Topic 18: Variation and Selection -- Challenge Exam 2
1 hour 15 minutes
80
7
75:00
0610
Instructions
Answer all questions in the spaces provided.
Show all working for calculations.
Use appropriate scientific terminology.
Your answers will be automatically graded when you submit.
Question Navigation
This paper covers the whole of Topic 18. Like a real Cambridge paper it ranges across every sub-topic — 18.1 variation and mutation, 18.2 adaptive features including hydrophytes, 18.3 natural selection, and 18.4 selective breeding and how artificial selection differs from natural selection — and it mixes them inside single questions. All three Topic 18 papers do; they differ in the angle they come at it from, not in what they cover.
Question 1 — Two Shapes of Difference
Total: 12 marks
Fig. 1.1 shows the results of two surveys carried out by a student. Chart A shows the heights of the 64 students in one school year group. Chart B shows the ABO blood groups of 1000 people living in the same town. Read the values off the axes carefully before you start — several parts of this question need a figure from the charts.
(a)[3]
Name the type of variation shown by Chart A and the type shown by Chart B. Using one figure from Chart B, give the evidence for your answer to Chart B.
Model Answer — 1(a)
Chart A shows continuous variation — a range of phenotypes between two extremes, here from the 145–150 cm class to the 175–180 cm class, with every class in between occupied [1]
Chart B shows discontinuous variation [1]
evidence: there are only four phenotypes (O 44%, A 34%, B 15%, AB 7%) and no intermediates — nobody falls between group B and group AB, and the four percentages add up to 100% with nothing left over [1]
⚠ If you missed marks here: Do not identify the type of variation from the shape of the bars alone — a chart can be drawn with gaps for tidiness. The mark is for the biology: are intermediates possible? A student could be 162.4 cm, so height is continuous. Nobody is halfway between blood group A and blood group B, so ABO is discontinuous. Also, “no intermediates” is the exact phrase Cambridge marks; “only a few options” is vaguer and often does not score.
(b)[3]
State the modal class of Chart A and the number of students in it. Then explain, in terms of what causes each pattern, why height varies in the way Chart A shows but blood group does not.
Model Answer — 1(b)
modal class = 160–165 cm, containing 19 students (the tallest bar; read it against the axis, where one small division is one student) [1]
height is caused by genes and the environment together — several genes contribute, and factors such as diet during childhood alter the value an individual reaches, so every value in between is possible [1]
ABO blood group is caused by genes only — the environment does not alter it, so an individual falls into one group or another with nothing in between [1]
⚠ If you missed marks here: Two traps. First, the modal class is the class with the most individuals, not the middle class and not the mean — and you must quote it as a class (160–165 cm), not as a single number. Second, an enormous number of candidates write that continuous variation is caused by the environment and discontinuous by genes. That is half wrong: continuous variation is caused by genes AND the environment. Genes are involved in both.
(c)[3]
The student now wants to find out whether hand span in her year group also shows continuous variation. Describe how she should collect and present her data. In your answer include one variable she must keep the same and one way of making her results more reliable.
Model Answer — 1(c)
measure the hand span of a large sample of individuals of the same species — ideally every student in the year group, and at least 50, because a small sample gives an unreliable shape [1]
a variable kept the same: the same method and the same instrument every time — the same ruler, the hand flat with fingers fully spread, measured between the same two points (thumb tip to little-finger tip), and the same hand (say, the right) for everyone; also keep the age group the same, since hand span changes with age [1]
reliability and presentation: repeat each measurement and take a mean, then group the results into equal-width classes and plot a bar chart or histogram, so the shape can be compared with Chart A [1]
⚠ If you missed marks here: Paper 4 does test experimental skill, and “measure lots of hands” is not an answer. The examiner is looking for a named control variable and a reason attached to it. The single commonest omission is the class width: if you group the data into classes of different widths the chart is meaningless, and if you plot 60 individual values you cannot see a shape at all.
(d)[3]
One student in the survey is 149 cm tall. Both of her parents are taller than 175 cm. Explain how this is possible.
Model Answer — 1(d)
offspring are not genetically identical to their parents: meiosis produces genetically different gametes, and which gamete happens to fuse with which at fertilisation is random, so she inherits one particular combination of alleles out of very many possible ones [1]
her parents may each carry alleles associated with shorter height without showing that height themselves, and she may have inherited that combination [1]
height is also affected by the environment — diet, illness during childhood — so two people with similar alleles can end up at different heights [1]
⚠ If you missed marks here: “It is just variation” scores nothing — the question is asking where the variation came from. Cambridge lists four sources: mutation, meiosis, random mating and random fertilisation. Two of those four are doing the work here. And do not forget the environment: this is a continuously varying feature, so genes alone were never going to be the whole answer.
Question 2 — The Population That Was Never Persuaded
Total: 12 marks
A patient in hospital has a bacterial infection. Fig. 2.1 shows the same population of bacteria at three times. A is a sample taken before any treatment. B is a sample taken a few hours after an antibiotic was given; cells drawn with a cross through them are dead. C is a sample taken two days later, after the population had grown back to its original size. The shading shows two types of cell, labelled type P and type Q in the key. Count the cells before you answer.
(a)[3]
Using Fig. 2.1, state the number of type Q cells in sample A. Describe what happened to the type P cells between A and B. State what type Q cells must be.
Model Answer — 2(a)
sample A contains 2 type Q cells out of 30 cells in total (the other 28 are type P) [1]
between A and B all 28 type P cells were killed by the antibiotic — every type P cell in B is crossed through — while both type Q cells survived unharmed [1]
type Q cells must be resistant to the antibiotic [1]
⚠ If you missed marks here: Quote the numbers. “Some survived” is worth nothing when the figure lets you say 2 out of 30. The word trap is writing that type Q cells are immune: immunity is what a person’s white blood cells do to a pathogen (Topic 10). A bacterium that survives a drug is resistant, and those two words are not interchangeable in a mark scheme.
(b)[5]
Explain fully, in terms of natural selection, how the population changed from A to C. There are five marks and there are five steps: set them out in order.
Model Answer — 2(b)
1 — variation already existed. Before the antibiotic was given, 2 of the 30 cells already carried an allele for resistance, produced by a random mutation in the base sequence of the DNA. Sample A proves it: the resistant cells are there in the figure before any treatment [1]
2 — many offspring are produced. Bacteria divide roughly every 20 minutes, so the population is enormous and produces far more cells than the patient can support [1]
3 — struggle for survival. The antibiotic kills the sensitive cells, and the cells that remain compete for nutrients and space [1]
4 — the better adapted are more likely to survive and reproduce. In an environment containing the antibiotic, the resistant cells are better adapted, so they are the ones that live long enough to divide [1]
5 — the alleles are passed on. The survivors pass the resistance allele to their offspring, so the proportion of resistant cells in the population rises generation after generation, until by C all 30 cells are type Q [1]
⚠ If you missed marks here: Five marks means five separate ideas, and step 1 is the one people leave out — without it the answer reads as though the antibiotic caused the resistance. It did not. It killed everything that was not already resistant. The diagram below is the same three panels with the five steps written on. Notice what none of the steps says: nothing in the sequence claims the bacteria wanted, tried or needed anything, and nothing claims an individual cell changed during its lifetime. That is exactly the point, and it is where almost every lost mark in this topic goes.
(c)[2]
A student answered part (b) like this:
“The bacteria realised the antibiotic was there, so they changed themselves to become resistant. After a while the whole population had got used to the drug.”
Rewrite this answer as two correct sentences. Each sentence should correct one thing that is wrong with it.
Model Answer — 2(c)
Correcting the first error — the bacteria did not change in response to the drug: “A few bacteria already carried an allele for resistance, produced by a random mutation, before the antibiotic was ever given.” [1]
Correcting the second error — no individual gets used to anything: “The antibiotic killed the cells that were not resistant, so the resistant cells were the ones that reproduced, and the proportion of resistant cells in the population rose over many generations.” [1]
(Accept any wording that (i) puts the mutation and the variation before the antibiotic and (ii) moves the change from the individual to the population across generations.)
⚠ If you missed marks here: The cure for a sentence like the student’s is always the same and it is worth learning as a rule: change the subject of the sentence from the individual to the population, and put the variation before the selection. If your rewrite still has a bacterium as the thing that changes — “the bacterium developed resistance”, “it built up a tolerance” — you have not fixed it. And “survival of the fittest” is not a repair either; it is a phrase students reach for instead of explaining, and it earns nothing.
(d)[2]
In Topic 15 you met MRSA, a strain of bacterium resistant to several antibiotics. Suggest why a change of this kind appears in a bacterial population within a few years, when a comparable change in a population of a large mammal would take many thousands of years.
Model Answer — 2(d)
bacteria have an extremely short generation time — a cell divides about every 20 minutes, so hundreds of generations pass in a week, whereas a large mammal may manage one generation in several years [1]
bacterial populations are vast, so although a mutation is rare per cell division, it happens often somewhere in the population; and resistance genes are frequently carried on plasmids, the small circular loops of DNA in a bacterial cell you met in Topic 2 [1]
⚠ If you missed marks here: The answer is not that bacteria mutate more readily than mammals do — the mutation rate per division is low in both. What differs is the number of divisions per unit time and the size of the population. Selection needs generations to work through, and bacteria supply generations by the hundred while a mammal supplies one.
Question 3 — A Chemical That Stopped Working
Total: 12 marks
Insecticide X has been sprayed in one district every year since 2012 to control mosquitoes. Each year a health team catches mosquitoes from the district, exposes them to a standard dose of insecticide X for one hour, and records the percentage still alive afterwards. Table 3.1 shows the results.
Year
2012
2014
2016
2018
2020
2022
percentage of mosquitoes surviving one hour of exposure / %
3
5
12
34
71
88
Table 3.1
(a)[3]
Describe the change shown in Table 3.1, and calculate the mean rate of change in the percentage surviving between 2016 and 2022. Give your answer in % per year, to one decimal place, and show your working.
Model Answer — 3(a)
the percentage surviving increased throughout, from 3% in 2012 to 88% in 2022 [1]
the increase was slow at first (3% to 12% over the first four years) and then much faster (12% to 88% over the next six), so the rate of increase itself rose — although between 2020 and 2022 it begins to level off as the percentage approaches 100 [1]
(88 − 12) ÷ (2022 − 2016) = 76 ÷ 6 = 12.7 % per year
correct working and answer 12.7 % per year (accept 12.67) [1]
⚠ If you missed marks here: A “describe” mark on a table is never given for “it went up”. Quote the first value, the last value and the unit, and say something about the shape of the change. On the calculation, the commonest error is dividing by the number of readings (4) instead of the number of years (6) — the rows are two years apart, not one.
(b)[3]
Explain the change shown in Table 3.1.
Model Answer — 3(b)
there was genetic variation in the mosquito population before 2012: a small number of individuals carried an allele, arising from a random mutation, that made them resistant to insecticide X — the 3% figure in 2012 is that variation [1]
each spraying killed the mosquitoes without the allele, so the resistant individuals were the ones left to breed — the insecticide acted as the selection pressure [1]
the survivors passed the allele on to their offspring, so the proportion of the population carrying it rose in every generation, reaching 88% by 2022 [1]
⚠ If you missed marks here: This is the same argument as the bacteria in Question 2 wearing different clothes, and Cambridge does that deliberately — if you can only tell the story about bacteria, you have memorised an example rather than learned a process. The first mark is the one people drop: the resistant mosquitoes were already there in 2012, and the table shows them. Writing that the mosquitoes “built up a resistance to the spray” describes an individual changing and scores nothing.
(c)[3]
In 2020 the health service doubled the dose of insecticide X used in the district. The number of mosquito bites fell sharply for one season and then rose again. Explain this result, and suggest one change to the control programme that would slow the loss of effectiveness.
Model Answer — 3(c)
resistance is not all-or-nothing — there is variation in how resistant individuals are. The doubled dose killed the weakly resistant individuals as well, which is why the bites fell for one season, but a few of the most resistant individuals still survived it [1]
those few survivors were the only parents of the next generation, so the doubled dose was a stronger selection pressure and the proportion of highly resistant mosquitoes rose faster than before — the population recovered, made up of the most resistant individuals [1]
improvement: alternate two unrelated insecticides in different years so that resistance to one is not continuously selected for, and combine this with non-chemical control — treated bed nets, removing the standing water in which the larvae develop — which reduces the mosquito population without applying a chemical selection pressure at all [1]
⚠ If you missed marks here: The counter-intuitive point is that using more chemical makes the problem arrive sooner. A stronger selection pressure does not defeat selection; it sharpens it, because it narrows the survivors down to the most resistant individuals of all. This is the same reasoning behind the Topic 15 advice to use antibiotics only when they are essential and to finish the course.
(d)[3]
Insecticide X has been used on the same fields for the same 30 years against a beetle that damages the crop. No resistance has appeared in the beetle population. Suggest three reasons for this difference.
Model Answer — 3(d)
mutation is random, so the particular mutation that would give resistance to insecticide X may simply not have occurred in the beetle population. Selection can only act on variation that already exists — it cannot produce the feature to order [1]
the beetle has a much longer generation time than a mosquito, so far fewer generations have passed in 30 years and there has been much less opportunity for any advantageous allele to spread through the population [1]
the beetle population may be much smaller, so the total number of cell divisions in which a mutation could occur is far lower; or beetles from untreated fields nearby may keep breeding into the population and diluting any resistance [1]
⚠ If you missed marks here: This part is testing the single most important sentence in the whole topic: selection acts on variation, it does not create it. If the resistant beetle never existed, thirty years of spraying cannot conjure one up. Any answer along the lines of “the beetles have not adapted yet” or “they have not had time to change” is describing individuals changing themselves and scores nothing.
Question 4 — The Herd the Farmer Chose
Total: 12 marks
A farmer keeps a herd of dairy cattle. Over five generations she has kept records of the mean milk yield of the herd and of the number of different bulls used as fathers of each generation. Table 4.1 shows her records.
generation
1
2
3
4
5
mean milk yield per cow / kg
4200
4900
5600
6200
6700
number of different bulls used as fathers
12
8
5
3
2
Table 4.1
(a)[4]
Describe how the farmer produced the change in mean milk yield shown in Table 4.1.
Model Answer — 4(a)
1 — selection by humans. The farmer identified the individuals with the desirable feature: the cows with the highest milk yields, and bulls whose mothers or daughters had high yields (a bull produces no milk, so he is judged on his female relatives) [1]
2 — crossing. She bred these selected individuals together [1]
3 — selection of offspring. From their offspring she again selected those with the highest yields and used them as the parents of the next generation [1]
4 — repetition. She repeated this over many generations, and the mean yield of the herd rose each time, from 4200 kg to 6700 kg over the five generations shown [1]
⚠ If you missed marks here: Four marks, four steps — and the fourth is the one people leave out, because “over many generations” sounds like padding. It is not: one round of breeding changes almost nothing, and Cambridge marks the repetition explicitly. Notice too that the farmer is not adding anything to the cattle. She is choosing between differences that were already in the herd, exactly as the environment does in natural selection.
(b)[2]
Calculate the percentage increase in mean milk yield per cow between generation 1 and generation 5. Show your working and give your answer to one decimal place.
correct method: divide the increase by the original value and multiply by 100 [1]
answer 59.5 % (accept 59.52 %) [1]
⚠ If you missed marks here: The classic slip is dividing by the final value (2500 ÷ 6700 = 37.3%). Percentage change is always measured against where you started. The other slip is quoting 159.5% — that is the yield as a percentage of generation 1, not the percentage increase.
(c)[3]
The number of different bulls used as fathers fell from 12 to 2. Explain what effect this has on the herd, and why it could become a serious problem.
Model Answer — 4(c)
using fewer parents means fewer different alleles are passed on, so the herd has much less genetic variation — the animals become increasingly similar to one another [1]
if a new disease reaches the herd, it is far less likely that any individual carries an allele giving resistance, so a large part of the herd could be lost at once; the same applies if the climate or the feed changes [1]
further improvement also becomes slow or impossible, because selection can only choose between differences that exist, and there are fewer and fewer differences left to choose between — the yield increase per generation is already falling (700, 700, 600, 500 kg) [1]
⚠ If you missed marks here: Loss of genetic variation is the standard drawback of selective breeding, and it is worth seeing why: every generation you breed only from the winners, which means you throw away every allele carried only by the losers — including alleles that had nothing to do with milk and might have mattered later. The third mark is available in the numbers: look at the gaps between the yields and notice they are shrinking.
(d)[3]
State three differences between natural selection and artificial selection. Do not use speed as one of your three.
Model Answer — 4(d)
who or what does the selecting: in natural selection the environment does the selecting; in artificial selection humans do [1]
what is selected for: natural selection favours features that improve survival and reproduction in that environment; artificial selection favours the features humans find desirable, which may actually make the organism less likely to survive in the wild [1]
the result: natural selection leaves a population better adapted to its environment; artificial selection leaves a population with the chosen feature but usually with less genetic variation [1]
⚠ If you missed marks here: The question banned speed on purpose, because “artificial selection is faster” is the answer everybody gives and it is the least important difference. The difference that matters is who or what does the selecting. And be clear about what is the same in both: the variation arises by mutation in both, and selection acts on variation that already exists in both. Neither process creates the feature.
Question 5 — The Leaf That Lies On The Water
Total: 10 marks
Fig. 5.1 shows a hydrophyte — a plant rooted in the mud at the bottom of a pond, with its leaves floating flat on the surface of the water. On the right is a magnified cross section through part of one lamina. Four features are labelled W, X, Y and Z. Nothing has been named for you.
(a)[4]
Name the features labelled W, X, Y and Z on Fig. 5.1.
Model Answer — 5(a)
W — the lamina (the flat blade of the leaf), lying on the surface of the water [1]
X — a stoma in the upper epidermis (accept: stomata on the upper surface) [1]
Y — a large air space in the mesophyll [1]
Z — the lower epidermis, the surface in contact with the water, which carries no stomata [1]
⚠ If you missed marks here: The trap in this figure is X. In every leaf diagram you met in Topic 6 the stomata were drawn on the lower surface, and it is very easy to label the top strip “lower epidermis” out of habit. Look at where the water line is: the pores are on the surface that is in the air. “Air pocket” is not the term for Y either — write air space.
(b)[3]
Explain how the positions of the stomata and the presence of the air spaces are adaptive features of a plant that lives in water.
Model Answer — 5(b)
the stomata are in the upper epidermis only, which is the surface in contact with the air, allowing carbon dioxide to diffuse into the leaf for photosynthesis and oxygen to diffuse out [1]
a stoma in the lower epidermis would be under water, and very little gas is dissolved in water, so gas exchange through it would be extremely slow — the plant with stomata below gains nothing [1]
the large air spaces run through the lamina and the stalk: they hold a reserve of the gases used in photosynthesis and respiration, and they make the leaf buoyant, which keeps the lamina at the surface in the light [1]
⚠ If you missed marks here: A hydrophyte is not a xerophyte with the answers reversed. There is no water shortage here, so “to reduce water loss” earns nothing — that is the xerophyte’s problem, and it is why this plant has no thick cuticle. This plant’s difficulty is getting hold of gases, because water holds very little dissolved gas. Explain every feature against that difficulty and the marks fall out.
(c)[1]
Define the term adaptive feature.
Model Answer — 5(c)
an inherited feature that helps an organism to survive and reproduce in its environment [1]
(all three ideas are marked: inherited — survive — reproduce)
⚠ If you missed marks here: Two words are dropped constantly. Inherited — a feature acquired during life, such as a scar or a trained muscle, is not adaptive in this sense because it cannot be passed on. And reproduce — surviving is not enough; an organism that survives beautifully and leaves no offspring contributes nothing to the next generation.
(d)[2]
A student wrote: “These plants grew big air spaces because they needed to float, and over time they developed stomata on top instead.” Explain why this answer would score no marks, and give the correct account.
Model Answer — 5(d)
an individual plant does not develop a feature because the feature would be useful — these features are inherited, and nothing an individual does in its lifetime changes the alleles it passes on [1]
the correct account: variation existed first. Random mutation produced plants with larger air spaces and with stomata differently placed; those individuals were more likely to survive and reproduce in a pond, and they passed their alleles on, so over many generations the whole population came to show these features. That process, the population becoming more suited to its environment over many generations, is adaptation [1]
⚠ If you missed marks here: The student’s sentence is the giraffe-stretching-its-neck idea in a pond, and it is the single most expensive habit in this topic. The repair is mechanical: make the population the subject of the sentence, and put the variation before the selection. Watch also for the two-sided vocabulary trap — an adaptive feature is the feature itself, while adaptation is the process that produced it across generations. Cambridge marks them as different words.
Question 6 — Where New Alleles Come From
Total: 12 marks
A laboratory grew 200 separate cultures of a single-celled organism at each of five doses of ionising radiation. After ten days each culture was examined and the team recorded whether any cell in it showed a phenotype that had never been seen in that organism before. Table 6.1 shows the results.
dose of ionising radiation / arbitrary units
0
1
2
4
8
number of cultures (out of 200) in which a new phenotype appeared
2
5
11
23
44
Table 6.1
(a)[2]
Define the term gene mutation, and state why mutation is essential if natural selection is to happen at all.
Model Answer — 6(a)
a gene mutation is a random change in the base sequence of DNA [1]
mutation is the way new alleles are formed, so it is the original source of the genetic variation that natural selection acts on — without it a population would have nothing new to select between [1]
⚠ If you missed marks here: Say base sequence of DNA, not “a change in the DNA” and certainly not “a change in the chromosome”. And the word random is part of the definition, not decoration — it is the word that separates mutation from selection. Mutation is random; selection is not.
(b)[3]
Describe the relationship shown in Table 6.1. Explain what the result at a dose of zero tells the team.
Model Answer — 6(b)
as the dose of ionising radiation increases, the number of cultures showing a new phenotype increases, from 2 out of 200 at a dose of zero to 44 out of 200 at a dose of 8 [1]
each doubling of the dose roughly doubles the number of cultures affected (11 → 23 → 44), so the relationship is approximately proportional over this range [1]
2 cultures showed a new phenotype even with no radiation at all, so mutation occurs spontaneously at a low rate anyway. Ionising radiation increases the rate of mutation; it does not start the process (some chemicals do the same) [1]
⚠ If you missed marks here: The zero-dose row is the whole point of the table and it is the row candidates skip. A control that gives a result of 2 rather than 0 is telling you something biological, not showing an error. The syllabus phrase is that ionising radiation and some chemicals increase the rate of mutation — the word rate is doing the work, because it concedes that mutation was happening already.
(c)[3]
This organism can also reproduce sexually. Name three sources of genetic variation in a sexually reproducing population other than mutation.
Model Answer — 6(c)
meiosis — the reduction division that produces gametes gives genetically different gametes, so no two are alike [1]
random mating — which individual breeds with which is not fixed, so different combinations of parental alleles are brought together [1]
random fertilisation — which male gamete happens to fuse with which female gamete is a matter of chance, so the combination in the zygote is one of an enormous number [1]
⚠ If you missed marks here: Cambridge names exactly four sources — mutation, meiosis, random mating and random fertilisation — and the question removed one, so the other three are the answer. Learn them as a list. Note that only mutation makes a new allele; the other three shuffle alleles that already exist into new combinations, which is still variation and is still something selection can act on. “The environment” is not accepted here: the question says genetic variation.
(d)[4]
A student wrote: “Mutations are always harmful, and natural selection tries to remove them so that the species gets better and better over time.” Discuss four things that are wrong with this statement.
Model Answer — 6(d)
1 — mutations are not always harmful. Most have no effect at all, some are harmful and a few are advantageous; the resistance alleles in Questions 2 and 3 are mutations and they were extremely advantageous to the organism carrying them [1]
2 — natural selection does not try to do anything. It has no goal and no direction. It is simply the outcome of the fact that individuals with an advantageous feature are more likely to survive and reproduce than those without [1]
3 — a species does not get “better and better”. A population becomes better adapted to the environment it is in at the time. There is no absolute standard of better [1]
4 — whether an allele is harmful depends on the environment, and environments change. An allele that is a disadvantage today may be an advantage when the environment changes — which is exactly what happened when an antibiotic or an insecticide arrived. (Accept also: the change is seen in the population over many generations, never in one individual during its lifetime) [1]
⚠ If you missed marks here: The word to hunt for in your own writing is tries, along with its relatives: wants, decides, aims, is trying to improve. Every one of them turns natural selection into a person making choices, and an examiner strikes them out. The other half of the statement — that a species gets “better” — hides the same mistake, because better implies someone measuring against a target. There is no target. There is only an environment, and it changes.
Question 7 — A Field of Identical Plants
Total: 10 marks
A fruit crop is grown commercially on large plantations. Every plant on every plantation is grown from a cutting taken from one original plant that was produced long ago by selective breeding, so all of the plants are genetically identical. A wild relative of the same crop grows in nearby forest; it reproduces sexually and the plants differ noticeably from one another in height, leaf shape and fruit size. A fungal disease that attacks this crop has recently spread into the region.
(a)[2]
Explain why every plant on the plantation is genetically identical.
Model Answer — 7(a)
the plants were produced by asexual reproduction from a single parent, and the new cells were produced by mitosis, which gives genetically identical cells [1]
so no gametes and no fertilisation were involved: every plant carries the same alleles as the original parent, and there is no genetic variation between them apart from any new mutation [1]
⚠ If you missed marks here: Do not write that there is no variation at all. Genetically identical plants can still differ in size and yield because of the environment — soil, water, light. What they lack is genetic variation, and that is the word the mark scheme wants. Mutation can still occur in a clone; it is simply the only source of new alleles left.
(b)[4]
The fungal disease kills every plant it infects on the plantations, but in the forest some wild plants survive and the wild population recovers. Explain this difference.
Model Answer — 7(b)
genetic variation already exists in the wild population, because it reproduces sexually (meiosis, random mating and random fertilisation) and because of mutation [1]
so some individuals are likely to carry an allele giving resistance to the fungus. On the plantation every plant has the same alleles, so if one plant is susceptible then every plant is susceptible, and the disease takes all of them [1]
in the forest the resistant individuals are better adapted to an environment that now contains the fungus, so they are more likely to survive and reproduce than the rest [1]
they pass the resistance allele on to their offspring, so the proportion of resistant plants in the wild population rises over many generations and the population recovers [1]
⚠ If you missed marks here: This is Question 2 again with plants and a fungus instead of bacteria and a drug, and that is the examiner’s point: one process, any number of costumes. If you found this hard after answering Question 2 well, what you learned was an example rather than a mechanism. The distinctive mark here is the second one — you must say why the plantation has no survivors, not only why the forest does.
(c)[2]
Suggest two reasons why growers use genetically identical plants even though they know the risk.
Model Answer — 7(c)
every plant shows exactly the feature the grower selected — the same fruit size, flavour and ripening time — so the crop is uniform, can be harvested in one operation and meets what buyers expect [1]
the original plant had already been improved by selective breeding, and taking cuttings keeps that exact combination of alleles. Breeding it sexually would shuffle the alleles and the offspring would not all show the desirable feature [1]
(accept also: growth from cuttings is faster and more predictable than growing from seed)
⚠ If you missed marks here: A “suggest” question wants you to reason from what you have been given rather than recall a fact. The reasoning here is that the very thing which makes a clone dangerous — no genetic variation — is the same thing that makes it commercially attractive. Uniformity and vulnerability are two descriptions of one property.
(d)[2]
State whether the change taking place in the wild forest population is an example of natural selection or of artificial selection. Justify your answer.
Model Answer — 7(d)
natural selection [1]
because the selecting is done by the environment — here, the fungal disease — and not by humans. Nobody chose which forest plants would breed, and the feature being selected for is one that improves survival and reproduction in that environment, not one that a human found desirable [1]
⚠ If you missed marks here: The presence of humans somewhere in a question does not make it artificial selection. People planted the plantation, people spread the fungus around the world, people will lose the crop — and none of that matters. Ask one question only: who or what decided which individuals reproduced? In the forest, the fungus did. That is the environment, so it is natural selection.
Self-Assessment
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80
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A* : 56+
A : 48-55
B : 40-47
C : 32-39
D : 24-31
E : 16-23
U : <16
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