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Theme A: Unity and diversity (HL) About this practice HL-only depth on nucleic acid structure and the mechanisms and evidence behind speciation.
Biology HL Theme A: Unity and diversity (HL)
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1 Distinguish between the structure of DNA and RNA in terms of their sugar, bases and number of strands. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 3 Outline how natural selection can lead to allopatric speciation. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 5 Outline the evidence from selective breeding (artificial selection) that supports the theory of evolution by natural selection. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 7 Describe the Meselson-Stahl experiment, and state what its results showed about the mechanism of DNA replication. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 9 Outline the purpose of the polymerase chain reaction (PCR), and state the three main temperature-controlled steps involved in a single PCR cycle. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 11 Describe how gel electrophoresis is used to separate fragments of DNA by size, and state one application of this technique. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 13 Explain how polyploidy can lead to sympatric speciation in plants, without any geographic separation being required. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 15 Distinguish between prezygotic and postzygotic reproductive isolating mechanisms, giving one example of each. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 17 Distinguish between homologous structures and analogous structures, giving one example of each, and state what each provides evidence for. Medium 4 marks No calculator + 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. Marking analysis Medium 4 marks No calculator + 19 Outline, using the example of Darwin's finches on the Galápagos Islands, what is meant by adaptive radiation. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 23 Explain how comparing the DNA base sequences of a particular gene across different species can be used as evidence for their evolutionary relationships. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 25 Distinguish between the gradualism and punctuated equilibrium models of the pace of evolutionary change. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Medium 4 marks Calculator + 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. Marking analysis Medium 4 marks Calculator + 29 State three assumptions that must hold for a population to be in Hardy-Weinberg equilibrium. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + 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. Easy 3 marks No calculator + 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. Marking analysis Easy 3 marks No calculator + Self-assessed Not marked yet
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