and difficulty of Secondary 3 Biology assessments.
Section A: Multiple Choice & Short Answer (Questions 1–10)
Questions 1–5: Choose the best answer. Each question carries 2 marks.
1. Which of the following best describes natural selection?
(a) Organisms choose which traits to pass on to offspring.
(b) Individuals with advantageous traits are more likely to survive and reproduce.
(c) All members of a population evolve simultaneously in response to the environment.
(d) Mutations occur only when the environment changes.
Answer: _______________ [2]
2. The diagram below shows variation in shell colour in a population of snails.
(Imagine: a bar chart showing a normal distribution of shell colours from light to dark, with the highest frequency in the middle range.)
Which type of variation does shell colour in this population most likely represent?
(a) Discontinuous variation only
(b) Continuous variation
(c) Mutation-driven variation only
(d) Environmental variation only
Answer: _______________ [2]
3. What is the primary source of new genetic variation in a population?
(a) Natural selection
(b) Selective breeding
(c) Mutation
(d) Genetic drift
Answer: _______________ [2]
4. Which statement about evolution by natural selection is correct?
(a) Evolution occurs within the lifetime of a single organism.
(b) Populations evolve when allele frequencies change over generations.
(c) Only physical traits can be selected; behavioural traits cannot.
(d) Natural selection always leads to more complex organisms.
Answer: _______________ [2]
5. The pentadactyl limb (five-digit limb) found in mammals, reptiles, and amphibians is an example of:
(a) Analogous structures
(b) Vestigial structures
(c) Homologous structures
(d) Convergent evolution
Answer: _______________ [2]
Questions 6–10: Short answer. Write your answer in the space provided.
6. Define the term species. [2]
7. State two conditions required for a population to be in Hardy-Weinberg equilibrium. [2]
(i) _________________________________________________________________________
(ii) ________________________________________________________________________
8. Distinguish between continuous variation and discontinuous variation, giving one example of each. [3]
9. Explain what is meant by the term adaptive radiation, using a named example. [3]
10. The peppered moth (Biston betularia) is a classic example of natural selection in action. During the Industrial Revolution in England, the frequency of the dark (melanic) form increased in polluted areas. Explain why the dark form had a selective advantage in these areas. [3]
Section B: Structured Response (Questions 11–17)
Answer all questions. Show your reasoning clearly.
11. The table below shows the number of individuals with different blood groups in a human population of 400 people.
| Blood Group | Number of Individuals |
|---|---|
| A | 120 |
| B | 80 |
| AB | 40 |
| O | 160 |
(a) Calculate the frequency of blood group O in this population. Express your answer as a percentage. [2]
(b) Blood group is an example of discontinuous variation. Explain why. [2]
(c) State one other example of discontinuous variation in humans. [1]
12. The diagram below shows the beak shapes of four species of finch found on different islands of the Galápagos archipelago.
(Imagine: four simplified diagrams showing — (i) a large, thick beak for cracking seeds; (ii) a long, thin beak for probing flowers; (iii) a pointed, sharp beak for catching insects; (iv) a curved beak for feeding on cactus.)
(a) Suggest how differences in food availability on different islands could have led to the evolution of different beak shapes. [4]
(b) Explain what evidence the finch beak shapes provide for the theory of evolution by natural selection. [2]
13. Antibiotic resistance in bacteria is an example of evolution occurring over a short time period.
(a) Explain, step by step, how antibiotic resistance evolves in a bacterial population. [5]
(b) State two practices that can slow the development of antibiotic resistance. [2]
(i) _________________________________________________________________________
(ii) ________________________________________________________________________
14. The diagram below shows a phylogenetic tree for five vertebrate species (A–E).
(Imagine: a cladogram where Species A and B share a recent common ancestor; Species C branches off earlier; Species D and E share a recent common ancestor that diverged from the A-B-C lineage.)
(a) Which two species are most closely related? Explain your reasoning. [2]
(b) What does a branching point (node) on the phylogenetic tree represent? [1]
(c) State one type of evidence that scientists use to construct phylogenetic trees. [1]
15. A scientist studied the leg length of two populations of lizards living on different islands. The results are shown in the table below.
| Island | Mean Leg Length (cm) | Standard Deviation (cm) | Sample Size |
|---|---|---|---|
| Island X | 8.5 | 1.2 | 50 |
| Island Y | 12.3 | 1.8 | 50 |
(a) Compare the leg lengths of lizards on the two islands. [2]
(b) Suggest one environmental factor that could explain the difference in leg length between the two populations. [2]
(c) Explain how natural selection could have led to the observed difference in leg length. [3]
16. Define the following terms: [4]
(a) Genetic drift: ___________________________________________________________
(b) Gene flow: ______________________________________________________________
(c) Founder effect: _________________________________________________________
(d) Bottleneck effect: ______________________________________________________
17. The diagram below shows the distribution of two subspecies of a bird species across a mountain range.
(Imagine: Subspecies 1 found on the western lowlands; Subspecies 2 found on the eastern highlands; the mountain range acts as a barrier between them.)
(a) Explain how geographical isolation could lead to the formation of these two subspecies. [4]
(b) What additional process would need to occur for the two subspecies to become separate species? [1]
Section C: Extended Response (Questions 18–20)
Answer all questions in continuous prose. Use appropriate biological terminology.
18. Charles Darwin and Alfred Russel Wallace independently proposed the theory of evolution by natural selection.
Describe the key observations and reasoning that led Darwin and Wallace to propose this theory. In your answer, include reference to:
- variation within populations
- overproduction of offspring
- struggle for existence
- differential survival and reproduction
- inheritance of advantageous traits
[8]
<stage5_quiz_md>
# Secondary 3 Biology Quiz - Evolution Diversity
**Name:** ___________________________
**Class:** ___________________________
**Date:** ___________________________
**Score:** ________ / 50
**Duration:** 55 minutes
**Total Marks:** 50
**Instructions:**
- Answer all questions in the spaces provided.
- The number of marks for each question is shown in brackets [ ].
- Where a question requires explanation or reasoning, write in complete biological sentences.
- Diagrams may be drawn in pencil; label clearly.
- This quiz is based on syllabus content for Evolution and Diversity. It is not derived from past-year papers but is designed to reflect the style and difficulty of Secondary 3 Biology assessments.
---
## Section A: Multiple Choice & Short Answer (Questions 1–10)
**Questions 1–5: Choose the best answer. Each question carries 2 marks.**
**1.** Which of the following best describes natural selection?
(a) Organisms choose which traits to pass on to offspring.
(b) Individuals with advantageous traits are more likely to survive and reproduce.
(c) All members of a population evolve simultaneously in response to the environment.
(d) Mutations occur only when the environment changes.
Answer: _______________ [2]
---
**2.** The diagram below shows variation in shell colour in a population of snails.
*(Imagine: a bar chart showing a normal distribution of shell colours from light to dark, with the highest frequency in the middle range.)*
Which type of variation does shell colour in this population most likely represent?
(a) Discontinuous variation only
(b) Continuous variation
(c) Mutation-driven variation only
(d) Environmental variation only
Answer: _______________ [2]
---
**3.** What is the primary source of new genetic variation in a population?
(a) Natural selection
(b) Selective breeding
(c) Mutation
(d) Genetic drift
Answer: _______________ [2]
---
**4.** Which statement about evolution by natural selection is correct?
(a) Evolution occurs within the lifetime of a single organism.
(b) Populations evolve when allele frequencies change over generations.
(c) Only physical traits can be selected; behavioural traits cannot.
(d) Natural selection always leads to more complex organisms.
Answer: _______________ [2]
---
**5.** The pentadactyl limb (five-digit limb) found in mammals, reptiles, and amphibians is an example of:
(a) Analogous structures
(b) Vestigial structures
(c) Homologous structures
(d) Convergent evolution
Answer: _______________ [2]
---
**Questions 6–10: Short answer. Write your answer in the space provided.**
**6.** Define the term *species*. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**7.** State two conditions required for a population to be in Hardy-Weinberg equilibrium. [2]
(i) _________________________________________________________________________
(ii) ________________________________________________________________________
---
**8.** Distinguish between *continuous variation* and *discontinuous variation*, giving one example of each. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**9.** Explain what is meant by the term *adaptive radiation*, using a named example. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**10.** The peppered moth (*Biston betularia*) is a classic example of natural selection in action. During the Industrial Revolution in England, the frequency of the dark (melanic) form increased in polluted areas. Explain why the dark form had a selective advantage in these areas. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
## Section B: Structured Response (Questions 11–17)
**Answer all questions. Show your reasoning clearly.**
**11.** The table below shows the number of individuals with different blood groups in a human population of 400 people.
| Blood Group | Number of Individuals |
|-------------|----------------------|
| A | 120 |
| B | 80 |
| AB | 40 |
| O | 160 |
(a) Calculate the frequency of blood group O in this population. Express your answer as a percentage. [2]
_____________________________________________________________________________
_____________________________________________________________________________
(b) Blood group is an example of discontinuous variation. Explain why. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(c) State one other example of discontinuous variation in humans. [1]
_____________________________________________________________________________
---
**12.** The diagram below shows the beak shapes of four species of finch found on different islands of the Galápagos archipelago.
*(Imagine: four simplified diagrams showing — (i) a large, thick beak for cracking seeds; (ii) a long, thin beak for probing flowers; (iii) a pointed, sharp beak for catching insects; (iv) a curved beak for feeding on cactus.)*
(a) Suggest how differences in food availability on different islands could have led to the evolution of different beak shapes. [4]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) Explain what evidence the finch beak shapes provide for the theory of evolution by natural selection. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**13.** Antibiotic resistance in bacteria is an example of evolution occurring over a short time period.
(a) Explain, step by step, how antibiotic resistance evolves in a bacterial population. [5]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) State two practices that can slow the development of antibiotic resistance. [2]
(i) _________________________________________________________________________
(ii) ________________________________________________________________________
---
**14.** The diagram below shows a phylogenetic tree for five vertebrate species (A–E).
*(Imagine: a cladogram where Species A and B share a recent common ancestor; Species C branches off earlier; Species D and E share a recent common ancestor that diverged from the A-B-C lineage.)*
(a) Which two species are most closely related? Explain your reasoning. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) What does a branching point (node) on the phylogenetic tree represent? [1]
_____________________________________________________________________________
(c) State one type of evidence that scientists use to construct phylogenetic trees. [1]
_____________________________________________________________________________
---
**15.** A scientist studied the leg length of two populations of lizards living on different islands. The results are shown in the table below.
| Island | Mean Leg Length (cm) | Standard Deviation (cm) | Sample Size |
|--------|----------------------|--------------------------|-------------|
| Island X | 8.5 | 1.2 | 50 |
| Island Y | 12.3 | 1.8 | 50 |
(a) Compare the leg lengths of lizards on the two islands. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) Suggest one environmental factor that could explain the difference in leg length between the two populations. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(c) Explain how natural selection could have led to the observed difference in leg length. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**16.** Define the following terms: [4]
(a) *Genetic drift*: ___________________________________________________________
_____________________________________________________________________________
(b) *Gene flow*: ______________________________________________________________
_____________________________________________________________________________
(c) *Founder effect*: _________________________________________________________
_____________________________________________________________________________
(d) *Bottleneck effect*: ______________________________________________________
_____________________________________________________________________________
---
**17.** The diagram below shows the distribution of two subspecies of a bird species across a mountain range.
*(Imagine: Subspecies 1 found on the western lowlands; Subspecies 2 found on the eastern highlands; the mountain range acts as a barrier between them.)*
(a) Explain how geographical isolation could lead to the formation of these two subspecies. [4]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) What additional process would need to occur for the two subspecies to become separate species? [1]
_____________________________________________________________________________
---
## Section C: Extended Response (Questions 18–20)
**Answer all questions in continuous prose. Use appropriate biological terminology.**
**18.** Charles Darwin and Alfred Russel Wallace independently proposed the theory of evolution by natural selection.
Describe the key observations and reasoning that led Darwin and Wallace to propose this theory. In your answer, include reference to:
- variation within populations
- overproduction of offspring
- struggle for existence
- differential survival and reproduction
- inheritance of advantageous traits
[8]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
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________
---
**19.** The diagram below shows the evolutionary relationships among five groups of organisms based on molecular evidence (DNA sequence comparisons).
*(Imagine: a simplified phylogenetic tree showing — Fungi and Animals sharing a more recent common ancestor with each other than with Plants; Protists forming a paraphyletic group; Bacteria as the outgroup.)*
(a) Explain how molecular evidence (such as DNA or protein sequences) can be used to determine evolutionary relationships between organisms. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(b) State one advantage and one limitation of using molecular evidence compared with anatomical (morphological) evidence when constructing phylogenetic trees. [2]
Advantage: _________________________________________________________________
_____________________________________________________________________________
Limitation: _________________________________________________________________
_____________________________________________________________________________
(c) Based on the tree, which two groups of organisms are most closely related? Explain your reasoning. [2]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
(d) Explain why the fossil record and molecular evidence are considered complementary lines of evidence for evolution. [3]
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
---
**20.** A population of flowering plants is separated into two populations by the formation of a new river.
*(Imagine: Population A on the north bank grows in dry, sandy soil; Population B on the south bank grows in wet, clay-rich soil.)*
(a) Explain how natural selection could cause the two populations to diverge over many generations. In your answer, refer to:
- variation in traits related to water availability
- differential survival and reproduction
- changes in allele frequencies
[6]
_____________________________________________________________________________
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_____________________________________________________________________________
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(b) Over a very long period of time, the two populations may become reproductively isolated. Explain how reproductive isolation can arise and why it is important in the formation of new species. [4]
_____________________________________________________________________________
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---
**END OF PAPER**
**Total: 50 marks**
Answers
Secondary 3 Biology Quiz - Evolution Diversity
Answer Scheme
Section A: Multiple Choice & Short Answer (Questions 1–10)
1. (b) Individuals with advantageous traits are more likely to survive and reproduce. [2]
Marking: 1 mark for selecting (b). 1 mark for no other option selected.
2. (b) Continuous variation [2]
Marking: 1 mark for selecting (b). 1 mark for no other option selected.
3. (c) Mutation [2]
Marking: 1 mark for selecting (c). 1 mark for no other option selected.
4. (b) Populations evolve when allele frequencies change over generations. [2]
Marking: 1 mark for selecting (b). 1 mark for no other option selected.
5. (c) Homologous structures [2]
Marking: 1 mark for selecting (c). 1 mark for no other option selected.
6. Define the term species. [2]
A species is a group of organisms that can interbreed (mate with one another) to produce fertile offspring. [2]
Marking: 1 mark for "interbreed / mate with one another". 1 mark for "produce fertile offspring".
Acceptable alternatives: "A group of organisms that share similar characteristics and can breed to produce fertile young."
7. State two conditions required for a population to be in Hardy-Weinberg equilibrium. [2]
(i) No mutation (or random mating / no natural selection / no genetic drift / no gene flow / large population size — any valid condition) [1]
(ii) A second valid condition different from (i) [1]
Acceptable answers include:
- No mutation
- Random mating
- No natural selection
- Very large population size (no genetic drift)
- No gene flow (no migration into or out of the population)
Marking: 1 mark per correct condition. Any two valid conditions accepted.
8. Distinguish between continuous variation and discontinuous variation, giving one example of each. [3]
Continuous variation is where a trait shows a range of values with no distinct categories, often influenced by both genes and the environment (e.g., human height / skin colour / body mass). [1.5]
Discontinuous variation is where a trait falls into two or more distinct categories with no intermediate forms (e.g., human blood group / ability to roll tongue / attached vs free earlobes). [1.5]
Marking: 1.5 marks for correct definition + example of continuous variation; 1.5 marks for correct definition + example of discontinuous variation.
9. Explain what is meant by the term adaptive radiation, using a named example. [3]
Adaptive radiation is the process by which a single ancestral species rapidly diversifies into many new species, each adapted to exploit different ecological niches. [1.5]
Named example: Darwin's finches in the Galápagos Islands — a single ancestral finch species colonised the islands and diversified into multiple species with different beak shapes adapted to different food sources. [1.5]
Marking: 1.5 marks for correct definition; 1.5 marks for named example with brief explanation.
Acceptable alternative examples: Hawaiian honeycreepers, cichlid fish in African lakes, marsupials in Australia.
10. The peppered moth (Biston betularia) is a classic example of natural selection in action. During the Industrial Revolution in England, the frequency of the dark (melanic) form increased in polluted areas. Explain why the dark form had a selective advantage in these areas. [3]
During the Industrial Revolution, soot from factories darkened tree bark by killing light-coloured lichens and depositing soot. [1]
The dark (melanic) form of the moth was better camouflaged against the darkened tree bark, making it less visible to predatory birds. [1]
As a result, dark moths were more likely to survive and reproduce, passing the dark colouration allele to their offspring, increasing the frequency of the dark form in the population. [1]
Marking: 1 mark for describing environmental change (soot/darkened bark); 1 mark for explaining camouflage advantage; 1 mark for linking survival/reproduction to increased frequency.
Section B: Structured Response (Questions 11–17)
11. (a) Calculate the frequency of blood group O in this population. Express your answer as a percentage. [2]
Frequency of blood group O = (160 / 400) × 100 = 40% [2]
Marking: 1 mark for correct calculation (160/400 = 0.4); 1 mark for correct percentage (40%).
(b) Blood group is an example of discontinuous variation. Explain why. [2]
Blood group falls into distinct categories (A, B, AB, O) with no intermediate forms. [1]
It is controlled by alleles of a single gene and is not significantly influenced by the environment. [1]
Marking: 1 mark for "distinct categories / no intermediates"; 1 mark for "controlled by genes / not influenced by environment".
(c) State one other example of discontinuous variation in humans. [1]
Example: Ability to roll the tongue / attached vs free earlobes / widow's peak / dimples / blood type Rhesus factor. [1]
Marking: 1 mark for any valid example of discontinuous variation in humans.
12. (a) Suggest how differences in food availability on different islands could have led to the evolution of different beak shapes. [4]
On each island, the available food sources differed (e.g., seeds, insects, nectar, cactus). [1]
Within the ancestral finch population, there was natural variation in beak shape due to genetic differences. [1]
Finches with beak shapes better suited to the available food on their island were more likely to obtain food, survive, and reproduce. [1]
Over many generations, advantageous beak shapes became more common in each population, leading to divergence in beak morphology between islands. [1]
Marking: 1 mark per valid point. Maximum 4 marks.
(b) Explain what evidence the finch beak shapes provide for the theory of evolution by natural selection. [2]
The different beak shapes show that species have adapted to different environmental conditions (food sources). [1]
The variation in beak shapes from a common ancestor demonstrates that natural selection can lead to divergence and the formation of new species over time. [1]
Marking: 1 mark for adaptation to environment; 1 mark for divergence from common ancestor / speciation.
13. (a) Explain, step by step, how antibiotic resistance evolves in a bacterial population. [5]
-
Within a bacterial population, there is genetic variation due to random mutations. [1]
-
Some bacteria may carry a mutation that confers resistance to a particular antibiotic. [1]
-
When the population is exposed to the antibiotic, non-resistant bacteria are killed. [1]
-
Resistant bacteria survive and reproduce, passing the resistance gene to their offspring. [1]
-
Over time, the frequency of the resistance allele increases in the population, and the population becomes predominantly resistant. [1]
Marking: 1 mark per correct step. Maximum 5 marks. Steps must be in logical sequence.
(b) State two practices that can slow the development of antibiotic resistance. [2]
(i) Complete the full course of antibiotics as prescribed (even if symptoms improve). [1]
(ii) Do not use antibiotics for viral infections (e.g., common cold) / avoid unnecessary use of antibiotics / do not use antibiotics in livestock as growth promoters. [1]
Marking: 1 mark per valid practice. Maximum 2 marks.
14. (a) Which two species are most closely related? Explain your reasoning. [2]
Species A and B are most closely related. [1]
They share the most recent common ancestor (their branching point/node is the most recent on the tree). [1]
Marking: 1 mark for identifying A and B; 1 mark for correct reasoning.
(b) What does a branching point (node) on the phylogenetic tree represent? [1]
A common ancestor of the species that diverge from that point. [1]
Marking: 1 mark for "common ancestor".
(c) State one type of evidence that scientists use to construct phylogenetic trees. [1]
DNA / molecular / amino acid sequence comparisons / fossil evidence / anatomical (morphological) comparisons. [1]
Marking: 1 mark for any valid type of evidence.
15. (a) Compare the leg lengths of lizards on the two islands. [2]
Lizards on Island Y have a greater mean leg length (12.3 cm) compared to lizards on Island X (8.5 cm). [1]
Lizards on Island Y also show greater variation in leg length (standard deviation 1.8 cm) compared to Island X (standard deviation 1.2 cm). [1]
Marking: 1 mark for comparing means; 1 mark for comparing variation/spread.
(b) Suggest one environmental factor that could explain the difference in leg length between the two populations. [2]
Island Y may have terrain that requires greater climbing ability (e.g., rocky outcrops, trees, steep surfaces), favouring longer legs for grip and reach. [2]
Acceptable alternatives:
- Island Y may have taller vegetation, favouring longer legs for climbing.
- Island X may have dense ground cover, favouring shorter legs for burrowing or moving through undergrowth.
- Different predator types on each island may select for different locomotor strategies.
Marking: 2 marks for a plausible environmental factor with brief explanation.
(c) Explain how natural selection could have led to the observed difference in leg length. [3]
Within each lizard population, there was natural variation in leg length due to genetic differences. [1]
On Island Y, lizards with longer legs were better able to climb / escape predators / catch prey, giving them a survival and reproductive advantage. [1]
Over many generations, the alleles for longer legs became more frequent in the Island Y population, resulting in a higher mean leg length. [1]
Marking: 1 mark for variation; 1 mark for selective advantage; 1 mark for change in allele frequency over generations.
16. Define the following terms: [4]
(a) Genetic drift: Random change in allele frequencies in a population due to chance events, especially significant in small populations. [1]
(b) Gene flow: The transfer of alleles from one population to another through migration and interbreeding. [1]
(c) Founder effect: A type of genetic drift that occurs when a small group of individuals establishes a new population, carrying only a small, random sample of the original population's genetic diversity. [1]
(d) Bottleneck effect: A type of genetic drift that occurs when a population is drastically reduced in size by a random event (e.g., natural disaster), resulting in reduced genetic diversity in the surviving population. [1]
Marking: 1 mark per correct definition.
17. (a) Explain how geographical isolation could lead to the formation of these two subspecies. [4]
The mountain range acts as a physical barrier that prevents the two bird populations from interbreeding (no gene flow between them). [1]
The environments on the western lowlands and eastern highlands differ (e.g., temperature, altitude, food sources, vegetation). [1]
Different selection pressures in each environment favour different traits in each population. [1]
Over many generations, the two populations diverge genetically and phenotypically, becoming distinct subspecies. [1]
Marking: 1 mark per valid point. Maximum 4 marks.
(b) What additional process would need to occur for the two subspecies to become separate species? [1]
Reproductive isolation — the two populations must become unable to interbreed and produce fertile offspring even if they come into contact again. [1]
Marking: 1 mark for "reproductive isolation".
Section C: Extended Response (Questions 18–20)
18. Describe the key observations and reasoning that led Darwin and Wallace to propose the theory of evolution by natural selection. [8]
Indicative content:
-
Variation within populations: Individuals within a population show variation in their traits (e.g., size, colour, speed). Much of this variation is heritable (passed from parents to offspring via genes). [1–2 marks]
-
Overproduction of offspring: Organisms produce more offspring than can possibly survive to adulthood (e.g., a single fish may lay thousands of eggs). [1 mark]
-
Struggle for existence: Because resources (food, water, shelter, mates) are limited, there is competition among individuals for survival. Many offspring die before reaching reproductive age. [1–2 marks]
-
Differential survival and reproduction: Individuals with traits that are better suited to their environment are more likely to survive and reproduce. These advantageous traits increase the individual's fitness. [1–2 marks]
-
Inheritance of advantageous traits: The advantageous traits are heritable, so they are passed on to the next generation. Over many generations, the frequency of advantageous traits in the population increases. [1–2 marks]
-
Conclusion: Over long periods, this process leads to changes in the characteristics of populations and can result in the formation of new species. [1 mark]
Marking: Award marks for each valid point made. Maximum 8 marks. Quality of written communication (QWC) is assessed — answers should be well-organised, use appropriate biological terminology, and be written in clear, coherent prose.
QWC assessment: The response should demonstrate sustained and accurate use of biological terminology (e.g., variation, natural selection, allele frequency, heritable, fitness, adaptation, speciation). The argument should flow logically from observation to conclusion.
19. (a) Explain how molecular evidence (such as DNA or protein sequences) can be used to determine evolutionary relationships between organisms. [3]
The DNA or protein sequences of different species can be compared. [1]
Species that share a more recent common ancestor will have more similar DNA/protein sequences because they have had less time to accumulate differences (mutations). [1]
The degree of similarity (or difference) in sequences can be used to estimate how closely related species are and to construct phylogenetic trees. [1]
Marking: 1 mark per valid point. Maximum 3 marks.
(b) State one advantage and one limitation of using molecular evidence compared with anatomical (morphological) evidence when constructing phylogenetic trees. [2]
Advantage: Molecular evidence provides a more objective and quantitative measure of relatedness; it can reveal relationships between organisms that look very different anatomically (e.g., whales and hippos); it is not subject to convergent evolution in the same way as anatomical features. [1]
Limitation: Molecular data requires sophisticated technology and expertise to obtain and analyse; DNA may not be available from extinct species (except in rare cases); different genes may give different evolutionary signals. [1]
Marking: 1 mark for a valid advantage; 1 mark for a valid limitation.
(c) Based on the tree, which two groups of organisms are most closely related? Explain your reasoning. [2]
Fungi and Animals are most closely related. [1]
They share a more recent common ancestor with each other than either does with Plants, as shown by their branching pattern on the phylogenetic tree. [1]
Marking: 1 mark for identifying Fungi and Animals; 1 mark for correct reasoning.
(d) Explain why the fossil record and molecular evidence are considered complementary lines of evidence for evolution. [3]
The fossil record provides direct evidence of organisms that lived in the past, showing how species have changed over geological time and revealing transitional forms. [1]
Molecular evidence allows comparisons between living organisms and can reveal evolutionary relationships even when the fossil record is incomplete. [1]
Together, they provide a more complete picture of evolutionary history — fossils give temporal and morphological context, while molecular data provides precise measures of genetic relatedness. [1]
Marking: 1 mark per valid point. Maximum 3 marks.
20. (a) Explain how natural selection could cause the two populations to diverge over many generations. [6]
Indicative content:
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Within the original plant population, there was genetic variation in traits related to water availability (e.g., root depth, leaf thickness, stomatal density, drought tolerance). [1–2 marks]
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On the north bank (dry, sandy soil), plants with traits that improve water uptake and retention (e.g., deeper roots, thicker cuticles, smaller leaves) were more likely to survive and reproduce. [1–2 marks]
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On the south bank (wet, clay-rich soil), plants with traits suited to waterlogged conditions (e.g., shallow roots, aerenchyma tissue, tolerance to waterlogging) were favoured. [1–2 marks]
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Over many generations, the frequency of alleles conferring drought tolerance increased in Population A, while alleles conferring waterlogging tolerance increased in Population B. [1 mark]
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The two populations diverged genetically and phenotypically, becoming increasingly different from each other. [1 mark]
Marking: Award marks for each valid point. Maximum 6 marks. QWC assessed.
(b) Explain how reproductive isolation can arise and why it is important in the formation of new species. [4]
Reproductive isolation can arise through various mechanisms: [1–2 marks]
- Temporal isolation: The two populations may flower at different times, preventing cross-pollination.
- Behavioural/mechanical isolation: Pollinators may differ between the two habitats, or flower structures may change so that pollen transfer between populations is no longer possible.
- Geographical isolation: The physical barrier (river) prevents gene flow between populations, allowing them to diverge independently.
Reproductive isolation is important because it prevents gene flow between populations. [1]
Without gene flow, the two populations continue to diverge independently under different selection pressures. [1]
Eventually, the populations become so genetically different that they can no longer interbreed to produce fertile offspring — at this point, they are considered separate species (speciation has occurred). [1]
Marking: Award marks for describing mechanisms of reproductive isolation and explaining its importance. Maximum 4 marks. QWC assessed.
END OF ANSWER SCHEME
Total: 50 marks