Biology SL

1. 1.

   State three features common to all cells, according to cell theory.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that all cells are bound by a plasma membrane that separates the interior from the environment. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that all cells contain cytoplasm and genetic material (DNA) that controls cell activities. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that all cells carry out metabolic reactions (e.g. respiration) needed to sustain life. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Cell theory states that living organisms are composed of cells, cells are the smallest unit of life, and all cells come from pre-existing cells.

   Marking points

   • States that all cells are bound by a plasma membrane that separates the interior from the environment.
   • States that all cells contain cytoplasm and genetic material (DNA) that controls cell activities.
   • States that all cells carry out metabolic reactions (e.g. respiration) needed to sustain life.

   Examiner tip: Cell theory states that living organisms are composed of cells, cells are the smallest unit of life, and all cells come from pre-existing cells.

2. 2.

   Marking analysis: A learner attempts the following task: “State three features common to all cells, according to cell theory.” Their response addresses only this point: “States that all cells are bound by a plasma membrane that separates the interior from the environment.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [3 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that all cells are bound by a plasma membrane that separates the interior from the environment. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that all cells contain cytoplasm and genetic material (DNA) that controls cell activities. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that all cells carry out metabolic reactions (e.g. respiration) needed to sustain life. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that all cells are bound by a plasma membrane that separates the interior from the environment.
   • Identifies the missing requirement: States that all cells contain cytoplasm and genetic material (DNA) that controls cell activities.
   • Identifies the missing requirement: States that all cells carry out metabolic reactions (e.g. respiration) needed to sustain life.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

3. 3.

   Distinguish between prokaryotic and eukaryotic cells in terms of their genetic material and membrane-bound organelles.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that prokaryotic cells have a single circular DNA molecule that is not enclosed by a nuclear membrane, free in the cytoplasm. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that eukaryotic cells have linear DNA enclosed within a membrane-bound nucleus. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Develop this part of the answer: States that prokaryotic cells lack membrane-bound organelles (e.g. mitochondria, endoplasmic reticulum). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   5. Develop this part of the answer: States that eukaryotic cells contain multiple membrane-bound organelles that compartmentalize different functions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: The single biggest distinguishing feature is the presence (eukaryotic) or absence (prokaryotic) of a true, membrane-bound nucleus.

   Marking points

   • States that prokaryotic cells have a single circular DNA molecule that is not enclosed by a nuclear membrane, free in the cytoplasm.
   • States that eukaryotic cells have linear DNA enclosed within a membrane-bound nucleus.
   • States that prokaryotic cells lack membrane-bound organelles (e.g. mitochondria, endoplasmic reticulum).
   • States that eukaryotic cells contain multiple membrane-bound organelles that compartmentalize different functions.

   Examiner tip: The single biggest distinguishing feature is the presence (eukaryotic) or absence (prokaryotic) of a true, membrane-bound nucleus.

4. 4.

   Marking analysis: A learner attempts the following task: “Distinguish between prokaryotic and eukaryotic cells in terms of their genetic material and membrane-bound organelles.” Their response addresses only this point: “States that prokaryotic cells have a single circular DNA molecule that is not enclosed by a nuclear membrane, free in the cytoplasm.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [4 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that prokaryotic cells have a single circular DNA molecule that is not enclosed by a nuclear membrane, free in the cytoplasm. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that eukaryotic cells have linear DNA enclosed within a membrane-bound nucleus. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that prokaryotic cells lack membrane-bound organelles (e.g. mitochondria, endoplasmic reticulum). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Requirement 4: Identifies the missing requirement: States that eukaryotic cells contain multiple membrane-bound organelles that compartmentalize different functions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that prokaryotic cells have a single circular DNA molecule that is not enclosed by a nuclear membrane, free in the cytoplasm.
   • Identifies the missing requirement: States that eukaryotic cells have linear DNA enclosed within a membrane-bound nucleus.
   • Identifies the missing requirement: States that prokaryotic cells lack membrane-bound organelles (e.g. mitochondria, endoplasmic reticulum).
   • Identifies the missing requirement: States that eukaryotic cells contain multiple membrane-bound organelles that compartmentalize different functions.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

5. 5.

   Outline two properties of water that make it important for living organisms.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that water is a good solvent for polar and ionic substances, due to its polarity, allowing many biochemical reactions to occur in aqueous solution. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States that water has a high specific heat capacity, helping to buffer organisms against rapid temperature fluctuations. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Water's properties (solvent, high specific heat capacity, cohesion, high latent heat of vaporization) all trace back to hydrogen bonding between its polar molecules.

   Marking points

   • States that water is a good solvent for polar and ionic substances, due to its polarity, allowing many biochemical reactions to occur in aqueous solution.
   • States that water has a high specific heat capacity, helping to buffer organisms against rapid temperature fluctuations.

   Examiner tip: Water's properties (solvent, high specific heat capacity, cohesion, high latent heat of vaporization) all trace back to hydrogen bonding between its polar molecules.

6. 6.

   Marking analysis: A learner attempts the following task: “Outline two properties of water that make it important for living organisms.” Their response addresses only this point: “States that water is a good solvent for polar and ionic substances, due to its polarity, allowing many biochemical reactions to occur in aqueous solution.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States that water is a good solvent for polar and ionic substances, due to its polarity, allowing many biochemical reactions to occur in aqueous solution. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that water has a high specific heat capacity, helping to buffer organisms against rapid temperature fluctuations. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States that water is a good solvent for polar and ionic substances, due to its polarity, allowing many biochemical reactions to occur in aqueous solution.
   • Identifies the missing requirement: States that water has a high specific heat capacity, helping to buffer organisms against rapid temperature fluctuations.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

7. 7.

   Outline two types of evidence used to support the theory of evolution.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States fossil evidence: fossils in successive rock strata show a chronological sequence of gradual change in organisms over geological time. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States molecular/genetic evidence: the degree of similarity in DNA or protein (e.g. amino acid) sequences between species reflects how closely related they are and how recently they shared a common ancestor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Other valid evidence types include selective breeding (showing heritable variation responds to selection) and homologous structures (shared anatomical features from common ancestry).

   Marking points

   • States fossil evidence: fossils in successive rock strata show a chronological sequence of gradual change in organisms over geological time.
   • States molecular/genetic evidence: the degree of similarity in DNA or protein (e.g. amino acid) sequences between species reflects how closely related they are and how recently they shared a common ancestor.

   Examiner tip: Other valid evidence types include selective breeding (showing heritable variation responds to selection) and homologous structures (shared anatomical features from common ancestry).

8. 8.

   Marking analysis: A learner attempts the following task: “Outline two types of evidence used to support the theory of evolution.” Their response addresses only this point: “States fossil evidence: fossils in successive rock strata show a chronological sequence of gradual change in organisms over geological time.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
   2. Requirement 1: Recognises credit for the stated point: States fossil evidence: fossils in successive rock strata show a chronological sequence of gradual change in organisms over geological time. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States molecular/genetic evidence: the degree of similarity in DNA or protein (e.g. amino acid) sequences between species reflects how closely related they are and how recently they shared a common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

   Marking points

   • Recognises credit for the stated point: States fossil evidence: fossils in successive rock strata show a chronological sequence of gradual change in organisms over geological time.
   • Identifies the missing requirement: States molecular/genetic evidence: the degree of similarity in DNA or protein (e.g. amino acid) sequences between species reflects how closely related they are and how recently they shared a common ancestor.

   Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

9. 9.

   Outline the biological species concept and state one limitation of this definition.

   [2 marks] · no calculator

   Answer explanation

   Draft walkthroughs are based on marking guidance, not independently verified derivations.

   1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
   2. Develop this part of the answer: States that a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   3. Develop this part of the answer: States a valid limitation, e.g. it cannot be applied to asexually reproducing organisms, or to fossils where interbreeding cannot be observed. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
   4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Because the biological species concept has real limitations, biologists also use other criteria (morphological, genetic/molecular) to define species, especially for asexual or extinct organisms.

   Marking points

   • States that a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions.
   • States a valid limitation, e.g. it cannot be applied to asexually reproducing organisms, or to fossils where interbreeding cannot be observed.

   Examiner tip: Because the biological species concept has real limitations, biologists also use other criteria (morphological, genetic/molecular) to define species, especially for asexual or extinct organisms.

10. 10.

    Marking analysis: A learner attempts the following task: “Outline the biological species concept and state one limitation of this definition.” Their response addresses only this point: “States that a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States a valid limitation, e.g. it cannot be applied to asexually reproducing organisms, or to fossils where interbreeding cannot be observed. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a species is defined as a group of organisms that can interbreed and produce fertile offspring under natural conditions.
    • Identifies the missing requirement: States a valid limitation, e.g. it cannot be applied to asexually reproducing organisms, or to fossils where interbreeding cannot be observed.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

11. 11.

    Explain the process of allopatric speciation.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a population becomes geographically separated into two groups by a physical barrier (e.g. a mountain range, river or ocean), preventing gene flow between them. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that each isolated population experiences different mutations, genetic drift and/or different selection pressures in its own environment. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that over time, the accumulated genetic differences become so great that the two populations can no longer interbreed successfully, even if reunited. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that at this point, the two populations are considered separate species, since reproductive isolation has occurred. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Allopatric speciation requires geographic isolation first; sympatric speciation (rarer, harder to explain) occurs without geographic separation, through other reproductive isolating mechanisms.

    Marking points

    • States that a population becomes geographically separated into two groups by a physical barrier (e.g. a mountain range, river or ocean), preventing gene flow between them.
    • States that each isolated population experiences different mutations, genetic drift and/or different selection pressures in its own environment.
    • States that over time, the accumulated genetic differences become so great that the two populations can no longer interbreed successfully, even if reunited.
    • States that at this point, the two populations are considered separate species, since reproductive isolation has occurred.

    Examiner tip: Allopatric speciation requires geographic isolation first; sympatric speciation (rarer, harder to explain) occurs without geographic separation, through other reproductive isolating mechanisms.

12. 12.

    Marking analysis: A learner attempts the following task: “Explain the process of allopatric speciation.” Their response addresses only this point: “States that a population becomes geographically separated into two groups by a physical barrier (e.g. a mountain range, river or ocean), preventing gene flow between them.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a population becomes geographically separated into two groups by a physical barrier (e.g. a mountain range, river or ocean), preventing gene flow between them. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that each isolated population experiences different mutations, genetic drift and/or different selection pressures in its own environment. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that over time, the accumulated genetic differences become so great that the two populations can no longer interbreed successfully, even if reunited. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that at this point, the two populations are considered separate species, since reproductive isolation has occurred. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a population becomes geographically separated into two groups by a physical barrier (e.g. a mountain range, river or ocean), preventing gene flow between them.
    • Identifies the missing requirement: States that each isolated population experiences different mutations, genetic drift and/or different selection pressures in its own environment.
    • Identifies the missing requirement: States that over time, the accumulated genetic differences become so great that the two populations can no longer interbreed successfully, even if reunited.
    • Identifies the missing requirement: States that at this point, the two populations are considered separate species, since reproductive isolation has occurred.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

13. 13.

    Outline the principle of binomial nomenclature used in the naming of species.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that each species is given a two-part scientific name: the genus name followed by the species name (e.g. Homo sapiens). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that this system provides a single, universally recognized name for each species, avoiding confusion caused by different common names in different languages or regions. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: By convention, the genus name is capitalized and the species name is not, and both are italicized (or underlined when handwritten).

    Marking points

    • States that each species is given a two-part scientific name: the genus name followed by the species name (e.g. Homo sapiens).
    • States that this system provides a single, universally recognized name for each species, avoiding confusion caused by different common names in different languages or regions.

    Examiner tip: By convention, the genus name is capitalized and the species name is not, and both are italicized (or underlined when handwritten).

14. 14.

    Marking analysis: A learner attempts the following task: “Outline the principle of binomial nomenclature used in the naming of species.” Their response addresses only this point: “States that each species is given a two-part scientific name: the genus name followed by the species name (e.g. Homo sapiens).” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that each species is given a two-part scientific name: the genus name followed by the species name (e.g. Homo sapiens). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that this system provides a single, universally recognized name for each species, avoiding confusion caused by different common names in different languages or regions. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that each species is given a two-part scientific name: the genus name followed by the species name (e.g. Homo sapiens).
    • Identifies the missing requirement: States that this system provides a single, universally recognized name for each species, avoiding confusion caused by different common names in different languages or regions.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

15. 15.

    Explain how a cladogram uses shared derived characteristics to represent evolutionary relationships between species.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a cladogram is a branching diagram that represents hypothesized evolutionary relationships based on shared characteristics. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that species sharing a derived characteristic (one not present in a common ancestor) are grouped together on the same branch, indicating they share a more recent common ancestor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the branching pattern (topology) of the cladogram therefore reflects the sequence in which different groups diverged from common ancestors over evolutionary time. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A cladogram groups organisms by shared derived characteristics (synapomorphies), not simply by overall similarity — two species can look very different but still be shown as close relatives if they share key derived traits.

    Marking points

    • States that a cladogram is a branching diagram that represents hypothesized evolutionary relationships based on shared characteristics.
    • States that species sharing a derived characteristic (one not present in a common ancestor) are grouped together on the same branch, indicating they share a more recent common ancestor.
    • States that the branching pattern (topology) of the cladogram therefore reflects the sequence in which different groups diverged from common ancestors over evolutionary time.

    Examiner tip: A cladogram groups organisms by shared derived characteristics (synapomorphies), not simply by overall similarity — two species can look very different but still be shown as close relatives if they share key derived traits.

16. 16.

    Marking analysis: A learner attempts the following task: “Explain how a cladogram uses shared derived characteristics to represent evolutionary relationships between species.” Their response addresses only this point: “States that a cladogram is a branching diagram that represents hypothesized evolutionary relationships based on shared characteristics.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a cladogram is a branching diagram that represents hypothesized evolutionary relationships based on shared characteristics. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that species sharing a derived characteristic (one not present in a common ancestor) are grouped together on the same branch, indicating they share a more recent common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the branching pattern (topology) of the cladogram therefore reflects the sequence in which different groups diverged from common ancestors over evolutionary time. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a cladogram is a branching diagram that represents hypothesized evolutionary relationships based on shared characteristics.
    • Identifies the missing requirement: States that species sharing a derived characteristic (one not present in a common ancestor) are grouped together on the same branch, indicating they share a more recent common ancestor.
    • Identifies the missing requirement: States that the branching pattern (topology) of the cladogram therefore reflects the sequence in which different groups diverged from common ancestors over evolutionary time.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

17. 17.

    Define biodiversity, and outline why biodiversity is important for ecosystem stability.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Defines biodiversity as the variety of life, including diversity within species (genetic diversity), between species, and of ecosystems. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that greater biodiversity generally increases an ecosystem's resilience/stability, since a wider range of species and genetic variation provides more ways for the ecosystem to respond and adapt to disturbance or environmental change. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Genetic diversity within a species matters as much as species diversity between different species — both contribute to overall biodiversity and resilience.

    Marking points

    • Defines biodiversity as the variety of life, including diversity within species (genetic diversity), between species, and of ecosystems.
    • States that greater biodiversity generally increases an ecosystem's resilience/stability, since a wider range of species and genetic variation provides more ways for the ecosystem to respond and adapt to disturbance or environmental change.

    Examiner tip: Genetic diversity within a species matters as much as species diversity between different species — both contribute to overall biodiversity and resilience.

18. 18.

    Marking analysis: A learner attempts the following task: “Define biodiversity, and outline why biodiversity is important for ecosystem stability.” Their response addresses only this point: “Defines biodiversity as the variety of life, including diversity within species (genetic diversity), between species, and of ecosystems.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Defines biodiversity as the variety of life, including diversity within species (genetic diversity), between species, and of ecosystems. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that greater biodiversity generally increases an ecosystem's resilience/stability, since a wider range of species and genetic variation provides more ways for the ecosystem to respond and adapt to disturbance or environmental change. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Defines biodiversity as the variety of life, including diversity within species (genetic diversity), between species, and of ecosystems.
    • Identifies the missing requirement: States that greater biodiversity generally increases an ecosystem's resilience/stability, since a wider range of species and genetic variation provides more ways for the ecosystem to respond and adapt to disturbance or environmental change.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

19. 19.

    Outline two human activities that threaten biodiversity, and for each, state the primary mechanism of harm.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States habitat destruction (e.g. deforestation, land conversion for agriculture) as a first threat. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Explains that habitat destruction removes or fragments the environments species depend on, reducing population sizes and reproductive isolation of remaining populations. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States a second valid threat, e.g. pollution, overexploitation/overhunting, introduction of invasive species, or climate change. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: Explains its mechanism of harm, e.g. invasive species outcompete or predate native species; overexploitation removes individuals faster than populations can replace them. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Habitat destruction is generally considered the single greatest overall threat to global biodiversity, ahead of overexploitation, invasive species, pollution and climate change.

    Marking points

    • States habitat destruction (e.g. deforestation, land conversion for agriculture) as a first threat.
    • Explains that habitat destruction removes or fragments the environments species depend on, reducing population sizes and reproductive isolation of remaining populations.
    • States a second valid threat, e.g. pollution, overexploitation/overhunting, introduction of invasive species, or climate change.
    • Explains its mechanism of harm, e.g. invasive species outcompete or predate native species; overexploitation removes individuals faster than populations can replace them.

    Examiner tip: Habitat destruction is generally considered the single greatest overall threat to global biodiversity, ahead of overexploitation, invasive species, pollution and climate change.

20. 20.

    Marking analysis: A learner attempts the following task: “Outline two human activities that threaten biodiversity, and for each, state the primary mechanism of harm.” Their response addresses only this point: “States habitat destruction (e.g. deforestation, land conversion for agriculture) as a first threat.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States habitat destruction (e.g. deforestation, land conversion for agriculture) as a first threat. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Explains that habitat destruction removes or fragments the environments species depend on, reducing population sizes and reproductive isolation of remaining populations. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States a second valid threat, e.g. pollution, overexploitation/overhunting, introduction of invasive species, or climate change. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: Explains its mechanism of harm, e.g. invasive species outcompete or predate native species; overexploitation removes individuals faster than populations can replace them. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States habitat destruction (e.g. deforestation, land conversion for agriculture) as a first threat.
    • Identifies the missing requirement: Explains that habitat destruction removes or fragments the environments species depend on, reducing population sizes and reproductive isolation of remaining populations.
    • Identifies the missing requirement: States a second valid threat, e.g. pollution, overexploitation/overhunting, introduction of invasive species, or climate change.
    • Identifies the missing requirement: Explains its mechanism of harm, e.g. invasive species outcompete or predate native species; overexploitation removes individuals faster than populations can replace them.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

21. 21.

    Outline one strategy used in conservation biology to protect biodiversity, and explain how it addresses a specific threat.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: Outlines a valid conservation strategy, e.g. establishing protected areas/nature reserves, captive breeding programmes, or seed banks. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Explains how the strategy addresses a specific threat, e.g. protected areas prevent further habitat destruction by restricting land use and human activity within their boundaries. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Link the specific conservation strategy explicitly to the specific threat it counters (e.g. protected areas counter habitat loss; captive breeding counters small, at-risk populations) rather than describing strategies generically.

    Marking points

    • Outlines a valid conservation strategy, e.g. establishing protected areas/nature reserves, captive breeding programmes, or seed banks.
    • Explains how the strategy addresses a specific threat, e.g. protected areas prevent further habitat destruction by restricting land use and human activity within their boundaries.

    Examiner tip: Link the specific conservation strategy explicitly to the specific threat it counters (e.g. protected areas counter habitat loss; captive breeding counters small, at-risk populations) rather than describing strategies generically.

22. 22.

    Marking analysis: A learner attempts the following task: “Outline one strategy used in conservation biology to protect biodiversity, and explain how it addresses a specific threat.” Their response addresses only this point: “Outlines a valid conservation strategy, e.g. establishing protected areas/nature reserves, captive breeding programmes, or seed banks.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: Outlines a valid conservation strategy, e.g. establishing protected areas/nature reserves, captive breeding programmes, or seed banks. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Explains how the strategy addresses a specific threat, e.g. protected areas prevent further habitat destruction by restricting land use and human activity within their boundaries. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: Outlines a valid conservation strategy, e.g. establishing protected areas/nature reserves, captive breeding programmes, or seed banks.
    • Identifies the missing requirement: Explains how the strategy addresses a specific threat, e.g. protected areas prevent further habitat destruction by restricting land use and human activity within their boundaries.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

23. 23.

    Outline the basic structure of DNA, referring to nucleotides and the double helix.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that DNA is composed of repeating subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (A, T, C, G). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that two strands of nucleotides are held together by hydrogen bonds between complementary base pairs (A with T, C with G). Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that the two strands are wound around each other to form a double helix. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Complementary base pairing (A-T, C-G) is what allows each DNA strand to act as a template for accurately copying the molecule during replication.

    Marking points

    • States that DNA is composed of repeating subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (A, T, C, G).
    • States that two strands of nucleotides are held together by hydrogen bonds between complementary base pairs (A with T, C with G).
    • States that the two strands are wound around each other to form a double helix.

    Examiner tip: Complementary base pairing (A-T, C-G) is what allows each DNA strand to act as a template for accurately copying the molecule during replication.

24. 24.

    Marking analysis: A learner attempts the following task: “Outline the basic structure of DNA, referring to nucleotides and the double helix.” Their response addresses only this point: “States that DNA is composed of repeating subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (A, T, C, G).” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that DNA is composed of repeating subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (A, T, C, G). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that two strands of nucleotides are held together by hydrogen bonds between complementary base pairs (A with T, C with G). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the two strands are wound around each other to form a double helix. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that DNA is composed of repeating subunits called nucleotides, each containing a sugar (deoxyribose), a phosphate group, and one of four nitrogenous bases (A, T, C, G).
    • Identifies the missing requirement: States that two strands of nucleotides are held together by hydrogen bonds between complementary base pairs (A with T, C with G).
    • Identifies the missing requirement: States that the two strands are wound around each other to form a double helix.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

25. 25.

    Explain why carbon is particularly suited to forming the wide variety of biological molecules found in living organisms.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that a carbon atom can form four covalent bonds, allowing it to bond with up to four other atoms. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that carbon atoms can bond to each other to form long chains, branched structures, or rings, creating diverse molecular skeletons. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that carbon can form stable bonds with many other elements, e.g. hydrogen, oxygen and nitrogen, enabling a huge variety of functional groups and biomolecules. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Carbon's ability to form stable chains and rings with itself is unique among common elements, and is the fundamental reason organic chemistry (and life itself) is built around it.

    Marking points

    • States that a carbon atom can form four covalent bonds, allowing it to bond with up to four other atoms.
    • States that carbon atoms can bond to each other to form long chains, branched structures, or rings, creating diverse molecular skeletons.
    • States that carbon can form stable bonds with many other elements, e.g. hydrogen, oxygen and nitrogen, enabling a huge variety of functional groups and biomolecules.

    Examiner tip: Carbon's ability to form stable chains and rings with itself is unique among common elements, and is the fundamental reason organic chemistry (and life itself) is built around it.

26. 26.

    Marking analysis: A learner attempts the following task: “Explain why carbon is particularly suited to forming the wide variety of biological molecules found in living organisms.” Their response addresses only this point: “States that a carbon atom can form four covalent bonds, allowing it to bond with up to four other atoms.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that a carbon atom can form four covalent bonds, allowing it to bond with up to four other atoms. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that carbon atoms can bond to each other to form long chains, branched structures, or rings, creating diverse molecular skeletons. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that carbon can form stable bonds with many other elements, e.g. hydrogen, oxygen and nitrogen, enabling a huge variety of functional groups and biomolecules. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that a carbon atom can form four covalent bonds, allowing it to bond with up to four other atoms.
    • Identifies the missing requirement: States that carbon atoms can bond to each other to form long chains, branched structures, or rings, creating diverse molecular skeletons.
    • Identifies the missing requirement: States that carbon can form stable bonds with many other elements, e.g. hydrogen, oxygen and nitrogen, enabling a huge variety of functional groups and biomolecules.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

27. 27.

    State the seven main taxonomic ranks used to classify organisms, in order from the broadest to the most specific.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States the correct order: kingdom, phylum, class, order, family, genus, species. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that each successive rank represents an increasingly specific (narrower) grouping, with species being the most specific. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: A classic mnemonic for remembering this order is 'King Philip Came Over For Good Soup' — the first letter of each word matches the first letter of each taxonomic rank.

    Marking points

    • States the correct order: kingdom, phylum, class, order, family, genus, species.
    • States that each successive rank represents an increasingly specific (narrower) grouping, with species being the most specific.

    Examiner tip: A classic mnemonic for remembering this order is 'King Philip Came Over For Good Soup' — the first letter of each word matches the first letter of each taxonomic rank.

28. 28.

    Marking analysis: A learner attempts the following task: “State the seven main taxonomic ranks used to classify organisms, in order from the broadest to the most specific.” Their response addresses only this point: “States the correct order: kingdom, phylum, class, order, family, genus, species.” Evaluate the response against the complete 2-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [2 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States the correct order: kingdom, phylum, class, order, family, genus, species. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that each successive rank represents an increasingly specific (narrower) grouping, with species being the most specific. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States the correct order: kingdom, phylum, class, order, family, genus, species.
    • Identifies the missing requirement: States that each successive rank represents an increasingly specific (narrower) grouping, with species being the most specific.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

29. 29.

    Outline the process of natural selection, using antibiotic resistance in bacteria as an example.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that genetic variation exists naturally within a population, e.g. some bacteria may already carry a mutation providing antibiotic resistance. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: States that when the antibiotic (a selection pressure) is applied, resistant individuals are more likely to survive and reproduce than non-resistant individuals. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: States that resistant individuals pass the resistance allele on to their offspring. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Develop this part of the answer: States that over successive generations, the proportion of resistant individuals in the population increases, since they have a survival and reproductive advantage. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Natural selection does not create new mutations in response to a challenge — the resistant variants already exist by chance beforehand; selection simply favours their survival and reproduction once the pressure is applied.

    Marking points

    • States that genetic variation exists naturally within a population, e.g. some bacteria may already carry a mutation providing antibiotic resistance.
    • States that when the antibiotic (a selection pressure) is applied, resistant individuals are more likely to survive and reproduce than non-resistant individuals.
    • States that resistant individuals pass the resistance allele on to their offspring.
    • States that over successive generations, the proportion of resistant individuals in the population increases, since they have a survival and reproductive advantage.

    Examiner tip: Natural selection does not create new mutations in response to a challenge — the resistant variants already exist by chance beforehand; selection simply favours their survival and reproduction once the pressure is applied.

30. 30.

    Marking analysis: A learner attempts the following task: “Outline the process of natural selection, using antibiotic resistance in bacteria as an example.” Their response addresses only this point: “States that genetic variation exists naturally within a population, e.g. some bacteria may already carry a mutation providing antibiotic resistance.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that genetic variation exists naturally within a population, e.g. some bacteria may already carry a mutation providing antibiotic resistance. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that when the antibiotic (a selection pressure) is applied, resistant individuals are more likely to survive and reproduce than non-resistant individuals. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that resistant individuals pass the resistance allele on to their offspring. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that over successive generations, the proportion of resistant individuals in the population increases, since they have a survival and reproductive advantage. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that genetic variation exists naturally within a population, e.g. some bacteria may already carry a mutation providing antibiotic resistance.
    • Identifies the missing requirement: States that when the antibiotic (a selection pressure) is applied, resistant individuals are more likely to survive and reproduce than non-resistant individuals.
    • Identifies the missing requirement: States that resistant individuals pass the resistance allele on to their offspring.
    • Identifies the missing requirement: States that over successive generations, the proportion of resistant individuals in the population increases, since they have a survival and reproductive advantage.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

31. 31.

    In a population, 9% of individuals show a recessive phenotype (genotype aa). Assuming the population is in Hardy-Weinberg equilibrium, (a) calculate the frequency of the recessive allele (q), (b) calculate the frequency of the dominant allele (p), and (c) calculate the percentage of the population that are heterozygous carriers (Aa).

    [5 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Work through this mathematical step: States the Hardy-Weinberg equations: p + q = 1 and p² + 2pq + q² = 1. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    3. Work through this mathematical step: States that the recessive phenotype frequency equals q², so q² = 0.09. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Work through this mathematical step: Calculates q = √0.09 = 0.3. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Work through this mathematical step: Calculates p = 1 − 0.3 = 0.7. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    6. Work through this mathematical step: Calculates the heterozygote frequency as 2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42% of the population. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Always start from the recessive phenotype frequency (q²), since it is the only genotype whose frequency can be read directly from the visible phenotype — p and the heterozygote frequency must then be derived from it.

    Marking points

    • States the Hardy-Weinberg equations: p + q = 1 and p² + 2pq + q² = 1.
    • States that the recessive phenotype frequency equals q², so q² = 0.09.
    • Calculates q = √0.09 = 0.3.
    • Calculates p = 1 − 0.3 = 0.7.
    • Calculates the heterozygote frequency as 2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42% of the population.

    Examiner tip: Always start from the recessive phenotype frequency (q²), since it is the only genotype whose frequency can be read directly from the visible phenotype — p and the heterozygote frequency must then be derived from it.

32. 32.

    Marking analysis: A learner attempts the following task: “In a population, 9% of individuals show a recessive phenotype (genotype aa). Assuming the population is in Hardy-Weinberg equilibrium, (a) calculate the frequency of the recessive allele (q), (b) calculate the frequency of the dominant allele (p), and (c) calculate the percentage of the population that are heterozygous carriers (Aa).” Their response addresses only this point: “States the Hardy-Weinberg equations: p + q = 1 and p² + 2pq + q² = 1.” Evaluate the response against the complete 5-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [5 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States the Hardy-Weinberg equations: p + q = 1 and p² + 2pq + q² = 1. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that the recessive phenotype frequency equals q², so q² = 0.09. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Calculates q = √0.09 = 0.3. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: Calculates p = 1 − 0.3 = 0.7. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    6. Requirement 5: Identifies the missing requirement: Calculates the heterozygote frequency as 2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42% of the population. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    7. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States the Hardy-Weinberg equations: p + q = 1 and p² + 2pq + q² = 1.
    • Identifies the missing requirement: States that the recessive phenotype frequency equals q², so q² = 0.09.
    • Identifies the missing requirement: Calculates q = √0.09 = 0.3.
    • Identifies the missing requirement: Calculates p = 1 − 0.3 = 0.7.
    • Identifies the missing requirement: Calculates the heterozygote frequency as 2pq = 2 × 0.7 × 0.3 = 0.42, i.e. 42% of the population.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

33. 33.

    The cladogram below shows the evolutionary relationships of four species, with species W and X sharing the most recent common ancestor, and this clade sharing an earlier common ancestor with species Y, which in turn shares an even earlier common ancestor with species Z: (((W,X),Y),Z). State which two species are most closely related, and explain how a cladogram represents evolutionary relationships using branching points.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Break the command into its requested parts. For each part, connect a relevant fact or observation to the conclusion it supports. Describing what happens and explaining why it happens are different tasks.
    2. Develop this part of the answer: States that species W and X are most closely related, since they share the most recent common ancestor. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Develop this part of the answer: Explains that each branching point (node) on a cladogram represents a common ancestor from which two lineages diverged. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    4. Develop this part of the answer: Explains that the relative closeness of a branching point to the tips of the tree indicates how recently two species diverged: a more recent (closer to the tips) branching point means a more recent common ancestor and a closer evolutionary relationship. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Relatedness on a cladogram is read from the branching pattern, not from the left-to-right order species happen to be drawn in — two species can appear far apart in a diagram yet still share the most recent common ancestor of all.

    Marking points

    • States that species W and X are most closely related, since they share the most recent common ancestor.
    • Explains that each branching point (node) on a cladogram represents a common ancestor from which two lineages diverged.
    • Explains that the relative closeness of a branching point to the tips of the tree indicates how recently two species diverged: a more recent (closer to the tips) branching point means a more recent common ancestor and a closer evolutionary relationship.

    Examiner tip: Relatedness on a cladogram is read from the branching pattern, not from the left-to-right order species happen to be drawn in — two species can appear far apart in a diagram yet still share the most recent common ancestor of all.

34. 34.

    Marking analysis: A learner attempts the following task: “The cladogram below shows the evolutionary relationships of four species, with species W and X sharing the most recent common ancestor, and this clade sharing an earlier common ancestor with species Y, which in turn shares an even earlier common ancestor with species Z: (((W,X),Y),Z). State which two species are most closely related, and explain how a cladogram represents evolutionary relationships using branching points.” Their response addresses only this point: “States that species W and X are most closely related, since they share the most recent common ancestor.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks] · no calculator

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that species W and X are most closely related, since they share the most recent common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Explains that each branching point (node) on a cladogram represents a common ancestor from which two lineages diverged. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Explains that the relative closeness of a branching point to the tips of the tree indicates how recently two species diverged: a more recent (closer to the tips) branching point means a more recent common ancestor and a closer evolutionary relationship. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that species W and X are most closely related, since they share the most recent common ancestor.
    • Identifies the missing requirement: Explains that each branching point (node) on a cladogram represents a common ancestor from which two lineages diverged.
    • Identifies the missing requirement: Explains that the relative closeness of a branching point to the tips of the tree indicates how recently two species diverged: a more recent (closer to the tips) branching point means a more recent common ancestor and a closer evolutionary relationship.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

35. 35.

    A sample of double-stranded DNA is found to be 30% adenine. (a) State the percentage of thymine in the sample. (b) Calculate the percentage of guanine and the percentage of cytosine in the sample, assuming they are present in equal amounts.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. List the given quantities and the requested unknown. Choose the relation that connects them, state any required assumptions, then substitute before rounding. Preserve exact expressions when the task asks for an exact result.
    2. Develop this part of the answer: States that, by complementary base pairing, adenine always pairs with thymine, so thymine must also be 30%. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    3. Work through this mathematical step: Calculates the remaining percentage for guanine and cytosine combined: 100% − 30% − 30% = 40%. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    4. Develop this part of the answer: States that since guanine always pairs with cytosine in equal amounts, each makes up half of this remaining percentage: 20% guanine and 20% cytosine. Show which detail or principle supports it and how it addresses the command; equivalent supported wording is acceptable.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Chargaff's base-pairing rule (%A = %T and %G = %C) is the key fact needed to solve any DNA base-composition problem — once you know one base's percentage, the percentage of its complementary partner is identical.

    Marking points

    • States that, by complementary base pairing, adenine always pairs with thymine, so thymine must also be 30%.
    • Calculates the remaining percentage for guanine and cytosine combined: 100% − 30% − 30% = 40%.
    • States that since guanine always pairs with cytosine in equal amounts, each makes up half of this remaining percentage: 20% guanine and 20% cytosine.

    Examiner tip: Chargaff's base-pairing rule (%A = %T and %G = %C) is the key fact needed to solve any DNA base-composition problem — once you know one base's percentage, the percentage of its complementary partner is identical.

36. 36.

    Marking analysis: A learner attempts the following task: “A sample of double-stranded DNA is found to be 30% adenine. (a) State the percentage of thymine in the sample. (b) Calculate the percentage of guanine and the percentage of cytosine in the sample, assuming they are present in equal amounts.” Their response addresses only this point: “States that, by complementary base pairing, adenine always pairs with thymine, so thymine must also be 30%.” Evaluate the response against the complete 3-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [3 marks]

    Answer explanation

    Draft walkthroughs are based on marking guidance, not independently verified derivations.

    1. Separate the learner's stated response from the complete task. Credit only what their response demonstrates, then identify each missing requirement; do not assume unstated working.
    2. Requirement 1: Recognises credit for the stated point: States that, by complementary base pairing, adenine always pairs with thymine, so thymine must also be 30%. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Calculates the remaining percentage for guanine and cytosine combined: 100% − 30% − 30% = 40%. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that since guanine always pairs with cytosine in equal amounts, each makes up half of this remaining percentage: 20% guanine and 20% cytosine. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Check the complete task again, including restrictions, units, precision and supporting evidence when relevant. Specific caution: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

    Marking points

    • Recognises credit for the stated point: States that, by complementary base pairing, adenine always pairs with thymine, so thymine must also be 30%.
    • Identifies the missing requirement: Calculates the remaining percentage for guanine and cytosine combined: 100% − 30% − 30% = 40%.
    • Identifies the missing requirement: States that since guanine always pairs with cytosine in equal amounts, each makes up half of this remaining percentage: 20% guanine and 20% cytosine.

    Examiner tip: Treat each marking point as a separate requirement. Do not award the same idea twice, and do not infer work the learner did not show.

Original, unofficial practice questions from SubjectScout — not official past-paper questions.