OxfordAQA A Level Biology: Diversity of Living Organisms — Practice Questions

These are original questions written for Marlbridge, in the style and at the standard of the examination. They are not reproduced past-paper questions — examination boards hold copyright in their own papers. Use these alongside the official past papers available free from your board.

Related: Diversity of Living Organisms revision notes

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Section A

1. State the taxonomic hierarchy from domain to species. [2]

2. Define the biological species concept and give one limitation of it. [3]

Section B

3. Explain how DNA and protein evidence has changed classification, giving two advantages over using observable features alone. [4]

4. A habitat contains 3 species with 40, 30 and 30 individuals respectively.

(a) Explain what an index of diversity measures beyond simple species richness. [3] (b) Calculate the index of diversity, d = N(N - 1) / Σn(n - 1), showing your working. [3] (c) Explain why a monoculture farmland has a much lower index of diversity than woodland, giving two reasons. [4]

5. A biologist studies two isolated populations of the same beetle species and wants to compare their genetic diversity.

(a) Describe two methods the biologist could use to measure genetic diversity within a population. [4] (b) Explain why comparing DNA base sequences gives a more objective measure of genetic diversity than comparing observable (phenotypic) characteristics. [3]

6. Explain what is meant by genetic diversity within a species and why low genetic diversity puts a population at risk. [4]

7. Two unrelated species of fish have independently evolved a very similar streamlined body shape for fast swimming.

(a) Name this phenomenon. [1] (b) Explain why this phenomenon makes classification based only on observable (physical) features potentially misleading, and how molecular evidence can resolve the problem. [2]

8. A student wants to investigate how plant species distribution changes across a rocky shore, from the top of the shore down to the sea. Explain why a transect would be more appropriate than randomly placed quadrats for this investigation. [2]

9. Explain two ways in which agricultural practices can reduce biodiversity. [2]

10. A mule (the offspring of a horse and a donkey) shares many features of both parent species. Explain why it is not classified as belonging to a species of its own, and state one way genetic diversity within a species can be measured. [3]

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Answers

1. Domain, kingdom, phylum, class, order, family, genus, species [2 — 1 mark if one is misplaced].

2. A species is a group of organisms that can interbreed to produce fertile offspring [1] [1]. Limitation: it cannot be applied to organisms that reproduce asexually, or to extinct species known only from fossils [1].

3. DNA base sequences and amino acid sequences can be compared directly and quantitatively, giving an objective measure of relatedness [1]. Organisms may look similar through convergent evolution while being unrelated, and DNA reveals this [1]. It shows how recently two species shared a common ancestor from the number of differences [1], and can distinguish species that are morphologically identical [1].

4. (a) It takes into account both the number of species (richness) and the evenness — how the individuals are distributed between them [1] [1]. A community dominated by one species has lower diversity than one with equal numbers, even with the same richness [1]. This index measures species diversity specifically — biodiversity can also be considered at the level of genetic diversity within a species, or diversity across different habitats. (b) N = 100, so N(N - 1) = 100 × 99 = 9900 [1]. Σn(n - 1) = (40 × 39) + (30 × 29) + (30 × 29) = 1560 + 870 + 870 = 3300 [1]. d = 9900 ÷ 3300 = 3.0 [1]. (c) Farmland grows one crop species over a large area, so richness is very low [1] and almost all individuals belong to that one species, so evenness is very low [1]. Hedgerows are removed and pesticides and herbicides kill other species [1], further reducing the number of habitats and niches available [1].

5. (a) Compare allele frequencies at specific gene loci between individuals in the population — the more different alleles present, and the more evenly their frequencies are distributed, the greater the genetic diversity [1] [1]; or compare DNA base sequences (or amino acid sequences in proteins) between individuals — greater variation in the sequences indicates greater genetic diversity [1] [1]. (Accept: the proportion of gene loci that are polymorphic.) (b) Phenotypic (observable) characteristics can be influenced by the environment as well as by genes, so genetically different individuals can look similar, or genetically similar individuals can look different [1]; DNA and protein sequences are a direct measure of genetic difference, unaffected by environmental factors [1], giving a more objective comparison between populations [1].

6. Genetic diversity is the number of different alleles present in the gene pool of a population [1]. If it is low, the population is less able to adapt to a change in the environment [1], because fewer individuals are likely to possess an advantageous allele [1], so a new disease or environmental change could kill the whole population — raising the risk of extinction [1].

7. (a) Convergent evolution [1]. (b) Convergent evolution means unrelated species can come to resemble each other because they have adapted to similar environments or ways of life, not because they share a recent common ancestor [1]; comparing DNA base sequences or amino acid sequences reveals the true evolutionary relationship, regardless of superficial physical resemblance [1].

8. Across a rocky shore there is a clear environmental gradient (e.g. changing exposure to air, wave action or salinity from top to bottom of the shore) [1], so the distribution being studied is not random — a transect samples systematically along this gradient, whereas randomly placed quadrats would not reveal how species composition changes with position [1].

9. Agricultural practices reduce biodiversity by removing natural habitats to create farmland, displacing the species that lived there [1], and by favouring monoculture — growing a single crop species over a large area — which supports far fewer species than a natural, varied habitat would [1].

10. A mule is sterile and cannot produce fertile offspring [1], so despite resulting from mating between a horse and a donkey it fails the biological species definition, which requires interbreeding to produce fertile offspring [1]. Genetic diversity within a species can be measured as the proportion of gene loci that are polymorphic within the population, or by comparing DNA base sequences or amino acid sequences between individuals [1].

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Where marks are usually lost

• Saying species interbreed to produce “offspring” — they must be fertile.
• Describing diversity index as species richness only.
• Comparing observable characteristics alone and treating that as an objective measure of genetic diversity, when the environment can also affect them.
• Confusing genetic diversity with species diversity.
• Writing binomial names without italics, or without capitalising the genus name.
• Saying quadrats should be “placed randomly by eye” rather than positioned using random coordinates — this still introduces bias.
• Assuming physical resemblance between species always indicates a recent common ancestor, rather than considering convergent evolution.