Topic 18: Variation and Selection -- Challenge Exam 3
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.
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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 xerophytes and hydrophytes, 18.3 natural selection, and 18.4 selective breeding and how it 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 Characteristics, One Population
Total: 12 marks
A biologist caught, measured and released 240 wild rabbits living on one small island. She recorded two characteristics of every rabbit: its body mass, and the pattern of its coat. Her results are in Table 1.1. Every rabbit on the island belongs to the same species.
body mass
coat pattern
mass / kg
number of rabbits
pattern
number of rabbits
1.00 – 1.19
8
plain
132
1.20 – 1.39
27
white blaze on forehead
76
1.40 – 1.59
61
white spots on back
32
1.60 – 1.79
78
no rabbit showed more than one of these patterns, and no rabbit was in between two of them
1.80 – 1.99
49
2.00 – 2.19
17
Table 1.1
(a)[3]
Name the type of variation shown by each of the two characteristics in Table 1.1. Use the data to justify each answer.
Model Answer — 1(a)
body mass shows continuous variation [1]
because it gives a range of phenotypes between two extremes — the masses run all the way from 1.00 kg to 2.19 kg with rabbits in every group in between, so intermediate values exist [1]
coat pattern shows discontinuous variation, because there is a limited number of phenotypes — only three — with no intermediates: 132 + 76 + 32 = 240, and the table states that no rabbit fell between two patterns [1]
⚠ If you missed marks here: Two words carry these marks and neither is optional. Cambridge wants “a range of phenotypes between two extremes” for continuous and “a limited number of phenotypes with no intermediates” for discontinuous. “Body mass is continuous because it can be anything” is too loose. The other common loss is not using the data at all — the question says use the data, so quote the range 1.00–2.19 kg and the three categories.
(b)[3]
Explain what causes each of these two patterns of variation. Include one specific factor that could make two rabbits with the same alleles for body mass end up with different masses.
Model Answer — 1(b)
continuous variation such as body mass is caused by genes and the environment together [1]
discontinuous variation such as coat pattern is usually caused by genes only [1]
a specific environmental factor: the amount of food available in the part of the island a rabbit feeds on — also acceptable: disease, parasite load, or temperature. A rabbit with plenty of grass reaches a greater mass than a genetically identical rabbit on poor grazing [1]
⚠ If you missed marks here: The single commonest error in the whole of 18.1 is writing that continuous variation is caused by the environment. It is caused by genes AND the environment — drop the genes and you drop the mark. Also note the word usually in front of “genes only” for discontinuous variation; Cambridge uses it and so should you. Naming “the environment” as your specific factor earns nothing — name an actual factor.
(c)[3]
Ten years later a rabbit appears on the island with a coat pattern that has never been recorded there before. Explain, in terms of DNA, how a new coat pattern can appear in a population.
Model Answer — 1(c)
a mutation is a genetic change [1]
a gene mutation is a random change in the base sequence of DNA [1]
mutation is the way new alleles are formed; the new allele is a new version of the coat-pattern gene, and if the mutation occurs in a cell that forms a gamete it can be inherited by the offspring and shown in their phenotype [1]
⚠ If you missed marks here: “A mutation is a change in a gene” is not enough at Extended level — the mark scheme wants a random change in the base sequence of DNA, and the word random is doing real work: nothing about the island caused this particular change. The second frequent loss is forgetting the sentence that makes mutation matter at all: it is the source of new alleles. Without new alleles there is nothing new for selection to act on, anywhere in this topic.
(d)[3]
The rabbits on the island reproduce sexually. Other than mutation, state three sources of genetic variation in this population and explain briefly how each one produces variation.
Model Answer — 1(d)
meiosis — the division that produces the gametes is a reduction division giving genetically different haploid cells, so no two gametes from one rabbit carry the same combination of alleles [1]
random mating — which male mates with which female is not fixed, so different combinations of alleles are brought together in each mating [1]
random fertilisation — which sperm happens to fuse with which egg cell is a matter of chance, so the genotype of each zygote is one of an enormous number of possibilities [1]
⚠ If you missed marks here: Cambridge lists exactly four sources: mutation, meiosis, random mating and random fertilisation. The question removed the first, so the other three are the answer — learn the list as a list. Watch the difference between the last two: random mating is about which two parents pair up; random fertilisation is about which two gametes fuse. Writing “sexual reproduction” once, on its own, is one idea, not three.
Question 2 — Three Snapshots of One Population
Total: 12 marks
Fig. 2.1 shows one population of a species of bacterium living in a patient, drawn at three times. Panel A is the population before the patient was given an antibiotic. Panel B is the same population two days into the course of antibiotic; cells drawn with a cross through them are dead. Panel C is the population a week later. The two shadings are two types of cell, labelled type P and type Q in the key. Thirty cells are drawn in each panel.
(a)[3]
Describe what Fig. 2.1 shows, using the numbers of cells in each panel. State which of the two types, P or Q, is resistant to the antibiotic, and how the figure tells you.
Model Answer — 2(a)
in A there are 28 type P and 2 type Q — type Q is already present, but it is a small minority, 2 out of 30 [1]
in B all 28 type P cells are dead and both type Q cells are alive; in C all 30 cells are type Q [1]
type Q is resistant, because type Q is the type that survives while the antibiotic is present; the proportion of the population that is resistant rises from 2 out of 30 (about 7%) to 30 out of 30 (100%) [1]
⚠ If you missed marks here: Do not skip panel A. The whole point of this figure is the 2 resistant cells that are there before any antibiotic is given, and a description that starts at panel B has thrown away the evidence the rest of the question depends on. Second trap: saying “the bacteria became type Q”. No cell changed type. The type P cells died and the type Q cells reproduced — those are different events.
(b)[5]
Explain, in terms of natural selection, how this population came to be made up entirely of resistant bacteria. There is one mark for each of the five steps, so set your answer out as five clear stages in the correct order.
Model Answer — 2(b)
1 — genetic variation already exists in the population. A few bacteria carry an allele for resistance, formed earlier by a random mutation. The allele is present before the antibiotic is used — panel A proves it [1]
2 — many offspring are produced. Bacteria divide roughly every 20 minutes, so the population is enormous and far more cells are produced than can survive [1]
3 — there is a struggle for survival. The antibiotic kills the sensitive cells, and the cells compete for resources such as nutrients and space [1]
4 — the individuals better adapted to that environment have a greater chance of reproduction. The resistant cells survive the antibiotic and so are more likely to reproduce than the sensitive cells were [1]
5 — the survivors pass on their alleles to the next generation, so the proportion of the population carrying the resistance allele increases with every generation until, in panel C, all of them carry it [1]
⚠ If you missed marks here: Almost every lost mark on a five-step question is step 1. If your answer begins at the antibiotic, the examiner reads it as “the antibiotic produced the resistance” and you lose the first mark and often the fourth as well. Start with the variation that was already there. Two phrases to strike out: “the strongest survive” (the word is better adapted) and “the bacteria became resistant so they could survive” (that sentence gives the bacteria a purpose and scores nothing). You met resistance in Topic 15, where you were simply told that the antibiotic does not create it — these five steps are the reason why.
(c)[2]
A newspaper reports: “The antibiotic caused the bacteria to become resistant.” Use Fig. 2.1 to explain why this statement is wrong.
Model Answer — 2(c)
the resistance allele was already present before the antibiotic was used — panel A shows 2 resistant cells in a population that had never met the antibiotic. It arose by random mutation, and no cell changed in response to the drug [1]
the antibiotic did not alter any bacterium: it killed the sensitive cells and so removed the competition, leaving the resistant cells free to reproduce. The antibiotic selects among variation that already existed; it does not create that variation [1]
⚠ If you missed marks here: The distinction is selects versus creates, and it is worth learning as those two words. Another version of the same error is “the bacteria built up a tolerance” or “they got used to it” — a bacterium cannot get used to anything; it either carries the allele or it does not. And note who is resistant: the bacterium is resistant to the antibiotic. The patient is not.
(d)[2]
A bacterium can divide about every 20 minutes. A rat produces a new generation about every three months. Explain why a change like the one in Fig. 2.1 takes days in bacteria but many years in rats.
Model Answer — 2(d)
the proportion of a population carrying an allele can only change from one generation to the next, so what matters is the number of generations, not the number of days. A bacterium passes through about 72 generations in a day; a rat passes through four in a year [1]
the bacterial population is also vastly larger, and every division is an opportunity for a mutation, so new alleles — including resistance alleles — appear far more often in the population [1]
⚠ If you missed marks here: “Because bacteria are faster” restates the question. Convert the times into generations — that is the currency natural selection is paid in, and it is the sentence the examiner is looking for. Do not write that bacteria mutate more readily than rats do; the point is that there are enormously more of them dividing enormously more often, so more mutations occur in the population as a whole.
Question 3 — The Rats of Welshpool
Total: 12 marks
Warfarin is a chemical used to control brown rats. It stops the rat’s blood from clotting, and a rat that eats it dies of internal bleeding. From 1958 one farming district put warfarin bait out continuously. Every four years a sample of rats was trapped and tested. Table 3.1 shows the percentage of the rats in each sample that survived a dose of warfarin large enough to kill an ordinary rat.
Later work showed that a rat carrying the resistance allele must take in about twenty times as much vitamin K in its food as a rat without it, because vitamin K is needed for blood clotting. Warfarin baiting in the district was stopped in 1978. When the rats were tested again in 1990, 34% were resistant.
year
1958
1962
1966
1970
1974
1978
percentage of rats resistant to warfarin
1
6
19
42
61
68
Table 3.1
(a)[2]
Describe the change in the percentage of resistant rats between 1958 and 1978. Use figures from Table 3.1.
Model Answer — 3(a)
the percentage increases, from 1% in 1958 to 68% in 1978 — a rise of 67 percentage points [1]
the rise is not steady: it is slow at first (1% to 6% in the first four years), fastest between 1966 and 1974 (19% to 61%, a rise of 42 points in eight years), and then begins to level off after 1974 (61% to 68%) [1]
⚠ If you missed marks here: “It goes up” is one third of an answer. A describe with figures question wants the start value, the end value and the shape of the change — and here the shape is the interesting part, because the levelling off tells you that most of the population already carries the allele, so there is little room left for the percentage to rise. Say “percentage points”, not “percent”, for the size of the rise.
(b)[5]
Explain how the change shown in Table 3.1 happened. One mark is available for each of the five steps of natural selection, so set your answer out as five numbered stages in the correct order and apply each one to these rats.
Model Answer — 3(b)
1 — genetic variation already existed in the rat population. A small number of rats carried an allele, formed by a random mutation, that made them resistant to warfarin. In 1958 that was 1% of them, and the allele was there before the baiting started [1]
2 — many offspring are produced. Rats breed several times a year and produce far more young than the district can support [1]
3 — there is a struggle for survival. The rats compete for resources such as food and shelter, and the warfarin bait kills the non-resistant rats that eat it [1]
4 — the better adapted individuals have a greater chance of reproduction. In a district full of warfarin, the resistant rats are more likely to survive to breeding age and so more likely to reproduce than non-resistant rats [1]
5 — the survivors pass on their alleles. The resistant rats pass the resistance allele to their offspring, so the proportion of the population carrying it rises generation after generation — from 1% to 68% in twenty years [1]
⚠ If you missed marks here: Check your step 5. The commonest half-answer stops at “the resistant rats survived” and never says the alleles were passed on, which is the whole mechanism — survival on its own changes nothing across generations. Check step 1 too: if you wrote that the rats became resistant after eating the bait, you have written the giraffe-stretching-its-neck idea, and the 1% in 1958 in your own data table contradicts it.
(c)[3]
A student was asked to explain Table 3.1 and wrote:
“The rats knew the poison was there, so they got used to it. Over time each rat became resistant in order to survive, and then it passed this on to its babies.”
Identify two things that are biologically wrong with this answer, then rewrite it as one or two sentences that would score marks.
Model Answer — 3(c)
Error 1 — an individual rat does not become resistant during its life. A rat has the alleles it was born with and cannot change them. Resistance is inherited; the allele arose by mutation before the warfarin was put out, and a rat either carries it or does not [1]
Error 2 — “knew”, “got used to it” and “in order to survive” give the rats an intention. Nothing in natural selection has a purpose. The warfarin does not cause the change; it selects among variation that already existed. (Accept also: it is the population that changes over generations, not the individual) [1]
Corrected version — for example: “A few rats already carried an allele for warfarin resistance, produced by a random mutation. When warfarin was put out, those rats were more likely to survive and reproduce, and they passed the allele on to their offspring, so the proportion of resistant rats in the population increased over many generations.” The rewrite must put the variation first, keep the population as the thing that changes, and include passing on the alleles [1]
⚠ If you missed marks here: This part is worth reading twice, because the student’s sentence is the exact answer thousands of candidates write every year. The cure is mechanical: change the subject of the sentence from the individual to the population, and put the variation before the selection. If your rewrite still contains “so that they could”, “needed to”, “tried to” or “learned to”, it has the same fault as the original and scores nothing. Note that the student’s last clause — “passed this on to its babies” — is the only part that is not wrong.
(d)[2]
Baiting stopped in 1978, when 68% of the rats were resistant. By 1990 only 34% were. Use the information about vitamin K to explain this fall.
Model Answer — 3(d)
once the warfarin was removed, resistance was no longer an advantage, and the very large vitamin K requirement became a disadvantage: a resistant rat that cannot obtain twenty times the normal amount of vitamin K is less likely to survive [1]
so in the new environment the non-resistant rats are the better adapted ones; they are more likely to survive and reproduce and pass on their alleles, and the proportion of resistant rats falls over the following generations, from 68% to 34% [1]
⚠ If you missed marks here: This part is the reason the whole topic insists on the phrase better adapted rather than “stronger” or “fitter”. The identical allele is an advantage in one environment and a disadvantage in another, and nothing about the rat changed — the environment did. A common wrong answer is “the resistant rats died out because there was no more poison to make them resistant”, which reverses cause and effect. Another is to say the resistance allele disappeared: it did not; a third of the population still carries it.
Question 4 — Two Leaves, Two Problems
Total: 12 marks
Fig. 4.1 and Fig. 4.2 show leaves from two plants growing in very different places. Fig. 4.1 is a cross section through the leaf of a plant that grows on a bare, sunny sand dune. Fig. 4.2 shows a plant rooted in the mud at the bottom of a pond, with a magnified cross section through one of its leaves. Neither figure has been named for you.
(a)[4]
Name the features labelled A, B and C on Fig. 4.1, and state the type of environment this plant is adapted to.
Model Answer — 4(a)
A — a thick waxy cuticle on the outer surface [1]
B — a sunken stoma (a stoma lying in a pit) [1]
C — hairs projecting from the surface into the enclosed space [1]
this plant is a xerophyte: it is adapted to very dry conditions, where water is scarce [1]
⚠ If you missed marks here: B loses more marks than anything else in this question, because “stoma” alone is not the feature — every leaf has stomata. The adaptive feature is that it is sunken, in a pit. Similarly A is not “cuticle” but a thick waxy one. And check the spelling Cambridge marks: one stoma, several stomata.
(b)[3]
The leaf in Fig. 4.1 is rolled, as shown at D, and its stomata are sunken. Explain how these two features together result in less water vapour being lost from the leaf.
Model Answer — 4(b)
rolling encloses the stomata inside the leaf, where the air is still, instead of exposing them to the moving air outside; the hairs hold that air still as well [1]
because the air is trapped, water vapour accumulates in the pits and in the enclosed space, so the water vapour concentration just outside each stoma stays high [1]
that makes the concentration gradient of water vapour between the inside of the leaf and the air outside smaller, so less water vapour diffuses out through the stomata and the rate of transpiration is lower [1]
⚠ If you missed marks here: There is a three-mark chain here and most answers stop after one link: “the stomata are covered so less water escapes”. Push through to the concentration gradient — that is the physics of Topic 8.3 doing the work, and it is where the third mark lives. A phrasing trap: do not write that the leaf rolls in order to save water. Say what the rolled shape does to the air and the gradient, and the mark follows.
(c)[3]
Fig. 4.2 shows a plant with a very different problem. Using the letters X, Y and Z, describe three features of this plant and explain the advantage of each one in its environment.
Model Answer — 4(c)
X — the stomata are in the upper surface only. The lower surface is under water, where a stoma could exchange almost no gas, so gas exchange takes place through the surface that is in contact with the air [1]
Y — large air spaces run through the lamina and the stalk. They hold the gases used in photosynthesis and respiration, and they make the leaf buoyant, so the lamina stays at the surface in the light [1]
Z — the lower surface is in contact with the water and has no stomata and no thick cuticle: losing water is not a problem for a plant surrounded by it, and a thin surface allows dissolved substances through [1]
(this plant is a hydrophyte — a plant adapted to living in water. W shows that the lamina floats at the surface, which is why the light and the air are available at all)
⚠ If you missed marks here: X is the feature examiners ask for most often and the one candidates get backwards, because in an ordinary land plant the stomata are mostly on the lower surface. Here that arrangement is reversed, and the reason is simply that the lower surface is under water. On Y, “air spaces so it can float” is only half the answer — they also hold the gases for photosynthesis and respiration. Do not describe the hydrophyte as the “opposite” of the xerophyte and stop there; describe the actual features.
(d)[2]
Define the term adaptive feature. Then explain why the sentence “the dune plant developed a rolled leaf because it needed to save water” would score no marks.
Model Answer — 4(d)
an adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment [1]
the sentence is wrong because no plant develops a feature because it needs it. The variation arose first, by random mutation, before it was of any use to anything; the plants that happened to have rolled leaves lost less water, survived and reproduced more, and passed the alleles on, so over many generations the population became more suited to the dry dune — that process is called adaptation [1]
⚠ If you missed marks here: The definition has three parts and candidates routinely drop the first: inherited. A feature you acquire — a scar, a well-developed muscle — is not adaptive in this sense, because it is not passed on. And note the two words Cambridge pairs: survive and reproduce. Surviving without reproducing changes nothing in the next generation. Two words separate the definition from the process: an adaptive feature is a thing an organism has; adaptation is the process a population goes through.
Question 5 — Counting the Accidents
Total: 10 marks
A fungus that normally grows as a dark green mould was used to investigate how new phenotypes arise. Five batches of one million spores were taken from the same parent culture. Batch 1 was left untreated. Batches 2, 3 and 4 were exposed to different doses of ionising radiation. Batch 5 was soaked in chemical M. Every batch was then spread on the same agar and kept at 25 °C for four days, and the number of colonies with a colour different from the parent mould was counted. Table 5.1 shows the results.
batch
treatment
number of colonies with a new colour, per million spores
1
none
4
2
ionising radiation, 200 units
21
3
ionising radiation, 400 units
47
4
ionising radiation, 800 units
96
5
chemical M
63
Table 5.1
(a)[2]
Describe the effect of the dose of ionising radiation on the number of colonies with a new colour. Use figures from Table 5.1.
Model Answer — 5(a)
as the dose of ionising radiation increases, the number of colonies with a new colour increases — from 21 at 200 units to 96 at 800 units, a rise of 75 per million spores [1]
the relationship is close to proportional: each time the dose is doubled the number roughly doubles (21 → 47 → 96), and all four radiation values are above the untreated value of 4 [1]
⚠ If you missed marks here: The second mark is for spotting the pattern, not just the direction. Doubling the dose doubles the count — check that yourself against the three numbers rather than taking it on trust, because a Cambridge data set will sometimes level off instead, and the description has to match the data in front of you. Do not include batch 5 in this answer: chemical M is a different treatment and is not part of the dose series.
(b)[2]
Explain the purpose of batch 1, and explain why the value for batch 1 is 4 rather than 0.
Model Answer — 5(b)
batch 1 is the control. It shows how many new-colour colonies appear with no treatment at all, so any increase above 4 can be attributed to the radiation or the chemical rather than to something else in the method [1]
it is not zero because mutation happens anyway: it is a random event that occurs at a low natural rate in every population. Ionising radiation and some chemicals increase the rate of mutation — chemical M raised it from 4 to 63 — but they do not start a process that was not already happening [1]
⚠ If you missed marks here: The exact syllabus wording is that radiation and some chemicals increase the rate of mutation. Writing that they “cause mutations” is close enough to be tempting and just wrong enough to lose the mark, because it implies there would be none without them — and your own control says there would be four. Do not describe batch 1 as “the normal one”; the word is control.
(c)[3]
Explain, in terms of DNA and proteins, how a treated spore can grow into a colony of a different colour from its parent.
Model Answer — 5(c)
a gene mutation is a random change in the base sequence of DNA [1]
the sequence of bases in a gene determines the sequence of amino acids in the protein it codes for, so a change in the bases can change the amino acid sequence and therefore the shape of the protein — here, an enzyme in the pathway that makes the colour, which then works differently or not at all [1]
mutation is the way new alleles are formed; the mutated spore divides by mitosis, so every cell of the colony it grows into carries the same new allele and the whole colony shows the new colour [1]
⚠ If you missed marks here: This part reaches back into Topic 17: a gene codes for a protein, and it is the base sequence that fixes the amino acid sequence. If your answer says the mutation “changes the colour gene” and stops, you have described the input and the output with no mechanism in between. The third mark is easy to forget: the colony is a clone produced by mitosis, which is why one changed spore gives one uniformly different colony rather than a speckled one.
(d)[3]
Most of the new-colour colonies grew more slowly than the parent mould. A student concluded: “This proves that all mutations are harmful.” Comment on this conclusion, and suggest one improvement to the method of the investigation.
Model Answer — 5(d)
the conclusion is not valid. Many mutations have no effect on the phenotype at all, and this method could not detect any of them — only colonies with a visible colour change were counted, so the sample is not a fair sample of all the mutations that occurred [1]
a few mutations are advantageous, and whether a mutation is harmful or advantageous depends on the environment: the same allele can be a disadvantage in one environment and an advantage in another, as the warfarin resistance allele is [1]
improvement: repeat each treatment several times and calculate a mean, so that an anomalous plate does not decide the result. (Accept: use identical numbers of spores from one parent culture for every batch; keep temperature, agar and time identical; count the colonies blind or have two people count them; add more doses to fill in the pattern) [1]
⚠ If you missed marks here: “All mutations are harmful” is one of the two or three commonest misconceptions in the whole syllabus, and this data set is designed to look as though it supports it. The killer objection is the one about what the method can see: a mutation with no visible effect is invisible to this count, so the investigation could never have found it. For the improvement mark, “be more accurate” and “do it properly” earn nothing — name a variable you would control or say what you would repeat.
Question 6 — Four Generations of Maize
Total: 12 marks
Fig. 6.1 shows four successive generations of a maize crop grown on one farm. Six plants are drawn from each generation and the size of each cob is drawn in proportion to the grain yield of that plant, measured in grams per plant. The mean yield of each generation is printed on the right. In every generation, the two plants inside the dashed rings were the only ones used to produce the next generation.
(a)[3]
Describe the change in the mean yield over the four generations shown in Fig. 6.1. Use figures.
Model Answer — 6(a)
the mean yield increases in every generation, from 14.0 g in generation 1 to 25.7 g in generation 4 [1]
a total increase of 11.7 g, which is a rise of about 84% of the starting value [1]
the increase is almost constant from one generation to the next — 3.7 g, then 4.0 g, then 4.0 g — so there is no sign yet of the improvement slowing down [1]
⚠ If you missed marks here: Three marks means three separate statements, and “the yield went up a lot” is one vague statement. Give the two end values, calculate the difference, then say something about the shape of the change. The percentage rise is 11.7 ÷ 14.0 × 100 = 83.6%, so “about 84%” is fine — but show that division, because an unexplained number in the margin cannot be given credit.
(b)[3]
Describe the process the farmer has carried out, as shown in Fig. 6.1.
Model Answer — 6(b)
this is selective breeding, also called artificial selection. First, humans select the individuals with the desirable feature — here the farmer rings the two highest-yielding plants in the generation [1]
those selected individuals are crossed with each other to produce the next generation [1]
the offspring showing the desirable feature are selected and crossed again, and the whole procedure is repeated over many generations [1]
⚠ If you missed marks here: These are the three steps Cambridge marks, and the third one is the one candidates leave out. Selecting and crossing once is not selective breeding; the repetition over many generations is what makes it work, and it is a mark. Human intention is correct biology here — “the farmer chose” and “the feature the farmer wanted” are exactly right in this question, and only become an error when written about natural selection.
(c)[3]
In generation 1 the yields of the six plants ranged from 10 g to 19 g. In generation 4 they ranged from 22 g to 30 g. The mean has risen by 11.7 g but the spread within a generation has hardly changed. Explain what this shows, and predict what would happen to the spread if the farmer continued in the same way for fifty generations.
Model Answer — 6(c)
it shows that variation is still present in every generation — selecting the two best plants does not make all their offspring identical. What has moved is the mean, not the existence of variation [1]
variation keeps being produced each generation by meiosis, random fertilisation and mutation, and yield is a continuous characteristic influenced by the environment as well as by genes, so the plants in one field will never all be the same [1]
after fifty generations the spread would be expected to narrow, because only two plants are used as parents each time, so the population would end up containing fewer different alleles than it started with. A crop bred this way has less genetic variation, which is a real disadvantage if a new disease appears [1]
⚠ If you missed marks here: Two different time scales are in play and it is easy to contradict yourself. Over four generations the spread stays wide, because meiosis and mutation keep making new combinations. Over fifty, with only two parents used each time, the number of different alleles in the population falls and the spread narrows. Both statements are true; they are answers to different questions. Note also that a wide spread in generation 1 is what made the whole exercise possible — with no variation there is nothing to select.
(d)[3]
The farmer says: “My maize has evolved by natural selection, the same as any wild plant — I have just speeded it up.” Explain why this is not correct.
Model Answer — 6(d)
the difference is who or what does the selecting. In natural selection the environment determines which individuals survive to reproduce; here the farmer decided, and the plants that were not ringed did not get to reproduce however well suited to the field they were [1]
the feature selected is one that humans find desirable, and it may actually reduce survival in the wild — a very large cob whose grains stay attached to the plant would not be dispersed, so this maize depends on being sown by people [1]
speed is not the difference: both act over many generations, both act on variation that already exists, and in both the source of new variation is the same — mutation. Selective breeding is usually faster only because the selection is deliberate and intense [1]
⚠ If you missed marks here: If your answer was “selective breeding is faster”, you have agreed with the farmer rather than corrected him. Speed is a consequence; the difference is the selecting agent. The other frequent error is claiming that selective breeding does not involve variation or does not involve alleles being passed on. It involves both, in exactly the same way. One thing changes, and it is who chooses.
Question 7 — Two Populations, Two Selectors
Total: 10 marks
A wild tomato plant grows on dry, stony, exposed hillsides. Its fruits are the size of a pea. A cultivated tomato variety has been produced from the same species by growers over about four hundred years; its fruits are large and heavy. Seeds of both were sown in two places: on the wild hillside, and in a watered, sheltered greenhouse. Table 7.1 shows the results.
number of seedlings surviving to flowering, out of 100 seeds sown on the hillside
mass of fruit produced per plant in the greenhouse / kg
wild tomato
68
0.4
cultivated tomato
5
4.2
Table 7.1
(a)[3]
Describe the results in Table 7.1, and explain how each of the two populations came to be as it is.
Model Answer — 7(a)
on the hillside the wild plant survives far better — 68 out of 100 against 5 out of 100, about 14 times as many — but in the greenhouse the cultivated variety yields far more fruit, 4.2 kg against 0.4 kg per plant, more than ten times as much [1]
the wild population was shaped by natural selection: over many generations, the plants that happened to carry alleles suiting them to dry, exposed, stony ground were more likely to survive and reproduce there, and passed those alleles on, so the population became better adapted to that environment [1]
the cultivated variety was shaped by selective breeding: growers selected the plants with the largest fruits, crossed them, and selected again from the offspring over many generations. Large fruit yield is a feature humans find desirable; it gives no advantage on a dry hillside, which is why only 5 of those seedlings survived there [1]
⚠ If you missed marks here: The question says describe and explain, so a pure description of the numbers can only reach one of the three marks. Notice what the table is really showing: the cultivated variety is not a worse plant, it is a plant suited to a different environment — the one its growers provide. “The wild one is stronger” earns nothing; the phrase is better adapted to that environment, and the greenhouse column shows how quickly that reverses.
(b)[4]
Give four differences between natural selection and selective breeding. Deal with: what does the selecting; the kind of feature that is selected; the effect on the genetic variation of the population; and how well suited the population ends up being to its natural environment.
Model Answer — 7(b)
what does the selecting: in natural selection the environment determines which individuals survive to reproduce; in selective breeding humans choose which individuals are allowed to breed [1]
the feature selected: natural selection favours features that improve survival and reproduction in that environment; selective breeding favours features that humans find desirable, which may reduce survival in the wild [1]
genetic variation: in a natural population variation is maintained, with new alleles arising by mutation each generation; in selective breeding only a few individuals are used as parents each time, so the population usually ends up with less genetic variation [1]
how well suited the population is: natural selection makes the population better adapted to its own environment; selective breeding makes it suited to what people want and often less able to survive without human care [1]
(what they share: variation arises by mutation in both, both act on variation that already exists, both need many generations, and in both the selected individuals pass their alleles to the next generation)
⚠ If you missed marks here: A comparison mark needs both sides in the same sentence. “Humans do the selecting” on its own is half a difference and is often not credited — write “in natural selection the environment selects, whereas in selective breeding humans select”. Avoid two answers that are not differences at all: “one is faster” and “one uses variation and the other does not”. The second is simply false.
(c)[3]
A gardener says: “If I just left my cultivated tomatoes out on that hillside for a few years, they would adapt to it.” Define the term adaptation, and explain two reasons why the gardener is wrong.
Model Answer — 7(c)
adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations [1]
first reason: an individual plant cannot adapt during its life. Each plant has the alleles it was born with. It is the population that changes, as the proportion of individuals carrying particular alleles shifts from one generation to the next — and “a few years” is only a few generations [1]
second reason: the variation has to be there first. Selection can only act on alleles that already exist in the population, and centuries of breeding for fruit size have left this variety with little genetic variation. If no plant carries alleles suiting it to dry stony ground, there is nothing for selection to act on and the population simply dies out — 95 of the 100 seedlings already did [1]
⚠ If you missed marks here: The definition is worth memorising word for word, and the three parts that carry it are process, populations and over many generations. On the second reason, this is the pay-off of the whole paper: selection does not create variation, it acts on variation that is already there. Extinction is a perfectly good biological answer — when the necessary variation is absent, the population does not adapt, it disappears. A wrong answer to avoid: “they would get used to the conditions”, which is the same error as the student made about the rats in Question 3.
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