Secondary 4 Pure Biology Evolution Diversity Quiz

Secondary 4 Pure Biology Quiz - Evolution Diversity

Answer Key

Section A: Multiple Choice Questions

1. (b) A change in the allele frequencies in a population over successive generations
Marking note: Evolution is defined at the population level, not the individual level. Option (c) reflects a common misconception ("survival of the fittest" misinterpreted). Option (d) reflects Lamarckian thinking.

2. (b) Analogous structures
Marking note: Analogous structures have similar functions but different structural origins and do not indicate common ancestry. Homologous structures share a common origin but may have different functions.

3. (c) Mutation
Marking note: Mutation is the ultimate source of new genetic variation. Natural selection, genetic drift, and gene flow act on existing variation but do not create new alleles.

4. (c) Traits acquired during an organism's lifetime can be passed to offspring.
Marking note: This describes Lamarck's theory of inheritance of acquired characteristics, which is not part of Darwin's theory of natural selection.

5. (b) Directional selection
Marking note: The population shifts toward one extreme phenotype (darker colour). Stabilising selection favours the intermediate; disruptive selection favours both extremes.

6. (b) These groups share a common ancestor
Marking note: The pentadactyl limb is a classic example of homologous structures indicating common ancestry, even though the limbs serve different functions.

7. (c) Geographic isolation of a small population followed by genetic divergence
Marking note: Speciation requires reproductive isolation and genetic divergence. Geographic isolation (allopatric speciation) is the most common mechanism.

8. (b) A progression of increasingly complex forms over geological time
Marking note: The fossil record shows transitional forms and increasing complexity over time, supporting evolutionary theory.

9. (b) Stabilising selection
Marking note: Stabilising selection favours intermediate phenotypes (pink) and acts against both extremes (red and white). This reduces variation in the population.

10. (b) Natural selection acts on phenotypes, not genotypes.
Marking note: Natural selection acts on the observable traits (phenotypes) of organisms. It does not create mutations (a), does not always lead to greater complexity (c), and does not guarantee survival (d).

Section B: Structured Response Questions

11.
(a) Dark wing colour has a selective advantage in the industrial environment. [1] This is because there are far more dark moths (415) compared to light moths (85) in the industrial environment, indicating that dark moths survive and reproduce more successfully there. [1]

(b) In the industrial environment, soot and pollution darken tree bark and surfaces. [1] Dark-coloured moths are better camouflaged against predators (such as birds) on darkened surfaces, so they are more likely to survive and reproduce, passing on the dark-wing allele to the next generation. [1]
Marking note: Accept any valid explanation linking camouflage, predation, and differential survival. The key idea is that the environment determines which phenotype is advantageous.

(c) Disruptive selection [1]

12.
(a) Homologous structures [1]

(b) The three animals share a common ancestor. [1] The similar bone arrangement indicates that these limbs were inherited from a common ancestor but have been modified over time to serve different functions (grasping, flying, swimming) through the process of adaptive evolution / descent with modification. [1]
Marking note: Students must mention common ancestor AND modification for different functions to receive full marks.

(c) Any one of the following: [1]

• DNA / molecular / genetic evidence (comparing DNA sequences)
• Embryological evidence (comparing embryonic development)
• Fossil evidence
• Biochemical evidence (e.g., comparing protein sequences)
  Marking note: Accept any valid type of evolutionary evidence apart from structural/comparative anatomy.

13.
(a) The average beak size in the finch population is likely to increase over several generations. [1] During the drought, large, hard seeds are more abundant. [1] Finches with larger, stronger beaks are better able to crack and eat the large seeds, so they are more likely to survive and reproduce. [1] These finches pass on the alleles for larger beaks to their offspring, causing the average beak size in the population to increase over generations. [1]
Award 3 marks for the chain of reasoning: larger beaks advantageous → differential survival/reproduction → allele frequency change. Award 1 mark for correct prediction alone.

(b) The drought is an environmental change that acts as a selective pressure but does not directly cause genetic changes in individual finches. [1] Individual finches cannot change their beak size in response to the environment during their lifetime. Natural selection acts on existing genetic variation in the population — finches that already possess alleles for larger beaks are favoured. The drought does not create new beak-size alleles; it only changes which existing alleles are advantageous. [1]
Marking note: Key point — the environment selects among existing variation; it does not induce beneficial changes in individuals. This addresses the Lamarckian misconception.

14.
(a) Mutation [1]
Marking note: Random mutations in bacterial DNA can produce alleles that confer resistance to antibiotics.

(b) Step-by-step explanation: [4 marks — 1 mark per valid step]

1. In a population of bacteria, genetic variation exists due to random mutations. Some bacteria carry a mutation that makes them resistant to the antibiotic. [1]
2. When the antibiotic is applied, non-resistant bacteria are killed. [1]
3. Resistant bacteria survive and reproduce, passing the resistance allele to their offspring. [1]
4. Over successive generations, the proportion of resistant bacteria in the population increases because they have a selective advantage in the presence of the antibiotic. [1]
   Marking note: Award marks for each logical step in the natural selection process. Students must show understanding that: (i) variation pre-exists, (ii) the antibiotic is the selective pressure, (iii) resistant individuals survive and reproduce, (iv) the allele frequency changes over generations.

Section C: Data Interpretation and Extended Response

15.
(a) When grown separately, P. aurelia population increases rapidly at first (exponential/log phase), then the rate of increase slows as resources become limited, and the population stabilises at a carrying capacity. [2]
Award 1 mark for describing initial rapid growth and 1 mark for describing the levelling off / reaching carrying capacity.

(b) P. caudatum population declines and eventually goes extinct (reaches zero) when grown together with P. aurelia. [1]

(c) Both species compete for the same limited food resource (they occupy the same ecological niche). [1] P. aurelia is the better competitor — it is more efficient at obtaining and using the shared resource. [1] As a result, P. aurelia depletes the food supply to a level that is too low for P. caudatum to survive, leading to the competitive exclusion of P. caudatum. [1]
Marking note: Key concepts: competition for same resource, competitive exclusion, one species outcompetes the other.

(d) Competitive exclusion principle [1]
Marking note: Also accept "competition" or "competitive exclusion."

16.
(a) During industrialisation, soot from factories darkened tree trunks and other surfaces. [1] Light-coloured moths were more visible to predators (birds) against the darkened bark and were eaten more frequently. [1] Dark-coloured (melanic) moths were better camouflaged on the darkened surfaces, so they were more likely to survive and reproduce, passing the dark-colour allele to offspring. [1] Over generations, the frequency of the dark-colour allele increased in the population. [1]
Award 3 marks for: pollution darkens surfaces → differential predation → differential survival/reproduction → allele frequency change.

(b) After clean air legislation, pollution decreased and tree bark became lighter again. [1] Light-coloured moths were once again better camouflaged against the lighter bark, giving them a selective advantage. Dark-coloured moths became more visible to predators and were eaten more frequently. Over generations, the frequency of the light-colour allele increased. [1]
Marking note: The key idea is that the selective advantage reversed when the environment changed back.

17.
(a) Arthropods and chordates [1]

(b) Jellyfish [1]
Marking note: Jellyfish branches off first (at the base of the cladogram), making it the most distantly related to chordates.

(c) Any one valid distinguishing feature: [1]

• Chordates have a notochord / dorsal nerve cord / post-anal tail / pharyngeal slits at some stage of development; arthropods do not.
• Arthropods have an exoskeleton / jointed appendages / segmented body with tagmata; chordates do not.
  Marking note: Accept any correct distinguishing feature.

18.
(a) Allopatric speciation [1]

(b) Any two of the following: [2 — 1 mark each]

• Geographic isolation: The physical separation by ocean prevents gene flow between the island and mainland populations.
• Different selection pressures: The island environment may have different food sources, predators, or climate, leading to different adaptations.
• Genetic drift: The small founding population has reduced genetic diversity, and random changes in allele frequencies cause divergence.
• Mutation: Different mutations arise independently in each population, contributing to genetic divergence.
• Reproductive isolation: Over time, accumulated genetic differences may result in inability to interbreed even if the populations come into contact again (e.g., behavioural, temporal, or mechanical isolation).
  Marking note: Award 1 mark per valid factor, maximum 2 marks.

19. Organisms that reproduce quickly have more generations in a given time period. [1] Each generation provides an opportunity for new mutations to arise and for natural selection to change allele frequencies. [1] Therefore, more generations per unit time means more opportunities for evolutionary change, resulting in a faster rate of evolution. In contrast, slowly reproducing organisms have fewer generations in the same time period, so allele frequencies change more slowly. [1]
Marking note: The key concept is the number of generations per unit time as the driver of evolutionary rate.

20. The student's statement is partially incorrect. [1] Evolution by natural selection does not necessarily lead to greater complexity — it leads to organisms that are better adapted to their specific environment. [1] In some cases, evolution can lead to simplification (e.g., loss of eyes in cave-dwelling organisms). Evolution also does not mean organisms become "better" in an absolute sense — it means they become better suited to their particular environment. If the environment changes, previously advantageous traits may become disadvantageous. Evolution has no direction or goal; it is driven by natural selection acting on random variation. [1]
Marking note: Award marks for: (i) identifying the statement as incorrect/misleading, (ii) explaining that evolution leads to adaptation (not complexity), (iii) providing a counterexample or explaining that evolution has no direction/goal.

End of Answer Key