Biology HL

1. 1.

   Distinguish between the structure of DNA and RNA in terms of their sugar, bases and number of strands.

   [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 contains deoxyribose sugar, while RNA contains ribose sugar. 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 DNA contains the base thymine, while RNA contains uracil in its place (both share adenine, cytosine and guanine). 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 DNA is typically double-stranded (a double helix), while RNA is typically single-stranded. 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: These three structural differences (sugar, one base, strand number) are the most commonly tested distinctions between DNA and RNA.

   Marking points

   • States that DNA contains deoxyribose sugar, while RNA contains ribose sugar.
   • States that DNA contains the base thymine, while RNA contains uracil in its place (both share adenine, cytosine and guanine).
   • States that DNA is typically double-stranded (a double helix), while RNA is typically single-stranded.

   Examiner tip: These three structural differences (sugar, one base, strand number) are the most commonly tested distinctions between DNA and RNA.

2. 2.

   Marking analysis: A learner attempts the following task: “Distinguish between the structure of DNA and RNA in terms of their sugar, bases and number of strands.” Their response addresses only this point: “States that DNA contains deoxyribose sugar, while RNA contains ribose sugar.” 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 contains deoxyribose sugar, while RNA contains ribose sugar. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that DNA contains the base thymine, while RNA contains uracil in its place (both share adenine, cytosine and guanine). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that DNA is typically double-stranded (a double helix), while RNA is typically single-stranded. 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 contains deoxyribose sugar, while RNA contains ribose sugar.
   • Identifies the missing requirement: States that DNA contains the base thymine, while RNA contains uracil in its place (both share adenine, cytosine and guanine).
   • Identifies the missing requirement: States that DNA is typically double-stranded (a double helix), while RNA is typically single-stranded.

   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.

   Outline how natural selection can lead to 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 geographic barrier (e.g. a river, mountain range or ocean) physically separates a population into two subpopulations, 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 the two subpopulations experience different selection pressures (or undergo different mutations/genetic drift) in their separate environments. 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 allele frequencies diverge between the two subpopulations over many generations as a result. 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 if the divergence becomes great enough that the two populations can no longer interbreed to produce fertile offspring even if reunited, they have become separate species (reproductive isolation). 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 defining feature of allopatric speciation is the geographic barrier acting first; distinguish this from sympatric speciation, where isolation arises without physical separation.

   Marking points

   • States that a geographic barrier (e.g. a river, mountain range or ocean) physically separates a population into two subpopulations, preventing gene flow between them.
   • States that the two subpopulations experience different selection pressures (or undergo different mutations/genetic drift) in their separate environments.
   • States that allele frequencies diverge between the two subpopulations over many generations as a result.
   • States that if the divergence becomes great enough that the two populations can no longer interbreed to produce fertile offspring even if reunited, they have become separate species (reproductive isolation).

   Examiner tip: The defining feature of allopatric speciation is the geographic barrier acting first; distinguish this from sympatric speciation, where isolation arises without physical separation.

4. 4.

   Marking analysis: A learner attempts the following task: “Outline how natural selection can lead to allopatric speciation.” Their response addresses only this point: “States that a geographic barrier (e.g. a river, mountain range or ocean) physically separates a population into two subpopulations, 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 geographic barrier (e.g. a river, mountain range or ocean) physically separates a population into two subpopulations, 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 the two subpopulations experience different selection pressures (or undergo different mutations/genetic drift) in their separate environments. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that allele frequencies diverge between the two subpopulations over many generations as a result. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Requirement 4: Identifies the missing requirement: States that if the divergence becomes great enough that the two populations can no longer interbreed to produce fertile offspring even if reunited, they have become separate species (reproductive isolation). 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 geographic barrier (e.g. a river, mountain range or ocean) physically separates a population into two subpopulations, preventing gene flow between them.
   • Identifies the missing requirement: States that the two subpopulations experience different selection pressures (or undergo different mutations/genetic drift) in their separate environments.
   • Identifies the missing requirement: States that allele frequencies diverge between the two subpopulations over many generations as a result.
   • Identifies the missing requirement: States that if the divergence becomes great enough that the two populations can no longer interbreed to produce fertile offspring even if reunited, they have become separate species (reproductive isolation).

   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 the evidence from selective breeding (artificial selection) that supports the theory of evolution by natural selection.

   [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 selective breeding of domesticated species (e.g. dog breeds, or crop varieties) has produced significant, heritable phenotypic change from a common ancestral population within a relatively short, observable time frame. 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 demonstrates that selecting individuals with desired heritable variations for reproduction, generation after generation, can change the allele frequencies and average phenotype of a population. 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 this supports natural selection as a plausible mechanism for evolutionary change, since natural selection operates by the same underlying principle (differential reproduction of heritable variants) but with the environment, rather than a human breeder, acting as the selecting agent. 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: Selective breeding is used as an analogy for natural selection because it isolates the same core mechanism (differential reproductive success of heritable variants) under human rather than environmental control.

   Marking points

   • States that selective breeding of domesticated species (e.g. dog breeds, or crop varieties) has produced significant, heritable phenotypic change from a common ancestral population within a relatively short, observable time frame.
   • States that this demonstrates that selecting individuals with desired heritable variations for reproduction, generation after generation, can change the allele frequencies and average phenotype of a population.
   • States that this supports natural selection as a plausible mechanism for evolutionary change, since natural selection operates by the same underlying principle (differential reproduction of heritable variants) but with the environment, rather than a human breeder, acting as the selecting agent.

   Examiner tip: Selective breeding is used as an analogy for natural selection because it isolates the same core mechanism (differential reproductive success of heritable variants) under human rather than environmental control.

6. 6.

   Marking analysis: A learner attempts the following task: “Outline the evidence from selective breeding (artificial selection) that supports the theory of evolution by natural selection.” Their response addresses only this point: “States that selective breeding of domesticated species (e.g. dog breeds, or crop varieties) has produced significant, heritable phenotypic change from a common ancestral population within a relatively short, observable time frame.” 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 selective breeding of domesticated species (e.g. dog breeds, or crop varieties) has produced significant, heritable phenotypic change from a common ancestral population within a relatively short, observable time frame. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that this demonstrates that selecting individuals with desired heritable variations for reproduction, generation after generation, can change the allele frequencies and average phenotype of a population. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that this supports natural selection as a plausible mechanism for evolutionary change, since natural selection operates by the same underlying principle (differential reproduction of heritable variants) but with the environment, rather than a human breeder, acting as the selecting agent. 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 selective breeding of domesticated species (e.g. dog breeds, or crop varieties) has produced significant, heritable phenotypic change from a common ancestral population within a relatively short, observable time frame.
   • Identifies the missing requirement: States that this demonstrates that selecting individuals with desired heritable variations for reproduction, generation after generation, can change the allele frequencies and average phenotype of a population.
   • Identifies the missing requirement: States that this supports natural selection as a plausible mechanism for evolutionary change, since natural selection operates by the same underlying principle (differential reproduction of heritable variants) but with the environment, rather than a human breeder, acting as the selecting agent.

   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.

   Describe the Meselson-Stahl experiment, and state what its results showed about the mechanism of DNA replication.

   [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 bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, so all their DNA contained ¹⁵N. 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 the bacteria were then transferred to a medium containing only light nitrogen (¹⁴N), and DNA was extracted after one and two rounds of replication. 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 DNA extracted after one round of replication had an intermediate density (half ¹⁵N, half ¹⁴N), rather than either fully heavy or fully light. 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 this result supported semi-conservative replication, in which each new DNA molecule consists of one original (parent) strand and one newly synthesised strand, ruling out conservative replication. 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 intermediate-density band after one replication round is the single most decisive piece of evidence distinguishing semi-conservative from conservative replication — conservative replication would have produced two separate, fully heavy and fully light bands instead.

   Marking points

   • States that bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, so all their DNA contained ¹⁵N.
   • States that the bacteria were then transferred to a medium containing only light nitrogen (¹⁴N), and DNA was extracted after one and two rounds of replication.
   • States that DNA extracted after one round of replication had an intermediate density (half ¹⁵N, half ¹⁴N), rather than either fully heavy or fully light.
   • States that this result supported semi-conservative replication, in which each new DNA molecule consists of one original (parent) strand and one newly synthesised strand, ruling out conservative replication.

   Examiner tip: The intermediate-density band after one replication round is the single most decisive piece of evidence distinguishing semi-conservative from conservative replication — conservative replication would have produced two separate, fully heavy and fully light bands instead.

8. 8.

   Marking analysis: A learner attempts the following task: “Describe the Meselson-Stahl experiment, and state what its results showed about the mechanism of DNA replication.” Their response addresses only this point: “States that bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, so all their DNA contained ¹⁵N.” 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 bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, so all their DNA contained ¹⁵N. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   3. Requirement 2: Identifies the missing requirement: States that the bacteria were then transferred to a medium containing only light nitrogen (¹⁴N), and DNA was extracted after one and two rounds of replication. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   4. Requirement 3: Identifies the missing requirement: States that DNA extracted after one round of replication had an intermediate density (half ¹⁵N, half ¹⁴N), rather than either fully heavy or fully light. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
   5. Requirement 4: Identifies the missing requirement: States that this result supported semi-conservative replication, in which each new DNA molecule consists of one original (parent) strand and one newly synthesised strand, ruling out conservative replication. 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 bacteria were grown in a medium containing heavy nitrogen (¹⁵N) for several generations, so all their DNA contained ¹⁵N.
   • Identifies the missing requirement: States that the bacteria were then transferred to a medium containing only light nitrogen (¹⁴N), and DNA was extracted after one and two rounds of replication.
   • Identifies the missing requirement: States that DNA extracted after one round of replication had an intermediate density (half ¹⁵N, half ¹⁴N), rather than either fully heavy or fully light.
   • Identifies the missing requirement: States that this result supported semi-conservative replication, in which each new DNA molecule consists of one original (parent) strand and one newly synthesised strand, ruling out conservative replication.

   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 purpose of the polymerase chain reaction (PCR), and state the three main temperature-controlled steps involved in a single PCR cycle.

   [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 PCR is used to make many identical copies (amplify) of a specific, small sample of DNA very quickly. 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 the denaturation step: the DNA sample is heated to a high temperature (around 95°C) to separate the double strands. 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 the annealing step: the mixture is cooled to allow short primers to bind to complementary sequences on each separated strand. 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 the extension step: DNA polymerase, a heat-stable enzyme, builds new complementary strands from the primers, and the cycle is then repeated many times. 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: Each PCR cycle doubles the amount of target DNA, so the amount of DNA increases exponentially — around 30 cycles can produce over a billion copies from a single starting molecule.

   Marking points

   • States that PCR is used to make many identical copies (amplify) of a specific, small sample of DNA very quickly.
   • States the denaturation step: the DNA sample is heated to a high temperature (around 95°C) to separate the double strands.
   • States the annealing step: the mixture is cooled to allow short primers to bind to complementary sequences on each separated strand.
   • States the extension step: DNA polymerase, a heat-stable enzyme, builds new complementary strands from the primers, and the cycle is then repeated many times.

   Examiner tip: Each PCR cycle doubles the amount of target DNA, so the amount of DNA increases exponentially — around 30 cycles can produce over a billion copies from a single starting molecule.

10. 10.

    Marking analysis: A learner attempts the following task: “Outline the purpose of the polymerase chain reaction (PCR), and state the three main temperature-controlled steps involved in a single PCR cycle.” Their response addresses only this point: “States that PCR is used to make many identical copies (amplify) of a specific, small sample of DNA very quickly.” 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 PCR is used to make many identical copies (amplify) of a specific, small sample of DNA very quickly. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States the denaturation step: the DNA sample is heated to a high temperature (around 95°C) to separate the double strands. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States the annealing step: the mixture is cooled to allow short primers to bind to complementary sequences on each separated strand. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States the extension step: DNA polymerase, a heat-stable enzyme, builds new complementary strands from the primers, and the cycle is then repeated many times. 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 PCR is used to make many identical copies (amplify) of a specific, small sample of DNA very quickly.
    • Identifies the missing requirement: States the denaturation step: the DNA sample is heated to a high temperature (around 95°C) to separate the double strands.
    • Identifies the missing requirement: States the annealing step: the mixture is cooled to allow short primers to bind to complementary sequences on each separated strand.
    • Identifies the missing requirement: States the extension step: DNA polymerase, a heat-stable enzyme, builds new complementary strands from the primers, and the cycle is then repeated many times.

    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.

    Describe how gel electrophoresis is used to separate fragments of DNA by size, and state one application of this technique.

    [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 DNA samples are placed into wells at one end of a gel, and an electric current is applied across the gel. 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 DNA fragments, being negatively charged due to their phosphate groups, migrate through the gel towards the positive electrode. 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 smaller DNA fragments move further (faster) through the gel than larger fragments in a given time, since they pass more easily through the pores of the gel, resulting in separation by size. 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 a valid application, e.g. DNA profiling for forensic investigations or paternity testing, or diagnosing genetic conditions. 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: Gel electrophoresis separates purely by size and charge, not by DNA sequence directly — distinct DNA sequences are compared by the pattern of fragment sizes they produce after being cut by specific enzymes.

    Marking points

    • States that DNA samples are placed into wells at one end of a gel, and an electric current is applied across the gel.
    • States that DNA fragments, being negatively charged due to their phosphate groups, migrate through the gel towards the positive electrode.
    • States that smaller DNA fragments move further (faster) through the gel than larger fragments in a given time, since they pass more easily through the pores of the gel, resulting in separation by size.
    • States a valid application, e.g. DNA profiling for forensic investigations or paternity testing, or diagnosing genetic conditions.

    Examiner tip: Gel electrophoresis separates purely by size and charge, not by DNA sequence directly — distinct DNA sequences are compared by the pattern of fragment sizes they produce after being cut by specific enzymes.

12. 12.

    Marking analysis: A learner attempts the following task: “Describe how gel electrophoresis is used to separate fragments of DNA by size, and state one application of this technique.” Their response addresses only this point: “States that DNA samples are placed into wells at one end of a gel, and an electric current is applied across the gel.” 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 DNA samples are placed into wells at one end of a gel, and an electric current is applied across the gel. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that DNA fragments, being negatively charged due to their phosphate groups, migrate through the gel towards the positive electrode. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that smaller DNA fragments move further (faster) through the gel than larger fragments in a given time, since they pass more easily through the pores of the gel, resulting in separation by size. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States a valid application, e.g. DNA profiling for forensic investigations or paternity testing, or diagnosing genetic conditions. 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 DNA samples are placed into wells at one end of a gel, and an electric current is applied across the gel.
    • Identifies the missing requirement: States that DNA fragments, being negatively charged due to their phosphate groups, migrate through the gel towards the positive electrode.
    • Identifies the missing requirement: States that smaller DNA fragments move further (faster) through the gel than larger fragments in a given time, since they pass more easily through the pores of the gel, resulting in separation by size.
    • Identifies the missing requirement: States a valid application, e.g. DNA profiling for forensic investigations or paternity testing, or diagnosing genetic conditions.

    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.

    Explain how polyploidy can lead to sympatric speciation in plants, without any geographic separation being required.

    [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 polyploidy is a condition in which an organism has more than two complete sets of chromosomes, e.g. arising from an error during meiosis or from the fusion of unreduced gametes. 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 a polyploid individual is usually unable to produce fertile offspring by breeding with the original (non-polyploid) diploid population, since their chromosomes cannot pair correctly during meiosis. 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 polyploid individual can still reproduce with other polyploid individuals or by self-fertilisation, so a new, reproductively isolated species can arise within the same geographic area as the original population, in a single generation. 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: Sympatric speciation via polyploidy is unusual in that reproductive isolation can arise instantly, in a single generation, rather than gradually accumulating over many generations as in allopatric speciation.

    Marking points

    • States that polyploidy is a condition in which an organism has more than two complete sets of chromosomes, e.g. arising from an error during meiosis or from the fusion of unreduced gametes.
    • States that a polyploid individual is usually unable to produce fertile offspring by breeding with the original (non-polyploid) diploid population, since their chromosomes cannot pair correctly during meiosis.
    • States that the polyploid individual can still reproduce with other polyploid individuals or by self-fertilisation, so a new, reproductively isolated species can arise within the same geographic area as the original population, in a single generation.

    Examiner tip: Sympatric speciation via polyploidy is unusual in that reproductive isolation can arise instantly, in a single generation, rather than gradually accumulating over many generations as in allopatric speciation.

14. 14.

    Marking analysis: A learner attempts the following task: “Explain how polyploidy can lead to sympatric speciation in plants, without any geographic separation being required.” Their response addresses only this point: “States that polyploidy is a condition in which an organism has more than two complete sets of chromosomes, e.g. arising from an error during meiosis or from the fusion of unreduced gametes.” 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 polyploidy is a condition in which an organism has more than two complete sets of chromosomes, e.g. arising from an error during meiosis or from the fusion of unreduced gametes. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a polyploid individual is usually unable to produce fertile offspring by breeding with the original (non-polyploid) diploid population, since their chromosomes cannot pair correctly during meiosis. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that the polyploid individual can still reproduce with other polyploid individuals or by self-fertilisation, so a new, reproductively isolated species can arise within the same geographic area as the original population, in a single generation. 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 polyploidy is a condition in which an organism has more than two complete sets of chromosomes, e.g. arising from an error during meiosis or from the fusion of unreduced gametes.
    • Identifies the missing requirement: States that a polyploid individual is usually unable to produce fertile offspring by breeding with the original (non-polyploid) diploid population, since their chromosomes cannot pair correctly during meiosis.
    • Identifies the missing requirement: States that the polyploid individual can still reproduce with other polyploid individuals or by self-fertilisation, so a new, reproductively isolated species can arise within the same geographic area as the original population, in a single generation.

    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.

    Distinguish between prezygotic and postzygotic reproductive isolating mechanisms, giving one example of each.

    [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 prezygotic isolating mechanism prevents mating or fertilisation from occurring between members of different species in the first place, e.g. differences in mating behaviour/courtship rituals, or differences in breeding season or habitat. 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 a postzygotic isolating mechanism operates after fertilisation has occurred, e.g. resulting in hybrid offspring that are infertile (such as a mule) or have reduced viability/fitness. 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 both types of mechanism prevent gene flow between the two populations, allowing them to continue diverging as separate species. 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: Prezygotic mechanisms stop a zygote from ever forming; postzygotic mechanisms allow fertilisation but prevent the resulting hybrid from successfully passing on genes — both achieve the same outcome of reproductive isolation by different means.

    Marking points

    • States that a prezygotic isolating mechanism prevents mating or fertilisation from occurring between members of different species in the first place, e.g. differences in mating behaviour/courtship rituals, or differences in breeding season or habitat.
    • States that a postzygotic isolating mechanism operates after fertilisation has occurred, e.g. resulting in hybrid offspring that are infertile (such as a mule) or have reduced viability/fitness.
    • States that both types of mechanism prevent gene flow between the two populations, allowing them to continue diverging as separate species.

    Examiner tip: Prezygotic mechanisms stop a zygote from ever forming; postzygotic mechanisms allow fertilisation but prevent the resulting hybrid from successfully passing on genes — both achieve the same outcome of reproductive isolation by different means.

16. 16.

    Marking analysis: A learner attempts the following task: “Distinguish between prezygotic and postzygotic reproductive isolating mechanisms, giving one example of each.” Their response addresses only this point: “States that a prezygotic isolating mechanism prevents mating or fertilisation from occurring between members of different species in the first place, e.g. differences in mating behaviour/courtship rituals, or differences in breeding season or habitat.” 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 prezygotic isolating mechanism prevents mating or fertilisation from occurring between members of different species in the first place, e.g. differences in mating behaviour/courtship rituals, or differences in breeding season or habitat. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a postzygotic isolating mechanism operates after fertilisation has occurred, e.g. resulting in hybrid offspring that are infertile (such as a mule) or have reduced viability/fitness. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that both types of mechanism prevent gene flow between the two populations, allowing them to continue diverging as separate species. 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 prezygotic isolating mechanism prevents mating or fertilisation from occurring between members of different species in the first place, e.g. differences in mating behaviour/courtship rituals, or differences in breeding season or habitat.
    • Identifies the missing requirement: States that a postzygotic isolating mechanism operates after fertilisation has occurred, e.g. resulting in hybrid offspring that are infertile (such as a mule) or have reduced viability/fitness.
    • Identifies the missing requirement: States that both types of mechanism prevent gene flow between the two populations, allowing them to continue diverging as separate species.

    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.

    Distinguish between homologous structures and analogous structures, giving one example of each, and state what each provides evidence for.

    [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 homologous structures have a similar underlying structure but may have different functions, because the species share a common ancestor, e.g. the forelimb bones of a human, bat and whale. 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 homologous structures provide evidence for divergent evolution from a shared 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 analogous structures have a similar function but a different underlying structure, because the species do not share a recent common ancestor, e.g. the wings of a bird and the wings of an insect. 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 analogous structures provide evidence for convergent evolution, where unrelated species independently evolve similar adaptations to similar environmental pressures. 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: A quick test to distinguish the two: homologous structures often look structurally alike but are used differently, while analogous structures are used similarly but are built completely differently underneath.

    Marking points

    • States that homologous structures have a similar underlying structure but may have different functions, because the species share a common ancestor, e.g. the forelimb bones of a human, bat and whale.
    • States that homologous structures provide evidence for divergent evolution from a shared common ancestor.
    • States that analogous structures have a similar function but a different underlying structure, because the species do not share a recent common ancestor, e.g. the wings of a bird and the wings of an insect.
    • States that analogous structures provide evidence for convergent evolution, where unrelated species independently evolve similar adaptations to similar environmental pressures.

    Examiner tip: A quick test to distinguish the two: homologous structures often look structurally alike but are used differently, while analogous structures are used similarly but are built completely differently underneath.

18. 18.

    Marking analysis: A learner attempts the following task: “Distinguish between homologous structures and analogous structures, giving one example of each, and state what each provides evidence for.” Their response addresses only this point: “States that homologous structures have a similar underlying structure but may have different functions, because the species share a common ancestor, e.g. the forelimb bones of a human, bat and whale.” 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 homologous structures have a similar underlying structure but may have different functions, because the species share a common ancestor, e.g. the forelimb bones of a human, bat and whale. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that homologous structures provide evidence for divergent evolution from a shared common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that analogous structures have a similar function but a different underlying structure, because the species do not share a recent common ancestor, e.g. the wings of a bird and the wings of an insect. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: States that analogous structures provide evidence for convergent evolution, where unrelated species independently evolve similar adaptations to similar environmental pressures. 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 homologous structures have a similar underlying structure but may have different functions, because the species share a common ancestor, e.g. the forelimb bones of a human, bat and whale.
    • Identifies the missing requirement: States that homologous structures provide evidence for divergent evolution from a shared common ancestor.
    • Identifies the missing requirement: States that analogous structures have a similar function but a different underlying structure, because the species do not share a recent common ancestor, e.g. the wings of a bird and the wings of an insect.
    • Identifies the missing requirement: States that analogous structures provide evidence for convergent evolution, where unrelated species independently evolve similar adaptations to similar environmental pressures.

    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, using the example of Darwin's finches on the Galápagos Islands, what is meant by adaptive radiation.

    [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 adaptive radiation is the diversification of a single ancestral species into multiple new species, each adapted to a different ecological niche, over a relatively short evolutionary time period. 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 a single ancestral finch species is thought to have colonised the Galápagos Islands and, in the absence of competing species, diversified. 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 natural selection favoured different beak shapes and sizes suited to the different available food sources on different islands, e.g. seeds, insects or cactus, producing the many distinct finch species observed today. 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: Adaptive radiation is essentially rapid speciation driven by the availability of many empty ecological niches with little competition — new islands or newly vacated habitats are classic settings for it.

    Marking points

    • States that adaptive radiation is the diversification of a single ancestral species into multiple new species, each adapted to a different ecological niche, over a relatively short evolutionary time period.
    • States that a single ancestral finch species is thought to have colonised the Galápagos Islands and, in the absence of competing species, diversified.
    • States that natural selection favoured different beak shapes and sizes suited to the different available food sources on different islands, e.g. seeds, insects or cactus, producing the many distinct finch species observed today.

    Examiner tip: Adaptive radiation is essentially rapid speciation driven by the availability of many empty ecological niches with little competition — new islands or newly vacated habitats are classic settings for it.

20. 20.

    Marking analysis: A learner attempts the following task: “Outline, using the example of Darwin's finches on the Galápagos Islands, what is meant by adaptive radiation.” Their response addresses only this point: “States that adaptive radiation is the diversification of a single ancestral species into multiple new species, each adapted to a different ecological niche, over a relatively short evolutionary time period.” 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 adaptive radiation is the diversification of a single ancestral species into multiple new species, each adapted to a different ecological niche, over a relatively short evolutionary time period. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that a single ancestral finch species is thought to have colonised the Galápagos Islands and, in the absence of competing species, diversified. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that natural selection favoured different beak shapes and sizes suited to the different available food sources on different islands, e.g. seeds, insects or cactus, producing the many distinct finch species observed today. 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 adaptive radiation is the diversification of a single ancestral species into multiple new species, each adapted to a different ecological niche, over a relatively short evolutionary time period.
    • Identifies the missing requirement: States that a single ancestral finch species is thought to have colonised the Galápagos Islands and, in the absence of competing species, diversified.
    • Identifies the missing requirement: States that natural selection favoured different beak shapes and sizes suited to the different available food sources on different islands, e.g. seeds, insects or cactus, producing the many distinct finch species observed today.

    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.

    A cladogram shows species B and C sharing a more recent common ancestor with each other than either does with species A. State what this indicates about the evolutionary relationships between the three species, and state one type of evidence commonly used to construct cladograms.

    [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 B and C are more closely related to each other than either is to species A. 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 means B and C diverged from a common ancestor more recently than the point at which A diverged from the lineage leading to B and C. 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 valid type of evidence, e.g. comparing DNA or protein (amino acid) sequences between species, or comparing anatomical/morphological characteristics. 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: On a cladogram, the relatedness between species is read from the position of their most recent common branch point (node), not from how close together the species' names happen to be drawn.

    Marking points

    • States that species B and C are more closely related to each other than either is to species A.
    • States that this means B and C diverged from a common ancestor more recently than the point at which A diverged from the lineage leading to B and C.
    • States a valid type of evidence, e.g. comparing DNA or protein (amino acid) sequences between species, or comparing anatomical/morphological characteristics.

    Examiner tip: On a cladogram, the relatedness between species is read from the position of their most recent common branch point (node), not from how close together the species' names happen to be drawn.

22. 22.

    Marking analysis: A learner attempts the following task: “A cladogram shows species B and C sharing a more recent common ancestor with each other than either does with species A. State what this indicates about the evolutionary relationships between the three species, and state one type of evidence commonly used to construct cladograms.” Their response addresses only this point: “States that species B and C are more closely related to each other than either is to species A.” 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 B and C are more closely related to each other than either is to species A. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that this means B and C diverged from a common ancestor more recently than the point at which A diverged from the lineage leading to B and C. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States a valid type of evidence, e.g. comparing DNA or protein (amino acid) sequences between species, or comparing anatomical/morphological characteristics. 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 B and C are more closely related to each other than either is to species A.
    • Identifies the missing requirement: States that this means B and C diverged from a common ancestor more recently than the point at which A diverged from the lineage leading to B and C.
    • Identifies the missing requirement: States a valid type of evidence, e.g. comparing DNA or protein (amino acid) sequences between species, or comparing anatomical/morphological characteristics.

    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.

    Explain how comparing the DNA base sequences of a particular gene across different species can be used as evidence for their evolutionary relationships.

    [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 which are more closely related are expected to share a more 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: States that the fewer differences (mutations accumulated) there are between the DNA sequences of two species for the same gene, the more recently they are likely to have diverged from a 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 this molecular evidence can be used to construct, or support, a phylogenetic tree showing the evolutionary relationships between species. 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: Molecular evidence (DNA or protein sequence comparison) is generally considered more objective and quantifiable than comparing anatomical structures, since it can be measured as a precise number of sequence differences.

    Marking points

    • States that species which are more closely related are expected to share a more recent common ancestor.
    • States that the fewer differences (mutations accumulated) there are between the DNA sequences of two species for the same gene, the more recently they are likely to have diverged from a common ancestor.
    • States that this molecular evidence can be used to construct, or support, a phylogenetic tree showing the evolutionary relationships between species.

    Examiner tip: Molecular evidence (DNA or protein sequence comparison) is generally considered more objective and quantifiable than comparing anatomical structures, since it can be measured as a precise number of sequence differences.

24. 24.

    Marking analysis: A learner attempts the following task: “Explain how comparing the DNA base sequences of a particular gene across different species can be used as evidence for their evolutionary relationships.” Their response addresses only this point: “States that species which are more closely related are expected to share a more 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 which are more closely related are expected to share a more recent common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that the fewer differences (mutations accumulated) there are between the DNA sequences of two species for the same gene, the more recently they are likely to have diverged from a common ancestor. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that this molecular evidence can be used to construct, or support, a phylogenetic tree showing the evolutionary relationships between species. 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 which are more closely related are expected to share a more recent common ancestor.
    • Identifies the missing requirement: States that the fewer differences (mutations accumulated) there are between the DNA sequences of two species for the same gene, the more recently they are likely to have diverged from a common ancestor.
    • Identifies the missing requirement: States that this molecular evidence can be used to construct, or support, a phylogenetic tree showing the evolutionary relationships between species.

    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.

    Distinguish between the gradualism and punctuated equilibrium models of the pace of evolutionary change.

    [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 gradualism proposes that evolutionary change occurs slowly and steadily (continuously) over long periods of 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 that punctuated equilibrium proposes that species remain largely unchanged (in stasis) for long periods, interspersed with relatively short bursts of rapid evolutionary change, e.g. following a speciation event. 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 both models are supported by different patterns observed in the fossil record for different lineages/species. 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: Neither model is universally 'correct' — different lineages in the fossil record show evidence for either pattern, and both are considered valid, coexisting descriptions of how evolution can proceed.

    Marking points

    • States that gradualism proposes that evolutionary change occurs slowly and steadily (continuously) over long periods of time.
    • States that punctuated equilibrium proposes that species remain largely unchanged (in stasis) for long periods, interspersed with relatively short bursts of rapid evolutionary change, e.g. following a speciation event.
    • States that both models are supported by different patterns observed in the fossil record for different lineages/species.

    Examiner tip: Neither model is universally 'correct' — different lineages in the fossil record show evidence for either pattern, and both are considered valid, coexisting descriptions of how evolution can proceed.

26. 26.

    Marking analysis: A learner attempts the following task: “Distinguish between the gradualism and punctuated equilibrium models of the pace of evolutionary change.” Their response addresses only this point: “States that gradualism proposes that evolutionary change occurs slowly and steadily (continuously) over long periods of time.” 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 gradualism proposes that evolutionary change occurs slowly and steadily (continuously) over long periods of time. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that punctuated equilibrium proposes that species remain largely unchanged (in stasis) for long periods, interspersed with relatively short bursts of rapid evolutionary change, e.g. following a speciation event. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that both models are supported by different patterns observed in the fossil record for different lineages/species. 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 gradualism proposes that evolutionary change occurs slowly and steadily (continuously) over long periods of time.
    • Identifies the missing requirement: States that punctuated equilibrium proposes that species remain largely unchanged (in stasis) for long periods, interspersed with relatively short bursts of rapid evolutionary change, e.g. following a speciation event.
    • Identifies the missing requirement: States that both models are supported by different patterns observed in the fossil record for different lineages/species.

    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.

    In a population, the frequency of the recessive allele (q) for a particular gene is 0.2. Assuming the population is in Hardy-Weinberg equilibrium, calculate the frequency of the dominant allele (p), and the percentage of the population expected to be heterozygous.

    [4 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 that p + q = 1, so p = 1 − 0.2. 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: Obtains p = 0.8. 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: Uses heterozygote frequency = 2pq, substituting 2 × 0.8 × 0.2. Write the intermediate operation, keep the units consistent where applicable, and check the relation against the quantities given in the question.
    5. Develop this part of the answer: Obtains 0.32, i.e. 32% of the population is expected to be heterozygous. 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 Hardy-Weinberg equation p² + 2pq + q² = 1 gives the genotype frequencies directly: p² is homozygous dominant, 2pq is heterozygous, and q² is homozygous recessive.

    Marking points

    • States that p + q = 1, so p = 1 − 0.2.
    • Obtains p = 0.8.
    • Uses heterozygote frequency = 2pq, substituting 2 × 0.8 × 0.2.
    • Obtains 0.32, i.e. 32% of the population is expected to be heterozygous.

    Examiner tip: The Hardy-Weinberg equation p² + 2pq + q² = 1 gives the genotype frequencies directly: p² is homozygous dominant, 2pq is heterozygous, and q² is homozygous recessive.

28. 28.

    Marking analysis: A learner attempts the following task: “In a population, the frequency of the recessive allele (q) for a particular gene is 0.2. Assuming the population is in Hardy-Weinberg equilibrium, calculate the frequency of the dominant allele (p), and the percentage of the population expected to be heterozygous.” Their response addresses only this point: “States that p + q = 1, so p = 1 − 0.2.” Evaluate the response against the complete 4-mark task. Identify what earns credit and state every additional requirement needed for full marks.

    [4 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 p + q = 1, so p = 1 − 0.2. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Obtains p = 0.8. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Uses heterozygote frequency = 2pq, substituting 2 × 0.8 × 0.2. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    5. Requirement 4: Identifies the missing requirement: Obtains 0.32, i.e. 32% of the population is expected to be heterozygous. 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 p + q = 1, so p = 1 − 0.2.
    • Identifies the missing requirement: Obtains p = 0.8.
    • Identifies the missing requirement: Uses heterozygote frequency = 2pq, substituting 2 × 0.8 × 0.2.
    • Identifies the missing requirement: Obtains 0.32, i.e. 32% of the population is expected to be heterozygous.

    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.

    State three assumptions that must hold for a population to be in Hardy-Weinberg equilibrium.

    [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 no mutations are occurring in the population. 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 no migration of individuals into or out of the population is occurring (no gene flow). 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 mating is random, no natural selection is acting on the gene concerned, and the population size is very large (effectively infinite), to avoid genetic drift. 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: Hardy-Weinberg equilibrium is a theoretical baseline (a population where allele frequencies do not change) — in reality, at least one of these assumptions is almost always violated to some degree, which is exactly why real populations evolve.

    Marking points

    • States that no mutations are occurring in the population.
    • States that no migration of individuals into or out of the population is occurring (no gene flow).
    • States that mating is random, no natural selection is acting on the gene concerned, and the population size is very large (effectively infinite), to avoid genetic drift.

    Examiner tip: Hardy-Weinberg equilibrium is a theoretical baseline (a population where allele frequencies do not change) — in reality, at least one of these assumptions is almost always violated to some degree, which is exactly why real populations evolve.

30. 30.

    Marking analysis: A learner attempts the following task: “State three assumptions that must hold for a population to be in Hardy-Weinberg equilibrium.” Their response addresses only this point: “States that no mutations are occurring in the population.” 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 no mutations are occurring in the population. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: States that no migration of individuals into or out of the population is occurring (no gene flow). Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: States that mating is random, no natural selection is acting on the gene concerned, and the population size is very large (effectively infinite), to avoid genetic drift. 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 no mutations are occurring in the population.
    • Identifies the missing requirement: States that no migration of individuals into or out of the population is occurring (no gene flow).
    • Identifies the missing requirement: States that mating is random, no natural selection is acting on the gene concerned, and the population size is very large (effectively infinite), to avoid genetic drift.

    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.

    A natural disaster drastically reduces a population of elephant seals to only 20 surviving individuals, after which the population recovers in number but shows unusually low genetic diversity compared to other seal populations that did not experience this event. Explain, using the term 'genetic bottleneck', why the surviving population's genetic diversity remains low even after its numbers recover.

    [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: Defines a genetic bottleneck as a sharp reduction in population size that eliminates most of the genetic variation that existed in the original, larger population, purely by chance rather than by selection. 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 the 20 surviving seals carry only a small, random subset of the alleles present in the original population, since many alleles were lost simply because the individuals carrying them did not survive the disaster. 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 even as the population grows back to a larger size, all later individuals descend from this small founding group, so the population's overall genetic diversity remains limited to whatever variation happened to survive in those 20 individuals. 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 genetic bottleneck reduces diversity through pure chance survival, not through any adaptive advantage — this distinguishes it clearly from natural selection, which removes alleles based on their effect on fitness.

    Marking points

    • Defines a genetic bottleneck as a sharp reduction in population size that eliminates most of the genetic variation that existed in the original, larger population, purely by chance rather than by selection.
    • Explains that the 20 surviving seals carry only a small, random subset of the alleles present in the original population, since many alleles were lost simply because the individuals carrying them did not survive the disaster.
    • Explains that even as the population grows back to a larger size, all later individuals descend from this small founding group, so the population's overall genetic diversity remains limited to whatever variation happened to survive in those 20 individuals.

    Examiner tip: A genetic bottleneck reduces diversity through pure chance survival, not through any adaptive advantage — this distinguishes it clearly from natural selection, which removes alleles based on their effect on fitness.

32. 32.

    Marking analysis: A learner attempts the following task: “A natural disaster drastically reduces a population of elephant seals to only 20 surviving individuals, after which the population recovers in number but shows unusually low genetic diversity compared to other seal populations that did not experience this event. Explain, using the term 'genetic bottleneck', why the surviving population's genetic diversity remains low even after its numbers recover.” Their response addresses only this point: “Defines a genetic bottleneck as a sharp reduction in population size that eliminates most of the genetic variation that existed in the original, larger population, purely by chance rather than by selection.” 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: Defines a genetic bottleneck as a sharp reduction in population size that eliminates most of the genetic variation that existed in the original, larger population, purely by chance rather than by selection. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Explains that the 20 surviving seals carry only a small, random subset of the alleles present in the original population, since many alleles were lost simply because the individuals carrying them did not survive the disaster. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Explains that even as the population grows back to a larger size, all later individuals descend from this small founding group, so the population's overall genetic diversity remains limited to whatever variation happened to survive in those 20 individuals. 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: Defines a genetic bottleneck as a sharp reduction in population size that eliminates most of the genetic variation that existed in the original, larger population, purely by chance rather than by selection.
    • Identifies the missing requirement: Explains that the 20 surviving seals carry only a small, random subset of the alleles present in the original population, since many alleles were lost simply because the individuals carrying them did not survive the disaster.
    • Identifies the missing requirement: Explains that even as the population grows back to a larger size, all later individuals descend from this small founding group, so the population's overall genetic diversity remains limited to whatever variation happened to survive in those 20 individuals.

    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.

    Dolphins (mammals) and sharks (fish) both have streamlined, torpedo-shaped bodies and fins, despite belonging to very different evolutionary lineages. Explain this similarity using the concept of convergent evolution, and distinguish it from the homologous similarity seen in the forelimbs of mammals.

    [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: Defines convergent evolution as the independent evolution of similar structures or features in unrelated lineages, as a result of adapting to similar environmental pressures or ways of life, rather than from shared ancestry. 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 dolphins and sharks independently evolved streamlined bodies and fins because both live in water and face the same selective pressure to minimize drag while swimming, even though their most recent common ancestor was not aquatic or streamlined. 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: Distinguishes this from homologous structures, which arise from a shared ancestor and may now look superficially different and serve different functions (e.g. a bat's wing and a human's arm), despite an underlying structural similarity inherited from that common ancestor. 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: The key test for convergent versus homologous similarity is the underlying cause: convergent features look alike because of similar function/environment despite different ancestry, while homologous features share an internal structural blueprint because of common ancestry, even when their final appearance and function differ.

    Marking points

    • Defines convergent evolution as the independent evolution of similar structures or features in unrelated lineages, as a result of adapting to similar environmental pressures or ways of life, rather than from shared ancestry.
    • Explains that dolphins and sharks independently evolved streamlined bodies and fins because both live in water and face the same selective pressure to minimize drag while swimming, even though their most recent common ancestor was not aquatic or streamlined.
    • Distinguishes this from homologous structures, which arise from a shared ancestor and may now look superficially different and serve different functions (e.g. a bat's wing and a human's arm), despite an underlying structural similarity inherited from that common ancestor.

    Examiner tip: The key test for convergent versus homologous similarity is the underlying cause: convergent features look alike because of similar function/environment despite different ancestry, while homologous features share an internal structural blueprint because of common ancestry, even when their final appearance and function differ.

34. 34.

    Marking analysis: A learner attempts the following task: “Dolphins (mammals) and sharks (fish) both have streamlined, torpedo-shaped bodies and fins, despite belonging to very different evolutionary lineages. Explain this similarity using the concept of convergent evolution, and distinguish it from the homologous similarity seen in the forelimbs of mammals.” Their response addresses only this point: “Defines convergent evolution as the independent evolution of similar structures or features in unrelated lineages, as a result of adapting to similar environmental pressures or ways of life, rather than from shared ancestry.” 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: Defines convergent evolution as the independent evolution of similar structures or features in unrelated lineages, as a result of adapting to similar environmental pressures or ways of life, rather than from shared ancestry. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    3. Requirement 2: Identifies the missing requirement: Explains that dolphins and sharks independently evolved streamlined bodies and fins because both live in water and face the same selective pressure to minimize drag while swimming, even though their most recent common ancestor was not aquatic or streamlined. Compare this requirement with the supplied learner response; missing evidence cannot earn credit.
    4. Requirement 3: Identifies the missing requirement: Distinguishes this from homologous structures, which arise from a shared ancestor and may now look superficially different and serve different functions (e.g. a bat's wing and a human's arm), despite an underlying structural similarity inherited from that common ancestor. 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: Defines convergent evolution as the independent evolution of similar structures or features in unrelated lineages, as a result of adapting to similar environmental pressures or ways of life, rather than from shared ancestry.
    • Identifies the missing requirement: Explains that dolphins and sharks independently evolved streamlined bodies and fins because both live in water and face the same selective pressure to minimize drag while swimming, even though their most recent common ancestor was not aquatic or streamlined.
    • Identifies the missing requirement: Distinguishes this from homologous structures, which arise from a shared ancestor and may now look superficially different and serve different functions (e.g. a bat's wing and a human's arm), despite an underlying structural similarity inherited from that 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.

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