Variation, Evolution and Selection - Worksheets, Questions and Revision

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GCSE · Biology

B6b Variation, Evolution and Selection

AQA 8464 · Calculator allowed · about 95 minutes

Total Marks

Name: _______________________________ Date: ____ / ____ / ______

Answer ALL questions. Show all your working.

Survival of the Fittest Bacteria

Original text written for Revision Library.

Every time a course of antibiotics is prescribed by a GP surgery or an NHS hospital, it puts a bacterial population under intense selection pressure. Most bacteria in an infection are killed by the drug, but a tiny number may already carry a random mutation that happens to protect them, for example by breaking down the antibiotic molecule before it can act. These resistant bacteria survive, face far less competition for space and nutrients, and reproduce rapidly by binary fission, doubling in number every twenty minutes or so under favourable conditions. Because bacteria reproduce so quickly, resistant strains can become established in a population within days rather than the many generations natural selection usually takes in animals or plants. Meticillin-resistant Staphylococcus aureus (MRSA) is one well-known example that now causes serious problems in hospitals across the United Kingdom. Understanding this process, essentially natural selection acting on a very short timescale, helps explain why doctors are urged to prescribe antibiotics only when necessary, and why patients are always told to complete the full course.

Variation is the differences that exist between individuals of the same species. Some variation is caused by genes, some by the environment, and some by both.

(a)State what is meant by the term 'genetic variation'.(1)

(b)Give two sources of genetic variation in a population.(2)

(Total for Question 1 is 3 marks)

The Linnaean system classifies living organisms into a hierarchy of groups.

(a)Put the following classification groups in order, starting with the largest: Family, Genus, Kingdom, Species.(2)

(b)The scientific (binomial) name for the field poppy is Papaver rhoeas. State which part of this name represents the genus.(1)

(Total for Question 2 is 3 marks)

Priya and her identical twin sister Zainab are monozygotic twins. Priya took up competitive swimming aged 8 and has a noticeably more muscular build than Zainab, who does little regular exercise. Both sisters have the same eye colour and the same blood group.

(a)Explain why Priya and Zainab have the same eye colour and blood group, but different builds.(2)

(b)Suggest why identical twins are useful to scientists studying the effect of the environment on variation.(2)

(Total for Question 3 is 4 marks)

A student, Ffion, measured the heights of 100 Year 11 students and recorded whether each student had attached or free earlobes. Height showed continuous variation; earlobe attachment showed discontinuous variation.

(a)State what is meant by 'continuous variation'.(1)

(b)State what is meant by 'discontinuous variation'.(1)

(c)Ffion plotted the height data as a bar chart with 10 cm class intervals, producing a smooth, bell-shaped spread of bars. She plotted the earlobe data as a bar chart with only two bars: 'attached' and 'free'. Suggest why the shapes of these two graphs are different.(2)

(Total for Question 4 is 4 marks)

A dairy farmer near Penrith, in Cumbria, wants to breed a herd of cows that produce a higher yield of milk.

(a)Describe how the farmer could use selective breeding, over several generations, to produce a herd of high milk-yielding cows.(4)

(b)Name the process described in part (a).(1)

(Total for Question 5 is 5 marks)

The diagram below is a simplified phylogenetic tree showing the evolutionary relationships between five species of finch, labelled P, Q, R, S and T, based on comparisons of their DNA. P and Q share the most recent common ancestor; R branches from an earlier point on the tree; S and T share a separate, older common ancestor branching near the base of the tree.

Figure (to be drawn): Diagram: a branching phylogenetic tree with five tips labelled P, Q, R, S and T. P and Q share the most recent common ancestor (the branching node closest to the tips); R branches from an earlier node; S and T share a separate, older common ancestor branching near the base of the tree.

(a)Which two of the finch species shown are most closely related to each other?(1)

(b)Explain how the tree shows that these two species are the most closely related.(1)

(c)Identify two types of evidence that scientists could use to construct a phylogenetic tree such as this one.(2)

(Total for Question 6 is 4 marks)

A population of grasshoppers living in a meadow near Harrogate shows variation in body colour, ranging from bright green to dull brown. After a moorland wildfire, the ground nearby was covered in black ash for several years. Over the next 40 generations, the proportion of dark-brown grasshoppers in the population increased from 15% to 78%.

(a)Use the theory of natural selection to explain the change in the grasshopper population described above.(4)

(b)State the process by which new alleles, such as the allele for dark body colour, first arise in a population.(1)

(Total for Question 7 is 5 marks)

Describe three ways of reducing the rate at which bacterial populations become resistant to antibiotics.

(Total for Question 8 is 3 marks)

Ffion investigated the effect of three different antibiotic discs (A, B and C) on the growth of a culture of harmless bacteria, using aseptic technique. Each antibiotic disc was placed on an agar plate that had been evenly spread with the bacteria, then incubated at 25 degrees C for 48 hours. After incubation, a clear zone (no bacterial growth) had formed around each disc. Results: disc A, clear zone diameter 12 mm; disc B, clear zone diameter 26 mm; disc C, clear zone diameter 8 mm; control disc (no antibiotic), clear zone diameter 0 mm.

Figure (to be drawn): Diagram: a Petri dish (agar plate) evenly spread with bacteria, with three paper antibiotic discs (A, B, C) and one control disc placed on the surface, each surrounded by a clear zone of inhibition (diameters as given in the stem).

(a)State two precautions Ffion should take when handling the bacterial culture, in order to prevent contamination of the culture or to prevent harm to herself.(2)

(b)Explain why the plate was incubated at a maximum temperature of 25 degrees C, rather than 37 degrees C (human body temperature).(1)

(c)Calculate the area of the clear zone produced by antibiotic disc B. Use the equation: area of a circle = π x r². Use π = 3.14. Give your answer to 3 significant figures.(2)

(d)Ffion concluded that antibiotic disc B was the 'strongest' antibiotic because it produced the largest clear zone. Evaluate this conclusion.(2)

(e)Explain the purpose of including the control disc (no antibiotic) in this investigation.(1)

(f)Ffion repeated the investigation three times for each antibiotic disc and calculated a mean zone diameter. Suggest why this improves the reliability of her results.(2)

(g)Ffion wants to test whether increasing the concentration of antibiotic B increases the diameter of its clear zone. Describe one change to the method needed to test this, and identify the independent variable.(2)

(h)Explain why it was important that the agar plate was spread evenly with the same concentration of bacteria before the discs were added.(2)

(Total for Question 9 is 14 marks)

MRSA (meticillin-resistant Staphylococcus aureus) is a strain of bacteria that is resistant to many antibiotics, including meticillin. Use the theory of natural selection to explain how a population of bacteria can become resistant to an antibiotic such as meticillin.

(Total for Question 10 is 6 marks)

A hospital microbiology laboratory monitored the percentage of Staphylococcus aureus samples that were resistant to meticillin (MRSA) over four decades. Year 1990: 2% resistant. Year 2000: 15% resistant. Year 2010: 42% resistant. Year 2020: 68% resistant.

(a)Calculate the percentage increase in the proportion of meticillin-resistant samples between 1990 and 2020.(2)

(b)Calculate the mean rate of increase in resistance, in percentage points per decade, between 1990 and 2020.(1)

(c)Use the theory of natural selection to explain the trend shown in the data above.(3)

(Total for Question 11 is 6 marks)

A population of bacteria in a wound reproduces by binary fission, doubling in number every 20 minutes under favourable conditions. A student begins observing a population of 4 bacteria.

(a)Calculate the size of the bacterial population after 2 hours.(2)

(b)Calculate the minimum time taken for the population to first exceed 1000 bacteria.(2)

(c)Suggest why this rapid rate of reproduction contributes to bacterial populations evolving resistance to antibiotics relatively quickly, compared with organisms such as mammals.(1)

(Total for Question 12 is 5 marks)

Explain one disadvantage of selective breeding for the long-term health of a population of farm animals.

(Total for Question 13 is 3 marks)

Bacteria can be genetically engineered to produce useful human proteins, such as human growth hormone (HGH), which is used to treat children with a growth hormone deficiency. Describe how a bacterium can be genetically engineered to produce human growth hormone.

(Total for Question 14 is 5 marks)

Charles Darwin published his theory of evolution by natural selection in 1859. Before this, Jean-Baptiste Lamarck had proposed a different theory: that characteristics an organism developed or used during its lifetime, and lost through disuse, could be passed on directly to its offspring.

(a)Give two reasons why Darwin's theory of natural selection was not immediately accepted by other scientists at the time.(2)

(b)Explain, using an example, why Lamarck's theory of inheritance of acquired characteristics is not accepted today.(2)

(Total for Question 15 is 4 marks)

The diagram shows a cross-section through undisturbed layers of sedimentary rock exposed in a coastal cliff near Lyme Regis, in Dorset. Fossils of three different ammonite species, labelled X, Y and Z, were found in three different rock layers: X in the lowest (deepest) layer, Y in the middle layer, and Z in the uppermost (shallowest) layer.

Figure (to be drawn): Diagram: a cross-section of a cliff face showing three horizontal, undisturbed sedimentary rock layers stacked on top of one another; fossil X is embedded in the bottom layer, fossil Y in the middle layer, and fossil Z in the top layer.

(a)Assuming the rock layers have not been disturbed since they formed, state which fossil, X, Y or Z, is the oldest, and explain how you know.(2)

(b)Very few fossils exist of the earliest forms of life on Earth. Explain why.(2)

(Total for Question 16 is 4 marks)

Traditionally, living organisms were classified into five kingdoms. Following advances in biochemical evidence, in particular the analysis of a molecule found in every living cell, Carl Woese proposed a new three-domain system of classification.

(a)Name the three domains in Woese's system of classification.(1)

(b)Name the molecule whose sequence was analysed to provide the evidence for this new system of classification.(1)

(c)Explain why analysing rRNA gave scientists more reliable evidence for classifying organisms than using only their visible (observable) characteristics.(2)

(Total for Question 17 is 4 marks)

Scientists can use genetic engineering to insert a gene from one organism into the genome of a different organism. Golden Rice is a genetically engineered variety of rice that has been modified to produce β-carotene (which the body converts to vitamin A) in its grains. It has been proposed as a way of reducing vitamin A deficiency in parts of the world where rice is a staple food and vitamin A deficiency is common, but its introduction has been debated and delayed for many years. Evaluate the benefits and risks of using genetic engineering to produce crops such as Golden Rice.

(Total for Question 18 is 6 marks)

Mark scheme · B6b Variation, Evolution and Selection

Question 1

(a) B1 differences between individuals of a species caused by differences in their genes/alleles/DNA, oe
(a) Answer: Differences between individuals caused by differences in their genes (alleles).
(b) B1 mutation (creates new alleles)
(b) B1 meiosis (independent assortment of chromosomes) or sexual reproduction / random fusion of gametes from two parents, oe; any two of these sources for 2 marks
(b) Answer: Mutation, and meiosis (or sexual reproduction / the random fusion of gametes from two parents).

Question 2

(a) M1 Kingdom first and Species last
(a) A1 fully correct order: Kingdom, Family, Genus, Species
(a) Answer: Kingdom, Family, Genus, Species.
(b) B1 Papaver (the genus name is always given first and capitalised)
(b) Answer: Papaver.

Question 3

(a) B1 eye colour and blood group are controlled only by genes, and the twins have identical genes (identical DNA), so these characteristics are the same
(a) B1 build/muscle mass is affected by the environment (e.g. exercise and diet) as well as by genes, so it can differ between the twins even though their genotype is identical, oe
(a) Answer: Eye colour and blood group depend only on genes, which are identical in identical twins; build is also affected by environmental factors such as exercise, so it can differ.
(b) B1 identical twins have identical genes/DNA (the same genotype)
(b) B1 so any differences observed between them must be caused by environmental factors rather than genetic differences, allowing the effect of the environment to be studied/isolated, oe
(b) Answer: Because identical twins have identical genes, any differences between them must be due to the environment, allowing its effect to be studied in isolation.

Question 4

(a) B1 variation where individuals can take any value within a range, with no distinct/separate categories, oe
(a) Answer: Variation where measurements can take any value within a range, with no clear-cut categories.
(b) B1 variation that falls into a small number of distinct/discrete categories, with no intermediate values, oe
(b) Answer: Variation that falls into a small number of distinct categories, with no intermediate values.
(c) B1 height is controlled by many genes acting together (polygenic) and is also affected by the environment (e.g. diet), producing a continuous range of values / a bell-shaped spread
(c) B1 earlobe attachment is controlled by a single gene (with a dominant and a recessive allele), producing only a small number of discrete outcomes, oe
(c) Answer: Height's graph is a smooth spread because height is polygenic and affected by environment; the earlobe graph has two separate bars because earlobe shape is controlled by a single gene, giving distinct categories.

Question 5

(a) B1 select/choose the cow(s) and bull from the current herd with the highest milk yield
(a) B1 breed the selected high-yielding individuals together
(a) B1 select the offspring with the highest milk yield from this generation
(a) B1 repeat this process (selecting and breeding the best-yielding individuals) over many generations, until the desired milk yield is reached in most/all of the herd, oe
(a) Answer: Select the highest-yielding cows and bull, breed them together, select the highest-yielding offspring, then repeat this selection and breeding process over many generations.
(b) B1 selective breeding (artificial selection)
(b) Answer: Selective breeding (artificial selection).

Question 6

(a) B1 P and Q
(a) Answer: P and Q.
(b) B1 they share the most recent common ancestor; their branches diverge from the node closest to the present day (the tips of the tree), oe
(b) Answer: They share the most recent common ancestor, shown by the branching point closest to the tips of the tree.
(c) B1 comparing DNA/RNA base sequences (genomes) between species
(c) B1 any second valid type of evidence, e.g. comparing amino acid sequences of shared proteins, anatomical/structural similarities, or similarities in embryonic development between species, oe; any two for 2 marks
(c) Answer: For example, comparing DNA base sequences and comparing the amino acid sequences of shared proteins between the species.

Question 7

(a) B1 there was already variation in body colour within the grasshopper population before the fire
(a) B1 dark-brown grasshoppers were better camouflaged against the black ash, so were less likely to be eaten by predators (had a survival advantage)
(a) B1 grasshoppers that survived were more likely to reproduce, passing on the allele(s) for dark colouring to their offspring
(a) B1 over many generations, the allele for dark colouring became more common (increased in frequency) in the population, oe
(a) Answer: Variation in colour already existed; dark grasshoppers were camouflaged and survived predation better; they reproduced more and passed on the dark-colour allele; over generations the allele became more frequent in the population.
(b) B1 mutation
(b) Answer: Mutation.

Question 8

B1 only prescribe antibiotics for serious bacterial infections, not for viral infections such as colds or flu
B1 patients should always complete the full course of antibiotics as prescribed, rather than stopping early once they feel better
B1 avoid unnecessary use of antibiotics, e.g. in agriculture, and improve hygiene/infection control to reduce the spread of resistant strains; any three distinct measures for 3 marks, oe
Answer: For example: only prescribe antibiotics for bacterial (not viral) infections; patients complete the full course; improve hygiene and avoid unnecessary agricultural use to limit the spread of resistant strains.

Question 9

(a) B1 sterilise the inoculating loop/spreader before and after use (e.g. by passing it through a Bunsen burner flame)
(a) B1 any second valid precaution, e.g. work near a Bunsen burner flame to create an updraught, tape (do not fully seal) the lid of the Petri dish, or wash hands/disinfect surfaces before and after, oe; any two for 2 marks
(a) Answer: For example, sterilise the inoculating loop before and after use, and tape rather than fully seal the lid of the Petri dish.
(b) B1 to prevent/reduce the growth of pathogens that could be harmful to humans, which grow best near human body temperature, oe
(b) Answer: To reduce the growth of pathogens that could harm humans, which grow best at human body temperature.
(c) M1 radius = 26 / 2 = 13 (mm); correct substitution shown: 3.14 x 13²
(c) A1 531 mm² cao (3 s.f.)
(c) Answer: 531 mm² (3 s.f.)
(d) B1 a larger zone does suggest disc B was more effective against this species of bacteria, under these conditions
(d) B1 however, this conclusion is only valid if all three discs contained the same concentration of antibiotic and diffused into the agar at a similar rate; if the concentrations differed, the comparison would not be fair, oe
(d) Answer: A larger zone does suggest greater effectiveness, but the comparison is only valid if all discs contained the same concentration of antibiotic and diffused at similar rates.
(e) B1 to show that any clear zones around the antibiotic discs were caused by the antibiotic, and not by another factor such as the paper disc itself or the general growth conditions, oe
(e) Answer: It shows that any clear zones on the other discs are caused by the antibiotic itself, not by some other factor.
(f) B1 repeating reduces the effect of anomalous results/random error
(f) B1 a mean is more representative of the true effect of the antibiotic, increasing confidence that the pattern observed is a genuine effect rather than due to chance, oe
(f) Answer: Repeats reduce the effect of random error or anomalies; the mean is more representative of the antibiotic's true effect, increasing reliability.
(g) B1 use discs soaked in a range of different concentrations of antibiotic B, keeping all other variables the same (e.g. same bacteria species, temperature and incubation time)
(g) B1 independent variable = concentration of antibiotic (on the disc)
(g) Answer: Test discs soaked in a range of concentrations of antibiotic B, keeping all other variables constant; the independent variable is the concentration of antibiotic.
(h) B1 so that any difference in the size of the clear zones is due to the effectiveness of the antibiotics (a fair test)
(h) B1 rather than being due to a difference in the starting number/concentration of bacteria on different parts of the plate, oe
(h) Answer: So any difference in zone size is due to antibiotic effectiveness (a fair test), rather than an uneven starting concentration of bacteria on the plate.

Question 10

B1 within a bacterial population there is variation; some bacteria already carry a mutation/allele giving resistance to the antibiotic, before the antibiotic is ever used
B1 when the antibiotic is used, non-resistant bacteria are killed
B1 resistant bacteria survive (are not killed) and face less competition for resources such as nutrients and space
B1 resistant bacteria reproduce (by binary fission), passing the resistance allele/gene on to their offspring
B1 because bacteria reproduce rapidly, the resistant allele/population increases in frequency quickly, over successive generations
B1 eventually most/all of the population becomes resistant to the antibiotic, oe; award any 6 of the marking points, max 6
Answer: Existing variation means some bacteria already carry a resistance mutation; the antibiotic kills non-resistant bacteria; resistant bacteria survive with less competition and reproduce, passing on the resistance allele; because bacteria reproduce so quickly, the resistant allele spreads through the population over successive generations until most bacteria are resistant.

Question 11

(a) M1 correct substitution shown: (68 - 2) / 2 x 100
(a) A1 3300% cao
(a) Answer: 3300%
(b) B1 22 (percentage points per decade) cao; ecf allowed from a correctly divided total increase carried forward from (a)
(b) Answer: 22 percentage points per decade.
(c) B1 mutations occurring in the bacterial population meant some individual bacteria already carried a gene/allele giving resistance to meticillin, even before it was widely used
(c) B1 when meticillin was used, non-resistant bacteria were killed, while resistant bacteria survived and faced less competition for resources
(c) B1 resistant bacteria reproduced (by binary fission), passing on the resistance allele to their offspring, so the proportion of resistant bacteria in the population increased over each decade, oe
(c) Answer: Some bacteria already carried a resistance mutation before meticillin was used; non-resistant bacteria were killed by the antibiotic while resistant ones survived with less competition; resistant bacteria reproduced and passed on the resistance allele, so the proportion of resistant bacteria increased each decade.

Question 12

(a) M1 number of divisions = 120 / 20 = 6 (or 2⁶ identified)
(a) A1 256 cao
(a) Answer: 256 bacteria.
(b) M1 correct number of doublings identified = 8 (population = 4 x 2⁸ = 1024, the first value to exceed 1000); ecf from a correct doubling calculation carried forward from (a)
(b) A1 160 minutes cao (accept 2 hours 40 minutes)
(b) Answer: 160 minutes (2 hours 40 minutes).
(c) B1 bacteria reproduce (and produce new generations, including any new mutations) far more quickly and frequently than slower-reproducing organisms, so natural selection can act, and resistant strains can become established, over a much shorter timescale, oe
(c) Answer: Because bacteria produce new generations, and therefore new mutations, far more quickly than slower-reproducing organisms, so natural selection can act over a much shorter timescale.

Question 13

B1 selective breeding reduces the genetic variation within the population, because only individuals with the desired characteristic(s) are bred from
B1 this can lead to inbreeding, increasing the chance that individuals inherit two copies of a harmful recessive allele (inbreeding depression)
B1 reduced genetic variation also makes the population less able to adapt to a new environmental change, or to resist a new disease, which could threaten the whole population, oe
Answer: Selective breeding reduces genetic variation, which can cause inbreeding depression (harmful recessive alleles becoming more common) and leaves the population less able to adapt to new diseases or environmental change.

Question 14

B1 the gene for human growth hormone is identified/isolated from human DNA, often using a restriction enzyme to cut the DNA at specific points either side of the gene
B1 a plasmid (a small circular piece of bacterial DNA) is removed from a bacterium and cut open using the same restriction enzyme, leaving matching/complementary ends
B1 the growth hormone gene is inserted into the cut plasmid using an enzyme called ligase, joining the two pieces of DNA together to form recombinant DNA
B1 the recombinant plasmid (acting as a vector) is inserted into a new bacterial cell
B1 the genetically modified bacteria are grown/cultured in large numbers (e.g. in fermenters); as they reproduce, they express the inserted gene, producing human growth hormone, which is then extracted and purified, oe
Answer: The HGH gene is cut from human DNA and a bacterial plasmid is cut open, both using the same restriction enzyme; the gene is inserted into the plasmid using ligase to form recombinant DNA; the recombinant plasmid (vector) is inserted into a bacterium, which is then cultured in large numbers to express and produce human growth hormone.

Question 15

(a) B1 the theory challenged the widely held religious belief that God had created all species and that species did not change
(a) B1 there was not enough evidence available at the time to fully convince other scientists, as the mechanism of inheritance (genes/DNA) was not yet known/understood; or the theory contradicted Lamarck's well-established theory of inheritance of acquired characteristics; any two for 2 marks, oe
(a) Answer: It challenged widely held religious beliefs about the fixed creation of species, and there was not yet enough scientific evidence (the mechanism of inheritance was unknown) to fully convince other scientists.
(b) B1 characteristics acquired during an organism's lifetime are not caused by a change to the genes/alleles in its sex cells (gametes), so they cannot be passed on to offspring through inheritance
(b) B1 example, e.g. a giraffe that stretches its neck to reach higher leaves does not produce offspring with longer necks, because stretching does not change the DNA/genes passed on in its gametes, oe (accept other valid example)
(b) Answer: Characteristics gained during a lifetime do not change the alleles in an organism's gametes, so they cannot be inherited; for example, a giraffe stretching its neck does not change the genes passed on to its offspring.

Question 16

(a) B1 X is the oldest
(a) B1 because, in undisturbed sedimentary rock, the lowest/deepest layers were deposited first (formed longest ago), so fossils found in the lowest layer must be the oldest, oe
(a) Answer: X is the oldest, because in undisturbed rock the lowest layers formed first, so contain the oldest fossils.
(b) B1 many early life forms were soft-bodied and had no hard parts (such as bone, shell or teeth), so they decayed away completely and left few or no traces to be preserved as fossils
(b) B1 many of the rocks that did contain early fossils have since been destroyed by geological activity (e.g. heat, pressure, erosion) over the vast amount of time that has passed, oe
(b) Answer: Most early life was soft-bodied, so it decayed without leaving traces; also, much of the rock that did contain early fossils has since been destroyed by geological processes.

Question 17

(a) B1 Archaea, Bacteria and Eukaryota (all three needed for the mark), oe
(a) Answer: Archaea, Bacteria and Eukaryota.
(b) B1 ribosomal RNA (rRNA)
(b) Answer: Ribosomal RNA (rRNA).
(c) B1 organisms can look similar, or different, due to factors such as convergent evolution or the environment affecting appearance, so this does not necessarily reflect how closely related they actually are
(c) B1 rRNA (molecular) evidence directly reflects genetic/evolutionary relatedness between organisms, and can reveal similarities or differences that are not visible from appearance alone, oe
(c) Answer: Appearance can be misleading, as similar features can evolve independently or be affected by environment, whereas rRNA sequences directly reflect genetic relatedness, revealing relationships not visible from appearance alone.