Topic 18: Variation and Selection -- Challenge Exam 1
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 xerophytes, 18.3 natural selection, and 18.4 selective breeding and the difference between natural and artificial 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 Charts, Two Kinds of Difference
Total: 12 marks
Fig. 1.1 shows two sets of data collected from human populations. Chart A comes from a survey of 64 students in one school. Chart B comes from a survey of 1000 people living in the same town. Nothing has been named for you.
(a)[4]
Name the type of variation shown by Chart A and the type shown by Chart B. For each one, give the feature of the chart that told you.
Model Answer — 1(a)
Chart A shows continuous variation [1]
because it shows a range of values between two extremes — every height from 145 cm to 180 cm is possible, and the bars touch with no gaps [1]
Chart B shows discontinuous variation [1]
because there is a limited number of phenotypes with no intermediates — a person is in group O, A, B or AB and nobody is between two of them, which is why the bars are drawn separately [1]
⚠ If you missed marks here: The reason marks are where this question is won, and “because the bars are touching” on its own is a description of the drawing, not of the biology. Say what the shape means: a range between two extremes, or a limited number of groups with no intermediates. That phrase — no intermediates — is the one Cambridge marks. Also watch the direction of the error: plenty of students write “continuous because there is a lot of it”. The amount of variation has nothing to do with which type it is.
(b)[2]
Using Chart A, state the modal class of height, and calculate the percentage of the 64 students who fell into it. Show your working.
Model Answer — 1(b)
the modal class is 160 cm to 165 cm — it is the tallest bar, with 19 students [1]
19 ÷ 64 × 100 = 29.7 %
29.7 % (accept 30 %) [1]
⚠ If you missed marks here: The modal class is the class, so it needs both ends of the bar — “160” on its own is not a class and “19” is the frequency, not the mode. The other trap is dividing by the wrong total: there are 64 students, not 100 and not 7. Adding the bars up (2 + 6 + 13 + 19 + 14 + 7 + 3 = 64) takes ten seconds and tells you which total to use.
(c)[3]
Explain what causes the pattern in Chart A and what causes the pattern in Chart B. Include one named factor in your answer.
Model Answer — 1(c)
the variation in Chart A is caused by genes and the environment together [1]
a named environmental factor: diet / nutrition (accept exercise, illness during childhood) — two students with the same alleles for height can end up different heights if one was better fed [1]
the variation in Chart B is caused by genes only; nothing a person eats or does changes their ABO blood group [1]
⚠ If you missed marks here: The 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 — the genes set a range and the environment determines where in that range you land. Drop the word “genes” and you lose the mark. Going the other way, “discontinuous variation is caused by genes” needs the word only to be worth anything, because genes are involved in both.
(d)[3]
All of the variation in Fig. 1.1 depends on there being different alleles in the population. There is only one process that produces a brand new allele. State what that process is in full, and state two factors that increase the rate at which it happens.
Model Answer — 1(d)
gene mutation — a random change in the base sequence of DNA [1]
ionising radiation increases the rate of mutation [1]
some chemicals increase the rate of mutation [1]
⚠ If you missed marks here: “A change in the DNA” is too vague to score; Cambridge wants a change in the base sequence, and the word random matters more here than anywhere else on the syllabus — it is what stops the rest of Topic 18 from turning into “the organism changed because it needed to”. Note also that meiosis, random mating and random fertilisation are sources of genetic variation, but they only shuffle alleles that already exist. Mutation is the only source of a new allele.
Question 2 — Thirty Cells, Three Times
Total: 12 marks
Fig. 2.1 shows the same population of bacteria drawn at three different times, A, B and C. Two types of cell are present, type P and type Q, and the key tells you which shading is which. A cell drawn with a dashed outline and a cross through it has been killed. Between A and B the population was treated with an antibiotic. Between B and C no antibiotic was used and the bacteria were left to reproduce.
(a)[3]
Using Fig. 2.1, state the number of type Q cells drawn in A and the number drawn in C, and describe what has happened to type P by the time of C.
Model Answer — 2(a)
in A there are 2 type Q cells (out of 30) [1]
in C there are 30 type Q cells — every cell drawn is type Q [1]
type P has disappeared from the population: all 28 type P cells were killed by the antibiotic, and there are none left in C [1]
⚠ If you missed marks here: Count, do not estimate — each panel is a 6 by 5 grid, so there are 30 cells in every one and the arithmetic is fixed. The reading error worth naming is describing panel C as “the type P cells turned into type Q”. No cell changed type. The type P cells died; the two type Q cells that were already there reproduced until they filled the panel.
(b)[2]
State which type of cell was resistant to the antibiotic, and give the evidence in Fig. 2.1 that led you to that answer.
Model Answer — 2(b)
type Q was resistant [1]
evidence: in panel B, after the antibiotic had been used, the only cells still alive are the two type Q cells — every cell that has been crossed out is a type P cell [1]
⚠ If you missed marks here: An “evidence” mark needs you to point at the figure, not at your own knowledge. “Because type Q is resistant” is circular and scores nothing. Say which panel you are reading and what is in it. Note also that the resistant cells are the rare ones at the start — resistance being common is the end of the story, not the beginning.
(c)[5]
Explain, in terms of natural selection, how the population changed from A to C. There is one mark for each of the five stages, so set your answer out in five steps.
Model Answer — 2(c)
1 — variation already existed. Before any antibiotic was used, two of the 30 cells in the population carried an allele for resistance, which had arisen by a random mutation [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 — struggle for survival. The antibiotic killed the sensitive type P cells, and the survivors also compete with each other for nutrients and space [1]
4 — the better adapted are more likely to survive and reproduce. The type Q cells were not harmed by the antibiotic, so they were far more likely than type P to survive and reproduce in these conditions [1]
5 — alleles are passed on. The surviving type Q cells passed the resistance allele to their offspring, so the proportion of the population that is resistant rose until, in C, the whole population is resistant [1]
⚠ If you missed marks here: Check step 1 before anything else. If your answer starts at the antibiotic rather than at the variation, you have written the story backwards and the examiner will read the rest as “the drug made them resistant”. The antibiotic selected; it did not create anything. Second, keep the subject of every sentence right: it is the population whose composition changes, never an individual cell that becomes resistant during its life. Third, step 5 must say the alleles were passed on to offspring — “so they all became resistant” skips the only part of the sentence that is about inheritance.
(d)[2]
A change of this size in a population of large mammals would take many thousands of years. In bacteria it can happen inside a hospital ward in a few days. Explain why.
Model Answer — 2(d)
bacteria reproduce roughly every 20 minutes, so an enormous number of generations passes in a few days, and selection acts once per generation [1]
the populations are also very large, so even a rare mutation appears in some cells, and every generation is another chance for the proportion carrying the resistance allele to rise [1]
⚠ If you missed marks here: The answer is about generation time, not about bacteria being small or simple. Say the number of generations, because natural selection is measured in generations and not in years — that one sentence is what turns a vague answer into a marked one. Note also that a single bacterium does not become more resistant as it ages; it divides, and the change is in what the next generation is made of.
Question 3 — Twenty-Eight Years of Samples
Total: 12 marks
One hospital tested samples of a single species of bacterium taken from its patients every four years from 1998 to 2026, and recorded the percentage of samples that were resistant to one particular antibiotic. The results are shown in Fig. 3.1. Over the same period that antibiotic was prescribed heavily on the wards.
(a)[3]
Use Fig. 3.1 to calculate the increase in the percentage of resistant samples between 1998 and 2026, and then calculate the mean rate of increase between 2010 and 2018 in per cent per year. Show your working.
Model Answer — 3(a)
reading the graph: 1998 = 2 %, 2026 = 63 %, so the increase is 61 % [1]
2010 = 21 %, 2018 = 47 %, a rise of 26 over 8 years — correct method shown [1]
(47 − 21) ÷ (2018 − 2010) = 26 ÷ 8
3.25 % per year (accept 3.2 to 3.3) [1]
⚠ If you missed marks here: Read the first point off the graph properly: 1998 sits just above the axis at 2 %, not at 0, and reading it as zero costs you this mark and the whole of part (b). For the rate, divide by 8 years — the points are four years apart, so the gap from 2010 to 2018 is two intervals, not one. A rate must carry its unit; 3.25 on its own is not a rate.
(b)[2]
In 1998 the value plotted was 2 % and not 0 %. Explain why this single data point matters to any explanation of the rest of the graph.
Model Answer — 3(b)
it shows that resistant bacteria were already present in the population at the start — the variation existed before the heavy use of the antibiotic [1]
so the antibiotic did not create the resistance; it acted as a selection pressure on variation that was already there, and selection can only act on variation that already exists [1]
⚠ If you missed marks here: This is the highest-value two marks in the topic, because everything else follows from them. Variation comes first; selection comes second. If you wrote that the bacteria became resistant after being exposed to the drug, go back and reread the 1998 point — the graph itself disproves that answer, which is exactly why it was drawn starting at 2 %.
(c)[4]
A student was asked to explain the trend in Fig. 3.1 and wrote:
“The bacteria found that the antibiotic was killing them, so over the years they got used to it and became resistant in order to survive. By 2026 they had all learned to resist the drug.” Identify two things that are biologically wrong with this answer, then rewrite it correctly.
Model Answer — 3(c)
error 1 — the bacteria did not change in response to the antibiotic. The resistance allele arose earlier, by a random mutation, and mutation does not happen because of what the environment is doing [1]
error 2 — an individual bacterium does not become resistant during its life, and nothing is “learned”. What changes is the proportion of the population that carries the resistance allele [1]
rewrite, first half — some bacteria in the population already carried an allele for resistance, produced by a random mutation. When the antibiotic was used the sensitive bacteria were killed and the resistant ones survived [1]
rewrite, second half — the survivors reproduced and passed the resistance allele to their offspring, so the percentage of the population that was resistant rose over many generations, from 2 % in 1998 to 63 % in 2026 [1]
(the two rewrite marks are only awarded for a version that avoids “got used to”, “learned to” and “in order to survive” altogether — softening those phrases is not enough)
⚠ If you missed marks here: The student answer above is not a strange one — it is what most people write, and it scores zero marks out of four in a real paper. The giveaway words are found that, got used to, in order to and learned: every one of them gives the bacteria a purpose. The repair is mechanical, so learn it as a habit. Change the subject of the sentence from the individual to the population, and put the variation before the selection. If your rewrite still has an organism doing something on purpose, it has not been rewritten, only reworded.
(d)[3]
In Topic 15 you met the advice that antibiotics should only be prescribed when they are essential. Use Fig. 3.1 to explain the biological reason behind that advice.
Model Answer — 3(d)
the more often the antibiotic is used, the more often the sensitive bacteria are killed, so the greater the selection pressure acting on the population [1]
this gives the resistant bacteria a larger survival and reproductive advantage, so the proportion of the population that is resistant rises — which is the trend from 2 % to 63 % shown on the graph [1]
using the antibiotic only when it is essential reduces that selection, so the resistant proportion rises far more slowly and the antibiotic stays useful for treating patients for longer [1]
⚠ If you missed marks here: Two things to be careful about. The first is language: it is the bacterium that is resistant, never the patient — a person does not become resistant to an antibiotic, and writing that they do will cost you a mark on any paper. The second is that this question asks for the biological reason, so cost and side effects earn nothing here. Topic 15 gave you this advice; Topic 18 is where you can finally say why it works.
Question 4 — A Leaf Built for a Sand Dune
Total: 12 marks
Fig. 4.1 is a cross section through the leaf of a grass that grows on open sand dunes, where the sand holds very little water and the wind rarely stops. The leaf is rolled. Five features are labelled A to E. You have met the structure of a normal broad leaf in Topic 6 and the factors that affect transpiration in Topic 8 — use both.
(a)[2]
State what is meant by an adaptive feature.
Model Answer — 4(a)
an inherited feature [1]
that helps an organism to survive and reproduce in its environment [1]
⚠ If you missed marks here: Two words carry the whole definition and both are routinely dropped. Inherited rules out anything an organism picks up during its life — a tan or a scar is not an adaptive feature. And it is survive and reproduce: an organism that survives brilliantly and leaves no offspring passes nothing on, so surviving alone is not enough for the feature to spread.
(b)[4]
Name the features labelled A, B, C and D on Fig. 4.1.
Model Answer — 4(b)
A — a thick waxy cuticle on the outer surface [1]
B — a sunken stoma, sitting in a pit rather than flush with the surface [1]
C — hairs projecting from the inner surface into the enclosed space [1]
D — the rolled leaf (the rolled edge that encloses the inner surface) [1]
⚠ If you missed marks here: B is the one people lose: “stoma” on its own is not worth the mark, because an ordinary leaf has stomata too. The adaptive part is that it is sunken. In the same way A is not just “cuticle” but a thick one. Notice where the stomata are: on the inner, enclosed surface. Compare that with the broad leaf you drew in Topic 6, where most stomata sit on the lower epidermis, open to the moving air.
(c)[4]
Explain how the features labelled A, B and C reduce the loss of water vapour from this leaf.
Model Answer — 4(c)
A — the cuticle is waxy and waterproof, and being thick it gives a longer diffusion distance, so almost no water vapour escapes through the epidermis itself [1]
B — a sunken stoma sits at the bottom of a pit, so water vapour collects in the pit instead of being carried away [1]
this makes the air just outside the stoma more humid, so there is a smaller concentration gradient of water vapour between the air spaces inside the leaf and the air outside, and diffusion out is slower [1]
C — the hairs trap a layer of still, humid air next to the stomata, so wind cannot sweep the water vapour away and the gradient stays small [1]
⚠ If you missed marks here: Every one of these features does the same single job, and once you see it the marks come quickly: keep the water vapour close to the stomata so the concentration gradient stays small. Answers that stop at “the hairs trap moisture” describe without explaining. The word that turns a description into an explanation is gradient. Also, do not write that the stomata are closed or missing — the leaf still needs carbon dioxide for photosynthesis, so the stomata are there and they open.
(d)[2]
State what is meant by adaptation. Your answer must make clear what it is that changes.
Model Answer — 4(d)
adaptation is the process, resulting from natural selection [1]
by which populations become more suited to their environment over many generations [1]
⚠ If you missed marks here: The subject of the sentence is the mark. It is a population that becomes adapted, over many generations — an individual grass plant on the dune cannot roll its leaves in response to a dry summer and pass that on. Notice too that adaptation is a process and an adaptive feature is a thing; Cambridge defines them separately and asks for them separately, so keep the two definitions apart in your head.
Question 5 — The Island That Changed Colour
Total: 10 marks
A small island is home to a population of ground beetles. Some individuals are pale brown and some are dark. The beetles rest on bare ground during the day and are hunted by birds that find them by sight. In 2014 a volcano on a neighbouring island covered most of the ground with dark grey ash, which has stayed there since. Table 5.1 shows the percentage of dark beetles found in samples of 200 beetles collected in four different years.
year
2012
2016
2020
2024
percentage of the sample that was dark
12
29
54
71
Table 5.1
(a)[2]
Describe the change shown in Table 5.1, and support your description with a calculation.
Model Answer — 5(a)
the percentage of dark beetles rose in every sample, from 12 % in 2012 to 71 % in 2024 [1]
71 − 12 = 59 (or 71 ÷ 12 = 5.9 times as many)
an increase of 59 %, or almost six times as many as in 2012 [1]
⚠ If you missed marks here: “It went up” is not a description — a description quotes the two end values and says the rise happened throughout, not only at the end. When a question says “support with a calculation” there is a mark waiting for a number, and any sensible one will do; a subtraction is the safest.
(b)[4]
Explain, in terms of natural selection, the change shown in Table 5.1.
Model Answer — 5(b)
variation in colour already existed in the population before the ash fell — the 2012 sample was already 12 % dark — and that variation arose by mutation [1]
once the ground was dark, pale beetles stood out against the ash and were seen and eaten by the birds, while dark beetles were harder for the birds to see [1]
the dark beetles were therefore better adapted to the new conditions and had a greater chance of surviving and reproducing [1]
the survivors passed on their alleles for dark colour to their offspring, so the proportion of dark beetles in the population rose in each generation [1]
⚠ If you missed marks here: Every context in 18.3 wants the same four moves, in the same order: variation first, then the selection pressure, then the survival and reproduction advantage, then the alleles being passed on. If you can name the selection pressure — here it is being eaten by birds — the middle two marks look after themselves. What you must not write is that the beetles changed colour to match the ash. They did not change; the mixture of the population changed.
(c)[2]
A student suggested that the alleles for dark colour first appeared in 2014, when the ash arrived. Explain why Table 5.1 shows that this cannot be correct.
Model Answer — 5(c)
the 2012 sample was 12 % dark, which is two years before the ash fell — so the alleles were clearly already present in the population [1]
mutation is random and does not happen in response to a change in the environment; selection can only act on variation that already exists, it cannot produce it [1]
⚠ If you missed marks here: This is the same mark as the 2 % point on the graph in Question 3, in a different costume. Examiners plant a low, non-zero starting value in the data precisely because it makes this question possible, so whenever you see one, ask yourself what it proves. And keep the two ideas separate: the data mark comes from quoting 2012, the biology mark comes from saying mutation is random.
(d)[2]
These beetles reproduce sexually. Apart from mutation, state two processes that are sources of genetic variation in this population.
Model Answer — 5(d)
meiosis — it produces genetically different gametes [1]
random mating or random fertilisation — which male mates with which female, and which sperm happens to fuse with which egg cell [1]
⚠ If you missed marks here: Cambridge lists exactly four sources: mutation, meiosis, random mating and random fertilisation. Two of those are named in that list, so learn the list rather than inventing an answer. Keep the distinction sharp: only mutation makes a new allele; the other three shuffle alleles that already exist into new combinations. “Sexual reproduction” on its own is too vague to score, because the question asks which processes within it.
Question 6 — Four Generations of Wheat
Total: 12 marks
A farmer grew wheat on one field for four successive generations. Fig. 6.1 shows six plants from each generation; the size of the head drawn is in proportion to the grain yield of that plant, in arbitrary units. In every generation the two plants inside the dashed rings were the only ones used to produce the next generation. The mean yield of each generation is printed on the right.
(a)[3]
Use Fig. 6.1 to calculate the percentage increase in mean yield from Generation 1 to Generation 4. Show your working.
Model Answer — 6(a)
both values read correctly from the figure: Generation 1 mean = 14.0, Generation 4 mean = 25.7 [1]
(25.7 − 14.0) ÷ 14.0 × 100
correct method — the increase is divided by the starting value, not by the final one [1]
83.6 % (accept 83 to 84 %) [1]
⚠ If you missed marks here: Dividing by 25.7 instead of 14.0 gives 45.5 %, which is the single commonest wrong answer to a percentage-increase question in any science paper. The rule never changes: divide the change by what you started with. Note also that the answer is not 11.7 %; 11.7 is the increase in yield units, and turning it into a percentage is the whole task.
(b)[3]
Describe the process shown in Fig. 6.1. There is one mark for each of the three stages.
Model Answer — 6(b)
1 — humans select the individuals with the desirable feature: in each generation the farmer picked out the two plants with the highest grain yield, shown by the dashed rings [1]
2 — those selected individuals are crossed (bred together) to produce the next generation [1]
3 — the offspring showing the desirable feature are selected and crossed again, and this is repeated over many generations — four are shown here, but a real breeding programme runs for far more [1]
⚠ If you missed marks here: Three marks means three separate stages, so number them. The stage most often left out is the third — repeated over many generations — and without it the answer describes one cross rather than a breeding programme. Naming the process is not enough on its own either: writing “selective breeding” and stopping earns nothing when the command word is describe.
(c)[3]
In Generation 4 the six plants still differ from one another by about as much as the six plants in Generation 1 did. State what has changed between the two generations, and explain both observations.
Model Answer — 6(c)
what has changed is the mean, which has risen from 14.0 to 25.7 — the whole set of yields has shifted upwards [1]
the mean rose because only the highest-yielding plants were allowed to reproduce, so a greater proportion of each new generation inherited alleles for high yield [1]
the spread stayed similar because variation is still present in every generation: the offspring of two high-yielding parents are not identical to their parents or to each other, so selective breeding shifts the mean rather than removing the variation [1]
⚠ If you missed marks here: Grain yield is continuous variation, caused by genes and the environment together, which is exactly why every generation still shows a range — look back at Chart A in Question 1 and you are looking at the same shape. A very common wrong answer is that selective breeding “removes the variation”. Over many generations it does reduce the genetic variation in a crop, which is the risk in part (d), but it never flattens a generation into identical plants.
(d)[3]
The farmer plans to continue this programme for another forty generations, using only the two best plants each time. Suggest and explain one disadvantage of doing this.
Model Answer — 6(d)
only two plants are parents each generation, so over forty generations the genetic variation in the crop falls and the plants become very similar to one another [1]
if a new disease or pest arrives, or the climate of the field changes, it is unlikely that any plant has alleles that allow it to survive [1]
so the whole crop could be affected in the same way and lost — and alleles discarded along the way, perhaps for disease resistance or drought tolerance, cannot be recovered [1]
⚠ If you missed marks here: “They would all get the disease” is the answer without the reason. The reason is low genetic variation, and it earns its mark only when you say what that means: no individual happens to carry the alleles that would let it survive the new condition. Do not write that the plants become clones — they are still produced sexually, so they are similar, not identical. Notice the link back to Question 5: variation is what lets a population survive a change, and this programme is throwing it away.
Question 7 — Who Or What Is Doing The Choosing
Total: 10 marks
This question is about the relationship between the two processes you have used in this paper: the natural selection of resistant bacteria in Questions 2 and 3, and the artificial selection of wheat in Question 6.
(a)[4]
State four differences between natural selection and artificial selection.
Model Answer — 7(a)
in natural selection the selecting is done by the environment; in artificial selection it is done by humans [1]
natural selection favours features that improve survival and reproduction in that environment; artificial selection favours features that humans find desirable, such as grain yield or milk yield [1]
a feature produced by artificial selection may reduce the organism’s chance of survival in the wild — a wheat plant with a very heavy head is of no use to a wild plant [1]
artificial selection usually produces the change in far fewer generations, because only the chosen individuals are allowed to breed at all; and it usually leaves the population with less genetic variation [1]
⚠ If you missed marks here: A difference needs both sides of it. “Humans choose” is half an answer; “humans choose, whereas in natural selection the environment does” is the mark. Write each point as one sentence with a “whereas” in the middle and you cannot lose them. Note that the first point is the one that actually defines the two processes; the others follow from it.
(b)[2]
State two ways in which the two processes are the same.
Model Answer — 7(b)
in both, the variation is already present in the population and arose by mutation — the source of the variation is exactly the same in the two processes [1]
in both, only some individuals reproduce and pass their alleles to the next generation, and the change builds up over many generations [1]
⚠ If you missed marks here: Students often assume that because artificial selection involves people, it must somehow create the feature. It does not. The farmer in Question 6 did not put anything into the wheat — the high-yield alleles were already there, and the farmer simply decided which plants got to reproduce. Once you hold on to that, part (c) writes itself.
(c)[4]
A student wrote:
“Selective breeding and natural selection are really the same thing. Selective breeding is just natural selection speeded up by farmers, and both of them work because the species wants to improve itself.” Explain why this statement is not correct.
Model Answer — 7(c)
the difference between the two is not speed: it is who or what does the selecting — the environment in natural selection, humans in artificial selection [1]
the two processes also select for different kinds of feature: natural selection favours whatever improves survival and reproduction in that environment, while a farmer selects a feature that is useful to people and which may be a disadvantage in the wild [1]
neither process involves a species wanting anything: a species has no aims, and natural selection has no goal — it is simply the outcome of some individuals surviving and reproducing more than others [1]
nor does an individual improve itself during its life. The variation arises randomly by mutation before selection acts, and what changes over generations is the proportion of the population carrying each allele [1]
⚠ If you missed marks here: Two separate errors are buried in that sentence and both are worth spotting on sight. The first is treating the difference as one of speed — speed is a consequence of deliberate human selection, not the definition of it. The second is the phrase “the species wants to improve itself”, which is the same fault as the bacteria in Question 3 and the plant that supposedly rolled its own leaves in Question 4. Watch out too for “survival of the fittest”: it is a phrase people reach for instead of an explanation, and on its own it earns nothing. Say better adapted, say what to, and say what happens to the alleles.
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