⚡ Challenge Paper Preparation

Challenge Prep: Variation and Selection

IGCSE Biology 0610 — Topic 18 — Extended

Topic 18 has almost no facts to forget. It has one sentence structure to get right, and nearly every mark in the topic is won or lost on it. The moment you write that an organism changed itself — that the bacteria toughened up, that the cactus grew spines because the desert is dry, that a species improved — the mark scheme stops giving you anything, however fluent the rest of the paragraph is. The cure is always the same two moves: make the population the subject of the sentence instead of the individual, and put the variation before the selection. Antibiotic resistance is the centrepiece here, and it is the case you already half know — Topic 15 told you that the antibiotic does not create resistance; this is the topic where you finally get to say why. Twelve traps, six data-led walkthroughs, six lookalike pairs, a concept map built on three frameworks, six badly-scoring answers and ten full practice questions below.

⚠️ Common Traps & Misconceptions

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Twelve traps, spread across 18.1, 18.2, 18.3 and 18.4. Six of them are the same fault wearing six different costumes — writing as though the organism did the changing. Learn to hear it.

⚠️ TRAP — 18.1

Trap 1: Saying continuous variation is caused by the environment

The Trap“Height is continuous variation because it depends on how much food you eat, and blood group is discontinuous because that one is genetic.” It sounds like a tidy rule — environment gives you a smooth range, genes give you separate boxes — and it is half of the syllabus statement with the important half missing.

The TruthContinuous variation is caused by genes AND the environment together. Genes set the range a person could fall in; diet, disease and other environmental factors determine where inside that range they end up. Discontinuous variation is usually caused by genes only, which is why the environment cannot move you from blood group A to blood group B no matter what you eat. Continuous variation gives a range of phenotypes between two extremes (body length, body mass); discontinuous variation gives a limited number of phenotypes with no intermediates (ABO blood groups, seed shape in peas, seed colour in peas).

Why It MattersThe two-mark version of this question is “state one difference between the causes of continuous and discontinuous variation”, and the word and is the whole mark. Writing “environment” on its own for continuous variation loses it, because it would mean two genetically different people raised identically must end up exactly the same height, and they do not.

Example Question“State what causes continuous variation and what causes discontinuous variation, and give one example of each. [4]”

⚠️ TRAP — 18.1

Trap 2: Treating every mutation as damage

The Trap“Mutations are harmful changes to DNA caused by radiation, so a mutation could never help an organism.” It is what the word sounds like, and it makes the whole of 18.3 impossible to explain, because if no mutation is ever useful there is nothing for selection to act on.

The TruthA mutation is a genetic change, and a gene mutation is a random change in the base sequence of DNA. Most mutations have no effect at all; some are harmful; a small number are advantageous in a particular environment. Mutation is the way in which new alleles are formed — it is the only original source of new variation, and everything in Topic 18 depends on it. Ionising radiation and some chemicals increase the rate of mutation, but they do not determine which mutation happens.

Why It Matters“New alleles are formed by mutation” is a marking point on its own, and it is the first step of every natural selection answer. Notice also that advantageous always means “in that environment”: the same allele that protects a bacterium from an antibiotic may slow its growth when there is no antibiotic about.

Example Question“Define gene mutation and explain why mutation is important to a population, even though many mutations are harmful. [3]”

⚠️ TRAP — 18.1 and 18.3

Trap 3: “The mutation happened because the antibiotic was there”

The Trap“When the antibiotic was added, a mutation happened in some of the bacteria, and that mutation made them resistant.” This is the most sophisticated wrong answer in the topic, because it uses the right vocabulary — mutation, resistant, allele — in exactly the wrong order.

The TruthMutation is random. It is not triggered by the thing it happens to protect against, and it does not arrive on demand. The resistance allele arises by a random mutation that has already happened, in a few individuals, before the antibiotic is used at all. What the antibiotic then does is kill the bacteria that do not carry it. The antibiotic is not the cause of the change; it is the selection pressure that determines which existing bacteria survive to reproduce.

Why It MattersChronology is the marking point. Cambridge writes questions where a graph starts at 2 % rather than at zero precisely so that you can say “resistant bacteria were already present before the drug was used”. If you make the mutation a response to the drug, you have also, without meaning to, said that the environment can direct the DNA — and that is the giraffe-stretching-its-neck idea in modern clothes.

Example Question“Fig. 3.1 shows that 2 % of samples were already resistant in 1998. Explain the significance of this. [2]”

⚠️ TRAP — 18.2

Trap 4: “The plant developed a thick cuticle because it needed to save water”

The TrapThe classic xerophyte answer: “the plant developed a thick waxy cuticle because it needed to save water, and it rolled its leaves so that they can keep the stomata inside.” Every fact in it is correct and the sentence still scores badly, because of the two words developed and because.

The TruthAn adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. The plant did not develop it; it inherited it. Write the feature, then the consequence, and let the consequence do the explaining: “the cuticle is thick and waxy, which gives a longer diffusion distance for water vapour, so less water is lost by evaporation from the leaf surface.” Feature → mechanism → benefit. No motive anywhere.

Why It MattersAdaptive-feature questions are usually worth a mark per feature and a mark per explanation, so a list of features with a motive attached to each can collect half the marks available. The safe sentence pattern is: “X is Y, which means Z, so less water is lost / more light is absorbed / more gas is exchanged.” It also connects straight back to Topic 8.3 — the factors that affect transpiration are exactly the factors a xerophyte leaf is arranged to reduce.

Example Question“Explain how two features visible in Fig. 4.1 reduce the loss of water from this leaf. [4]”

⚠️ TRAP — 18.2

Trap 5: “The species adapted itself to the desert”

The Trap“Over time the species adapted itself to the desert, so now it has spines and shallow roots.” Or the same idea with a different subject: “the cactus adapted to living in dry conditions.” Both sentences make the organism the agent of its own change.

The TruthTwo different words are hiding here and Cambridge marks them separately. An adaptive feature is a thing — an inherited feature that helps the organism survive and reproduce. Adaptation is a process: the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Notice every word of that definition. It is populations, not individuals; it happens over many generations, not within a lifetime; and its cause is named — natural selection, not effort.

Why It Matters“The population became adapted” is a scoring sentence. “The species adapted itself” is not, and it is not a style point — the two describe genuinely different claims about what happened to the DNA. Rewrite it every time as: “the individuals that already had the feature were more likely to survive and reproduce, so a greater proportion of each generation carried it.”

Example Question“Define adaptation, and explain the difference between an adaptive feature and adaptation. [3]”

⚠️ TRAP — 18.3

Trap 6: “The bacteria became resistant so they could survive”

The Trap“The bacteria became resistant so they could survive the antibiotic”, and its close relatives “they got used to it”, “they built up a tolerance” and “they learned to survive it”. This is the single commonest sentence in the whole topic and it scores zero every time.

The TruthNo bacterium ever becomes resistant. A few bacteria were already resistant, because a random mutation had produced a resistance allele — often carried on a plasmid (Topic 2.1). When the antibiotic is used, the non-resistant bacteria are killed. The resistant ones survive, reproduce, and pass the allele to their offspring, so the proportion of the population that is resistant increases. The population changed. No individual did.

Why It MattersThis is the link back to Topic 15. There you learned that antibiotics kill bacteria but not viruses, that resistant strains such as MRSA reduce the effectiveness of antibiotics, and that you should only use antibiotics when they are essential and always finish the course. Topic 15 had to state the selection logic in the question stems, because this topic had not been taught yet. This is where it belongs, and the reason for finishing the course now makes sense: stopping early leaves behind the bacteria that were hardest to kill, and those are exactly the ones that then reproduce.

Example Question“Explain how a population of bacteria in a hospital became almost entirely resistant to one antibiotic. [5]”

⚠️ TRAP — 18.3

Trap 7: “The giraffe stretched its neck and passed it on”

The Trap“Giraffes stretched their necks to reach the higher leaves, their necks got longer, and they passed the longer necks on to their offspring.” It is a genuinely reasonable-sounding story, it was seriously proposed a very long time ago, and it is wrong in a way worth understanding rather than just avoiding.

The TruthA feature you acquire during your life is not inherited, because it is not in the alleles in your gametes. Stretching does not rewrite DNA. The correct account reverses the order: there was already variation in neck length in the population, caused by different alleles. When food low down was scarce, the individuals with longer necks were better adapted — they could reach more food, so they were more likely to survive and reproduce and passed those alleles on. Over many generations the proportion of the population with longer necks increased.

Why It MattersTest any explanation you write with one question: would this still work if the individual never changed at all? A correct natural selection answer survives that test, because nothing in it depends on an individual changing — only on which individuals reproduced. Anything that fails the test is this trap.

Example Question“A student suggests that the ancestors of modern giraffes stretched their necks to reach food and passed longer necks to their offspring. Explain why this is not correct, and give the correct explanation. [4]”

⚠️ TRAP — 18.3

Trap 8: Letting an individual evolve during its own lifetime

The Trap“The mosquitoes that were sprayed evolved resistance”, “the rat adapted after eating the poison”, “this bacterium has changed into a resistant one”. Not a motive this time — just the wrong noun. A single organism is being made to do something only a population can do.

The TruthAn individual is stuck with the alleles it was born with. It can grow, get ill, get stronger, get sunburnt — none of that changes what it passes on. Only populations change across generations, and they change because the mix of individuals in them changes: some reproduce, some do not. A useful mental picture is a bag of counters. Selection never repaints a counter. It only takes counters out and lets the ones remaining make copies of themselves.

Why It MattersThe word to watch in your own writing is any singular subject: the bacterium, the mosquito, the plant. If your explanation has one organism at the start and the same organism at the end, it is the wrong shape. Start with “in the population there was variation…” and the grammar itself will keep you correct.

Example Question“A student writes: ‘the mosquitoes changed and became resistant to the insecticide’. Rewrite this using the correct biological explanation. [3]”

⚠️ TRAP — 18.3 and 18.4

Trap 9: Making selection create the variation instead of acting on it

The Trap“Natural selection produces the useful features”, or, on the artificial side, “selective breeding gave the maize longer cobs”. Both make the selecting step the source of the new feature, when selection can only ever choose among features that are already present.

The TruthThere are two separate stages and they happen in a fixed order. Variation comes first, and it comes from mutation — plus, in organisms that reproduce sexually, meiosis, random mating and random fertilisation, which shuffle existing alleles into new combinations. Selection comes second, and all it does is determine which of those individuals reproduce. That is true whether the selecting is done by the environment or by a farmer. The farmer who bred longer maize cobs did not add anything to the maize; the longer-cobbed plants were already in the field.

Why It MattersThis is why every good answer opens with variation. It is also why selective breeding eventually slows down: after enough generations of choosing the same extreme, there is less variation left in the population to select from, and further improvement gets harder. A question that says “the mean stopped increasing after generation 12 — suggest why” is asking for exactly that.

Example Question“Explain why selective breeding cannot continue to increase yield indefinitely. [3]”

⚠️ TRAP — 18.3

Trap 10: Using “survival of the fittest” as if it were an explanation

The Trap“It is survival of the fittest — the strongest ones survive and the weak ones die out.” The phrase feels like the answer to the whole topic, which is why it gets written instead of the answer to the whole topic.

The TruthThe phrase explains nothing, because it does not say what made those individuals more likely to survive, and “fittest” in everyday speech means strongest or healthiest, which is often simply not the case. The advantage may be a thicker shell, a longer root, an enzyme that breaks down a drug, or a colour that a predator does not notice. Cambridge wants “better adapted”, and it wants you to name the feature and say what advantage it gives in that particular environment. “Stronger”, “fitter” and “superior” are not marking points.

Why It MattersBeing better adapted is always relative to one environment. The bacterium with the resistance allele is better adapted while the antibiotic is present, and may be at a disadvantage when it is withdrawn, because making the extra protein costs it resources. A challenge question that removes the selection pressure and asks what happens next is testing whether you understood that, or whether you thought resistance was simply an improvement.

Example Question“An answer states that resistant bacteria survive because they are stronger. Explain why this is not an adequate explanation. [2]”

⚠️ TRAP — 18.3 and 18.4

Trap 11: “Natural selection tries to improve the species”

The Trap“Natural selection tries to improve the species so it is better suited to its habitat”, or “nature selects the best organisms”, or “the aim of natural selection is to make the population fitter”. It gives the process a purpose and a target, and neither exists.

The TruthNatural selection has no goal and no foresight. It is simply the outcome of the fact that more offspring are produced than can survive and that some individuals are more likely than others to reproduce. Nothing is being aimed at. This is precisely where artificial selection is different: in selective breeding there genuinely is an aim, because a human has one and chooses which individuals are crossed. That contrast — purpose in one, no purpose in the other — is the sharpest way to remember which is which.

Why It MattersThe consequence is worth marks: because natural selection has no target, a population is only ever adapted to the environment it has had, not the one that is coming. If the environment changes quickly, being well adapted to the old one is no protection at all. That is why variation within a population matters so much, and it is the answer to “suggest why a population with little variation is at greater risk”.

Example Question“Explain why natural selection cannot prepare a population for a change in its environment that has not yet happened. [3]”

⚠️ TRAP — 18.4

Trap 12: “Natural and artificial selection are the same thing, just at different speeds”

The Trap“Artificial selection is the same as natural selection but faster because humans speed it up.” Speed is the difference most students reach for, and on its own it is not the difference the mark scheme is looking for.

The TruthThe difference is who or what does the selecting. In natural selection the environment selects, and what is selected for is whatever improves survival and reproduction in that environment. In artificial selection humans select, and what is selected for is whatever humans find desirable — which may actually make the organism worse at surviving in the wild, as with a wheat plant whose head is too heavy to disperse its own seed. Speed is a consequence, not the definition, and even artificial selection takes many generations. And note what is the same in both: the variation is already there, produced by mutation, and selection acts on it.

Why It Matters“State two differences between natural and artificial selection” is a standard Supplement question, and “one is faster” will get at most one of them. Have three ready: who selects, what is selected for, and the effect on variation — artificial selection repeatedly crossing the same chosen type tends to leave the population with less variation.

Example Question“Outline three differences between natural selection and artificial selection. [3]”

🔍 Step-by-Step Walkthroughs

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Six challenge-level questions with real data, worked in the order you should actually think about them. Try each part yourself before you reveal the next step — the reveals get harder, not easier.

Walkthrough 1 — Resistance in One Hospital, 1998 to 2026

(a) Describe the change shown in Fig. 3.1 between 1998 and 2026. [2] (b) Calculate the mean increase in the percentage of resistant samples per year between 2010 and 2018. Show your working. [2] (c) The value for 1998 is 2 % and not 0 %. Explain why this is important. [2] (d) Explain, in terms of natural selection, why the percentage of resistant samples increased. [4] (e) The hospital reduced its use of this antibiotic in 2023. Suggest what the graph might show for the years after 2026, and explain your suggestion. [2]

Quote the two ends and then say something about the shape in between

A “describe” mark is almost never given for “it goes up”. Read the two endpoints off the axis: 2 % in 1998 rising to 63 % in 2026, an increase of 61 percentage points. Then say what the shape does: the curve is not a straight line — it is shallow at first, steepest in the middle (roughly 2010–2018) and it begins to level off after 2022. Two marks, and the second one lives entirely in that sentence about the shape.

Read both points off the graph first, then divide

In 2010 the value is 21 %; in 2018 it is 47 %. The change is 47 − 21 = 26 percentage points, over 2018 − 2010 = 8 years. So the mean increase is 26 ÷ 8 = 3.25 % per year. Write the subtraction and the division down: on a Paper 4, a correct answer with no working gets the marks, but a wrong final number with correct working usually still collects the method mark, and a naked wrong number gets nothing. Give the unit: % per year.

That 2 % is the variation, sitting there before the selection starts

The starting value is deliberate. It tells you that resistant bacteria were already present in 1998, before the heavy use of this antibiotic. So the resistance was not produced by the antibiotic; it had already arisen by random mutation, producing a new allele. The antibiotic did not cause the change — it acted as the selection pressure that determined which bacteria survived to reproduce. If the graph had started at zero, you could not have said any of that, which is exactly why it does not.

The five-step chain, compressed to fit a four-mark box

1. There was variation in the bacterial population: a few individuals carried a resistance allele produced by a random mutation, often on a plasmid. 2. Bacteria reproduce very rapidly, producing far more offspring than can survive, and they compete for nutrients and space. 3. When the antibiotic is used, the bacteria without the allele are killed; those with it are better adapted to this environment and are more likely to survive and reproduce. 4. The survivors pass the allele to their offspring, so the proportion of the population that is resistant increases with every generation. Nothing in those four sentences has a bacterium changing itself.

Resistance is an advantage only while the antibiotic is there

If the antibiotic is used far less, the resistant bacteria lose their advantage. Producing the extra protein that gives resistance costs resources, so where there is no antibiotic the non-resistant bacteria may grow and reproduce faster and out-compete them. The sensible prediction is therefore that the curve levels off and then falls slowly — slowly, because the resistance allele does not disappear; it is simply no longer favoured. Saying “it will drop back to 2 % immediately” misses that. This part is really a test of Trap 10: being better adapted is always relative to this environment.

Full Mark-Scheme Answer(a) Rises from 2 % in 1998 to 63 % in 2026 (an increase of 61 percentage points) [1]; the increase is not constant — slow at first, steepest between about 2010 and 2018, levelling off after 2022 [1]. (b) 47 − 21 = 26; 26 ÷ 8 [1] = 3.25 % per year [1]. (c) Resistant bacteria were already present before the antibiotic was used heavily [1]; so resistance arose by random mutation and was not caused by the antibiotic — the antibiotic acted as the selection pressure [1]. (d) Variation existed: some bacteria carried a resistance allele from a random mutation [1]; the antibiotic killed the bacteria without the allele, while those with it survived because they were better adapted [1]; the survivors reproduced [1]; and passed the allele to their offspring, so the proportion of resistant bacteria in the population increased [1]. (e) The percentage would level off and then slowly decrease [1], because without the antibiotic the resistant bacteria no longer have an advantage and producing the resistance protein costs resources, so non-resistant bacteria compete successfully with them [1].

Examiner’s NotePart (c) is the highest-value two marks on the page and almost everybody wastes them by writing “it shows resistance was low”. It is not about the size of the number; it is about the date. Also: this is the graph Topic 15 could not fully explain. In Topic 15 you were told that resistant strains such as MRSA reduce the effectiveness of antibiotics and that a course must be finished. Part (d) is the reason.

Walkthrough 2 — One Population of Bacteria, Photographed Three Times

Fig. 2.1 shows one population of bacteria in a patient at three times. Between A and B an antibiotic was given. Between B and C several hours passed and no further antibiotic was given.
(a) State the number of type Q cells in A and in C. [1] (b) Suggest what type P and type Q represent. [2] (c) Explain what happened between A and B. [2] (d) Explain what happened between B and C, and why the population in C is different from the population in A. [3] (e) A student says the antibiotic turned type P bacteria into type Q bacteria. Explain why this is wrong. [2]

There are 30 cells in A, and 2 of them are type Q

Count them. Panel A has 30 cells: 28 type P and 2 type Q. Panel B has the same 2 type Q, plus 28 that are drawn crossed out — dead. Panel C has 30 cells and all of them are type Q. That counting is the answer to almost every part below, and it is worth doing before you write a single word of explanation, because the numbers tell you that the two type Q cells in A are the ancestors of everything in C.

The ones that died were sensitive; the ones that lived carried the allele

Type P cells are killed when the antibiotic arrives, so type P are the bacteria that are not resistant (sensitive to this antibiotic). Type Q cells survive, so type Q are the bacteria that carry an allele giving resistance to it — an allele produced by a random mutation, and in bacteria often carried on a plasmid. Notice you were not told this. You worked it out from which cells were still alive in B, which is exactly the kind of reading a challenge paper is testing.

Two different processes, drawn as two different arrows

A → B: the antibiotic kills the 28 sensitive bacteria, leaving only the 2 that already carried the resistance allele. That is the selecting step, and it is the only thing the antibiotic does. B → C: the 2 survivors reproduce. Bacteria divide roughly every 20 minutes, so 2 cells become 4, 8, 16, 30 within a couple of hours — and because they divide by mitosis-like division they produce genetically identical offspring, all carrying the resistance allele. The population in C is therefore the same size as in A but is now entirely resistant: the individuals were replaced, and the proportion carrying the allele went from 2 in 30 to 30 in 30.

No cell in this diagram ever changes type

Follow any individual cell through the three panels and it never changes. The type Q cells in A are already type Q, before the antibiotic arrives — that is the variation, and it was produced by a random mutation, not by the drug. The antibiotic cannot alter the DNA of a living bacterium into a resistant form; all it can do is kill the ones that lack the allele. What changed is the composition of the population, not any individual in it. If you can say that sentence about this diagram, you can say it about every natural selection question on the paper.

Full Mark-Scheme Answer(a) 2 in A and 30 in C [1]. (b) P = bacteria that are not resistant to the antibiotic, since they are killed [1]; Q = bacteria carrying an allele for resistance, produced by a random mutation, since they survive [1]. (c) The antibiotic killed the bacteria that were not resistant [1]; the 2 resistant bacteria were unaffected and survived [1]. (d) The surviving resistant bacteria reproduced, rapidly, and passed on the resistance allele [1]; their offspring are genetically identical to them and therefore also resistant [1]; so the population is the same size but all of it is now resistant, whereas only 2 out of 30 were before [1]. (e) The antibiotic cannot change one type of bacterium into another — it does not alter their DNA [1]; the resistant bacteria were already present in A, before the antibiotic was given, and the antibiotic only removed the rest [1].

Examiner’s NotePart (d) is worth three marks and the third one is the comparison — “2 out of 30 before, 30 out of 30 after”. Quoting figures from a diagram earns marks that a general statement does not. Here is the same sequence with the five steps written on it:

Walkthrough 3 — Two Surveys, Two Shapes

(a) State which chart shows continuous variation and which shows discontinuous variation, and give one piece of evidence from the charts for each. [3] (b) Using Chart A, calculate the percentage of the 64 students who were 165 cm or taller. [2] (c) Using Chart B, calculate how many of the 1000 people surveyed were blood group B. [1] (d) Explain why height varies in the way shown in Chart A but blood group does not. [3] (e) Both charts show variation within a single species. Explain why variation of either kind matters to a population living in a changing environment. [2]

Touching bars against separated bars

Chart A shows continuous variation: the bars touch and cover a continuous range of heights from 145 to 180 cm, with every value in between possible — a range of phenotypes between two extremes. Chart B shows discontinuous variation: there are only four categories with no intermediates — there is no blood group between A and B, and the bars are separated for exactly that reason. Say “the bars touch” or “there are no values in between”; that is the evidence mark, and it is the part most answers leave out.

Add the right bars, then convert

For (b), the classes at 165 cm and above are 165–170 (14 students), 170–175 (7) and 175–180 (3). That is 14 + 7 + 3 = 24 students. As a percentage of 64: 24 ÷ 64 × 100 = 37.5 %. For (c), Chart B is already in percentages, so 15 % of 1000 = 150 people. Two easy marks, and the only way to lose them is to mis-read which bars the boundary includes — check the axis labels before you add.

Genes and the environment, versus genes only

Height is continuous because it is caused by genes and the environment together: many genes contribute, and on top of that diet, health and other environmental factors move an individual within the range their alleles allow. That combination of many small influences is what produces a smooth spread with most people near the middle. ABO blood group is discontinuous because it is determined by genes only — and by a single gene, so the phenotype falls into a limited number of categories with no intermediates. No amount of diet, exercise or illness moves anybody from group O to group A.

Variation is the raw material selection has to work with

Natural selection cannot act on a population in which every individual is the same — there would be nothing to select between. Because there is variation, some individuals will by chance be better adapted when the environment changes; those are more likely to survive and reproduce and to pass their alleles on, so the population can become better suited to the new conditions over many generations. A population with very little variation has no such individuals available, and a single new disease or a change in climate can remove all of it. That is one sentence of 18.1 and one of 18.3, and challenge papers join them constantly.

Full Mark-Scheme Answer(a) Chart A: continuous, because the bars touch / heights form a range between two extremes with all intermediate values possible [1–2]; Chart B: discontinuous, because there are only four separate categories and no intermediates [1]. (b) 14 + 7 + 3 = 24 [1]; 24 ÷ 64 × 100 = 37.5 % [1]. (c) 15 % of 1000 = 150 people [1]. (d) Height is controlled by genes and the environment [1], for example diet, so an individual can take any value within a range [1]; ABO blood group is controlled by genes only, so there are a limited number of phenotypes with no intermediates [1]. (e) Variation means some individuals may already be better adapted to the new conditions [1]; those individuals are more likely to survive and reproduce and pass on their alleles, so the population can become better suited over many generations [1].

Walkthrough 4 — Nine Years of Spraying One Village

(a) Describe the relationship between the two sets of results in Table 7.1. [2] (b) Calculate the percentage decrease in the proportion of mosquitoes killed between year 1 and year 9. [2] (c) Explain, in terms of natural selection, why the percentage killed fell. [4] (d) The village council proposes to double the dose of the same insecticide. Suggest why this may not work in the long term. [2] (e) Suggest one other method of reducing the number of cases of malaria in the village. [1]

Link the columns, and quote figures from both

The command word is “relationship”, so one column must be described in terms of the other. As the percentage killed falls from 99 % to 26 %, the number of malaria cases rises — but not smoothly. It falls at first (240 to 205 by year 3, while the spray is still killing 95 %) and then rises steeply to 470 by year 9, nearly double where it started. That non-monotonic shape is deliberate: the spraying did work at the beginning, and the marks are for noticing both halves.

Divide by the starting value, not by 100

The trap in this calculation is answering “73 %” because 99 − 26 = 73. That is the decrease in percentage points, not the percentage decrease. A percentage change is always change ÷ original value × 100: 73 ÷ 99 × 100 = 73.7 % (to 1 d.p.). Read the question wording every time — “percentage decrease” and “decrease in percentage” are two different sums and Cambridge uses both.

Write the four sentences and change only the nouns

Variation: in the mosquito population there was already genetic variation — a few individuals carried an allele giving resistance to the insecticide, produced by a random mutation before spraying began. Many offspring and a struggle to survive: mosquitoes produce very large numbers of offspring, most of which do not survive. Selection: each spraying kills the mosquitoes without the allele; the resistant ones are better adapted to this environment and are more likely to survive and reproduce. Inheritance: they pass the allele to their offspring, so the proportion of the population that is resistant increases every generation — which is exactly what a falling percentage killed measures. Notice this is word for word Walkthrough 1 with “bacteria” and “antibiotic” swapped out. That is the point of the topic.

A stronger selection pressure selects harder

Doubling the dose will kill more mosquitoes at first, including some of the less resistant ones. But it does not remove the resistance allele from the population — it applies an even stronger selection pressure, so the only survivors are the most resistant individuals of all, and they are the ones that reproduce. The population becomes more resistant, faster. There are also costs the question will accept: harm to other species including the mosquitoes’ predators and useful insects, and the expense. The examiner-friendly alternative is to use a different insecticide, because a mosquito resistant to one is not automatically resistant to another, or to reduce reliance on insecticide altogether.

Full Mark-Scheme Answer(a) As the percentage of mosquitoes killed falls (99 % to 26 %), the number of malaria cases rises (240 to 470) [1]; but the cases fall at first, from 240 to 205 by year 3, while the insecticide is still killing over 90 % [1]. (b) 99 − 26 = 73; 73 ÷ 99 × 100 [1] = 73.7 % [1]. (c) There was already genetic variation: some mosquitoes carried an allele for resistance, from a random mutation [1]; the insecticide killed the mosquitoes that did not carry it [1]; the resistant mosquitoes were better adapted and were more likely to survive and reproduce [1]; they passed the allele to their offspring, so the proportion of resistant mosquitoes in the population increased over the generations [1]. (d) A higher dose applies a stronger selection pressure, so only the most resistant survive and reproduce, and the population becomes more resistant still [1]; it may also harm other organisms, including the natural predators of mosquitoes, or be too expensive to sustain [1]. (e) Any one of: use a different insecticide; remove standing water where the mosquitoes breed; use insecticide-treated bed nets; treat infected people so the pathogen is not passed on [1].

Examiner’s NotePart (d) separates grades. The weak answer is “they will just get used to the higher dose too” — the same teleology in a new place. The strong answer names the mechanism: a stronger selection pressure means a narrower set of survivors, and those survivors are the parents of the next generation.

Walkthrough 5 — A Leaf You Have Never Seen Before

Fig. 4.1 shows a cross section through the leaf of a plant that grows on a sand dune, where water drains away rapidly and the wind is almost constant.
(a) Name the features labelled A, B and D. [3] (b) Explain how B and D each reduce the loss of water from this leaf. [4] (c) Suggest the function of the features labelled C. [2] (d) Define the term adaptive feature. [2] (e) Explain how a population of this species came to have these features. [4]

The leaf is rolled, and everything important is on the inside

Start with the shape. The leaf is rolled into a cylinder, so one surface is now on the outside, exposed to the air, and the other is enclosed. A is the thick band along the outer surface: a thick waxy cuticle. B is a stoma sitting at the bottom of a pit rather than flush with the surface: a sunken stoma. D marks the rolled edge itself: the rolled leaf. C are the fine projections growing into the enclosed space: hairs. E points at the thickness of the leaf tissue. A plant like this — adapted to survive in very dry conditions — is a xerophyte.

Two marks each, and the second mark is always the mechanism

B, the sunken stoma: it lies in a pit, so water vapour collects in the pit just outside the stoma. That makes the air there more humid, which reduces the concentration gradient of water vapour between the inside of the leaf and the air outside, so less water vapour diffuses out. D, the rolled leaf: rolling encloses the stomata inside a chamber, away from the moving air, so the humid air is not blown away and, again, the gradient stays small. This is Topic 8.3 in disguise: the factors that increase transpiration are wind, low humidity and heat, and every feature here attacks one of them.

Hairs trap still air; an adaptive feature is inherited

The hairs trap a layer of still, humid air next to the stomata, which does the same job as the rolling by a different route: less air movement over the stomata means the water vapour is not carried away, so the gradient is smaller and less water is lost. Now the definition, which is worth having word-perfect: an adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment. Two marking points: inherited, and survive and reproduce. Answers that say “a feature that helps it live in its habitat” get one at best, because they leave out both.

“How did it come to have them” is never answered by describing them again

Most of the marks in (e) are lost by candidates re-describing the features. The question asks about origin, so it wants the chain. There was variation in the ancestral population — caused by mutation, some plants had thicker cuticles or more sunken stomata than others. More offspring were produced than could survive, and there was competition for water. The plants with those features lost less water, so they were better adapted to the dry conditions and were more likely to survive and reproduce. They passed those alleles to their offspring, so over many generations the proportion of the population with the features increased. That is adaptation: a process, acting on a population, over generations.

Full Mark-Scheme Answer(a) A = thick waxy cuticle [1]; B = sunken stoma (a stoma in a pit) [1]; D = rolled leaf [1]. (b) B: water vapour is trapped in the pit so the air outside the stoma is more humid [1], reducing the concentration gradient so less water vapour diffuses out [1]. D: rolling encloses the stomata away from moving air [1], so the humid air is not removed and the gradient is kept small [1]. (c) The hairs trap a layer of still, humid air next to the stomata [1], reducing the concentration gradient and so reducing water loss [1]. (d) An inherited feature [1] that helps an organism to survive and reproduce in its environment [1]. (e) There was variation in the population, caused by mutation, so some plants already had thicker cuticles / more sunken stomata [1]; more offspring were produced than could survive and there was competition for water [1]; those plants lost less water, were better adapted and were more likely to survive and reproduce [1]; they passed the alleles on, so the proportion of the population with these features increased over many generations [1].

Examiner’s NoteNever write that the plant developed these features to cope with the dryness. Write what the feature is, what it does to the gradient, and what follows. Here is the same figure with the mechanisms written on:

Walkthrough 6 — A Drought on One Island

(a) Calculate the percentage decrease in the number of birds between year 1 and year 3. [2] (b) Calculate the percentage increase in the mean beak depth of the birds present. [2] (c) Explain, using the data, why the mean beak depth of the surviving birds was greater than that of the original population. [4] (d) The chicks hatched in year 3 had a mean beak depth of 10.0 mm rather than 9.4 mm. Explain what this shows. [2] (e) A student writes that “the finches adapted their beaks to the harder seeds”. Explain why this is not an acceptable answer. [2]

Same formula both times: change divided by the original

(a) The number falls from 751 to 90, a change of 661. 661 ÷ 751 × 100 = 88.0 % — nearly nine birds in ten died. (b) Mean beak depth rises from 9.4 to 10.1 mm, a change of 0.7 mm. 0.7 ÷ 9.4 × 100 = 7.4 %. Hold on to the contrast between those two numbers: an enormous death rate produced only a small shift in the mean. That is what real selection looks like, and it is why it takes many generations to produce anything dramatic.

The seeds got harder, and that is the whole cause

The third row is the row most candidates skip. The mean hardness of the seeds available rose from 4.5 to 6.8, because the drought killed the plants that produced the small soft seeds and left the tough ones. The footnote tells you a deeper beak can crack a harder seed. So the environment changed in a specific, measurable way, and that change is what determined which birds could feed. Always look for the row that describes the environment — that is the selection pressure, and quoting its figures is usually a mark.

The variation was already there — the footnote says so

Variation: beak depth shows continuous variation, so before the drought the population already contained birds with beaks deeper and shallower than 9.4 mm. Nothing new was created. Struggle for survival: the drought reduced the food supply and the remaining seeds were harder (4.5 → 6.8), so there was intense competition for food. Selection: birds with deeper beaks could crack the harder seeds, so they were better adapted to these conditions and were more likely to survive; birds with shallower beaks were more likely to starve, which is why 88 % of the population died. Result: the survivors were not a random sample — they were the deeper-beaked ones, so the mean of the survivors is higher at 10.1 mm.

10.0 mm in birds that never experienced the drought

The chicks hatched after the drought. They never had to crack a hard seed, and yet their mean beak depth is 10.0 mm, close to their parents’ 10.1 and well above the original 9.4. That is the evidence that beak depth is inherited — the surviving birds passed on the alleles for deeper beaks, so the change is carried into the next generation. Without this row, all you could show is that deep-beaked birds survived; with it, you can show that the population itself has changed. And it kills part (e) stone dead: no bird altered its own beak. The birds that could already feed survived and reproduced, and their offspring inherited the alleles. An individual cannot adapt; a population becomes adapted, over generations.

Full Mark-Scheme Answer(a) 751 − 90 = 661; 661 ÷ 751 × 100 [1] = 88.0 % [1]. (b) 10.1 − 9.4 = 0.7; 0.7 ÷ 9.4 × 100 [1] = 7.4 % [1]. (c) Beak depth shows continuous variation, so birds with deeper and shallower beaks were already present before the drought [1]; the drought made food scarce and the seeds harder (4.5 to 6.8), so there was competition for food [1]; birds with deeper beaks could crack the harder seeds, were better adapted and were more likely to survive [1]; the survivors were therefore mainly the deeper-beaked birds, raising the mean to 10.1 mm [1]. (d) The chicks did not experience the drought, so their deeper beaks cannot be a response to it [1]; beak depth is inherited — the surviving birds passed the alleles for deeper beaks to their offspring, so the change is passed to the next generation [1]. (e) No individual finch changed its own beak; an individual cannot adapt during its lifetime [1]; the variation existed first, and the population became adapted because the deeper-beaked birds survived and reproduced and passed on their alleles [1].

Examiner’s NoteThe row about the chicks is the row that turns this from a story into evidence, and it is where the highest mark sits. Also notice what is not claimed anywhere: nothing here says a new species appeared. One population changed slightly in one measurement over two years, which is all the data supports and all Cambridge asks for.

🔍 Spot the Difference

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Six pairs that look like the same question and are not. In Topic 18 the pair is nearly always a thing against a process, or an individual against a population.

Question A

What is continuous variation, what causes it, and give two examples.

A range of phenotypes between two extremes, with all the intermediate values possible. Caused by genes and the environment together. Examples: body length and body mass. On a chart the bars touch, and the shape is usually a smooth spread with most individuals near the middle.

Question B

What is discontinuous variation, what causes it, and give two examples.

A limited number of phenotypes with no intermediates. Usually caused by genes only. Examples: ABO blood groups, seed shape in peas, seed colour in peas. On a chart the bars are separate, because there is nothing that could sit between the categories.

Key DifferenceTwo differences, not one: the shape (a range against separate categories) and the cause (genes and environment against genes only). Most answers give the shape and stop, so the cause is the mark that is actually available. And the word and in “genes and the environment” is not decoration — leaving it out is Trap 1.

Question A

Describe natural selection.

(a) Genetic variation within the population; (b) production of many offspring; (c) a struggle for survival, including competition for resources; (d) individuals that are better adapted have a greater chance of reproduction; (e) those individuals pass on their alleles to the next generation. The selecting is done by the environment.

Question B

Describe selective breeding (artificial selection).

(a) Humans select the individuals with the desirable features; (b) those individuals are crossed; (c) the offspring showing the desirable features are selected and crossed again — repeated over many generations. Used to improve crop plants and domesticated animals.

Key DifferenceWho or what does the selecting — the environment, or a human being. From that one difference everything else follows: natural selection favours whatever improves survival and reproduction in that environment, artificial selection favours whatever humans find desirable, which may leave the organism less able to survive in the wild. Artificial selection also tends to leave the population with less variation. What is the same in both: the variation comes first, from mutation, and selection only acts on what is already there. It is not just a difference of speed (Trap 12).

Question A

What is a mutation?

A genetic change; a gene mutation is a random change in the base sequence of DNA. It is the way in which new alleles are formed — the only original source of new genetic material. Its rate is increased by ionising radiation and some chemicals. It is a cause.

Question B

What is variation?

Differences between individuals of the same species. It is what you can see or measure in a population — the spread of heights, the four blood groups, the range of beak depths. Its sources are mutation, meiosis, random mating and random fertilisation. It is an observed result.

Key DifferenceOne is the event in the DNA; the other is the differences you can observe across a population. Mutation produces new alleles; meiosis, random mating and random fertilisation only reshuffle alleles that already exist — which is why mutation is the one that must be named when a question asks where something genuinely new came from. Note also that some variation is not genetic at all: a plant kept in the dark is shorter, and that difference is not inherited.

Question A

Define an adaptive feature.

An inherited feature that helps an organism to survive and reproduce in its environment. It is a structure or characteristic you could point to: a sunken stoma, a thick cuticle, a deep beak, an enzyme that breaks down a drug.

Question B

Define adaptation.

The process, resulting from natural selection, by which populations become more suited to their environment over many generations. It is not a thing at all — it is what happened, across generations, to produce the things in Question A.

Key DifferenceA noun you can point at against a process you can only describe. Three words in definition B carry the marks: populations (not individuals), many generations (not a lifetime) and natural selection (its named cause). If a question says “define adaptation” and you describe a cactus spine, you have answered the other question.

Question A

What is a xerophyte, and how is it adapted?

A plant adapted to survive in very dry conditions. Thick waxy cuticle (longer diffusion distance, less evaporation); sunken stomata in pits and hairs (trap humid air, reducing the concentration gradient); rolled leaves (stomata enclosed, away from moving air); small surface area such as spines or needles; few stomata. Every feature reduces water loss.

Question B

What is a hydrophyte, and how is it adapted?

A plant adapted to living in water. Stomata on the upper surface only (the lower surface is under water, where a stoma could exchange nothing); large air spaces in the leaf and stalk (buoyancy, and a store of gases for photosynthesis and respiration); flexible stalk (bends with moving water instead of breaking); thin or no cuticle and little supporting tissue, because the water supports the plant.

Key DifferenceThey face opposite problems, so several features are exact mirror images. The one examiners ask for most is the position of the stomata: a land plant has most of them on the lower surface, a hydrophyte with a floating leaf has them on the upper surface. The hydrophyte’s difficulty is not obtaining water — it is obtaining gases, since very little is dissolved in water, and staying at the surface in the light.

Question A

What does “the population became adapted” mean, and why is it acceptable?

It means the proportion of individuals carrying a particular allele increased over many generations, because the individuals carrying it were more likely to survive and reproduce. The subject is a population, the timescale is generations, and the mechanism is inheritance of alleles. This is the sentence the mark scheme is written around.

Question B

What does “the individual adapted” claim, and why does it score nothing?

It claims that one organism changed itself during its own lifetime and then passed that change on. Nothing an organism acquires in its lifetime alters the alleles in its gametes, so nothing is passed on. It is the giraffe-stretching-its-neck idea, and it is refused wherever it appears — including in its polite disguises: “it got used to it”, “it built up resistance”, “it evolved”.

Key DifferenceThe subject of the sentence. Populations change across generations; individuals do not change what they pass on. Use this as a proofreading rule on every long answer you write in this topic: if the subject is singular — the bacterium, the plant, the finch — and the sentence describes a change, rewrite it with the population as the subject before you move on.

🔗 Variation and Selection Concept Map

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Click each node. The whole topic is three frameworks: where the variation comes from, what the environment then does to it, and what changes when a human does the selecting instead.

⭐ CORE FRAMEWORK 1

Random mutation → new alleles → reshuffled by meiosis, random mating and random fertilisation → variation you can measure

Mutation: the Only Source of Anything New ▶

Mutation = a genetic change. Gene mutation = a random change in the base sequence of DNA. Because a gene codes for a protein, changing the base sequence can change the sequence of amino acids and therefore the shape and function of the protein — which is Topic 17 doing the work underneath Topic 18.

Mutation is the way new alleles are formed. Nothing else on the syllabus creates a new allele. Meiosis, random mating and random fertilisation are powerful, but they only produce new combinations of alleles that already exist. If a question asks where a completely new characteristic came from, the answer is always mutation.

Most mutations have no effect; some are harmful; a few are advantageous in a particular environment. Ionising radiation and some chemicals increase the rate of mutation — but they do not choose which mutation occurs. Randomness is the point: the mutation is not produced by the problem it happens to solve.

Four Sources of Genetic Variation, and One That Is Not Genetic ▶

The Supplement list, worth learning as four: mutation (new alleles); meiosis (a reduction division producing genetically different gametes — Topic 17.3); random mating (which two individuals happen to breed); and random fertilisation (which gamete happens to fuse with which). The last three shuffle; only the first deals a new card.

There is also variation that is not genetic at all: a plant grown in the shade is taller and paler, a person who trains is more muscular. These differences are real but not inherited, so natural selection cannot act on them across generations. This is exactly why the giraffe idea fails, and it is why continuous variation being caused partly by the environment does not make the environment a source of new alleles.

Asexual reproduction, which you met in the reproduction topic, produces genetically identical offspring, so mutation is the only source of variation in an asexually reproducing population — which is precisely the situation with bacteria, and precisely why their enormous numbers and rapid division matter so much.

Two Shapes on a Chart, Two Different Causes ▶

Continuous variation: a range of phenotypes between two extremes — body length, body mass. Caused by genes AND the environment. Drawn with touching bars, because every intermediate value is possible.

Discontinuous variation: a limited number of phenotypes with no intermediates — ABO blood groups, seed shape in peas, seed colour in peas. Usually caused by genes only. Drawn with separated bars.

Why this is in the same topic as selection: variation is the raw material. Selection cannot act on a population whose individuals are all the same — there would be nothing to select between. Every natural selection answer therefore starts here, with the sentence “there was variation in the population, caused by mutation”.

⭐ CORE FRAMEWORK 2

Variation exists → many offspring → struggle for survival → the better adapted reproduce → alleles passed on

The Five Steps, in Cambridge’s Own Order ▶

1. Genetic variation within the population. The alleles are already there, produced by random mutation, before the environmental factor arrives. Selection does not create variation; it acts on it.

2. Production of many offspring — more than can possibly survive. 3. A struggle for survival, including competition for resources such as food, water, light, space and mates, plus predation and disease.

4. Individuals better adapted than others have a greater chance of reproduction. Say better adapted, and name the feature and the advantage it gives in that environment. 5. These individuals pass on their alleles to the next generation, so the proportion of the population showing the feature increases over many generations.

Learn it as five sentences you can write about any organism at all, changing only the nouns. That is what makes it worth five or six marks on demand, and it is why an unfamiliar organism in the question stem should be a relief rather than a shock.

Antibiotic Resistance: the Chain With Real Nouns In It ▶

1. A few bacteria carry an allele for resistance, produced by a random mutation and often carried on a plasmid (Topic 2.1). 2. Bacteria divide roughly every 20 minutes, so the population is enormous and so is the number of mutations occurring in it. 3. The antibiotic is used and the bacteria without the allele are killed; the rest compete for nutrients and space.

4. The resistant bacteria are better adapted to this environment and are more likely to survive and reproduce. 5. They pass the allele on, so the proportion of the population that is resistant rises — and because generations are 20 minutes long, a shift that would take a large animal thousands of years takes days.

The link to Topic 15. There you learned that antibiotics kill bacteria but have no effect on viruses, that resistant strains such as MRSA reduce the effectiveness of antibiotics, and that antibiotics should be used only when essential. This framework is the reason for all three. Finishing a course matters because stopping early leaves behind the bacteria that were hardest to kill — the exact individuals that then reproduce. And note carefully: the patient never becomes resistant. The bacterium does.

Adaptive Feature, Adaptation, and Why the Two Are Not Interchangeable ▶

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment — a thing. Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations — the history that produced the thing.

So framework 2 is the bridge between them: run the five steps for long enough and adaptive features accumulate in a population. That is the only mechanism on this syllabus by which they appear, which is why “how did the species come to have this feature” is always answered with the chain and never with a description of the feature.

Two consequences worth having ready. A population is adapted to the environment it has had, never to one that has not happened yet — natural selection has no goal and no foresight. And “better adapted” is always relative to a particular environment: the resistance allele is an advantage while the antibiotic is present and can be a disadvantage once it is withdrawn, because producing the protein costs resources.

⭐ CORE FRAMEWORK 3

Humans select → the chosen individuals are crossed → the best offspring are selected and crossed again → repeat over many generations

The Same Chain With One Step Replaced ▶

Put framework 2 and framework 3 side by side and only step 4 has changed. In natural selection, which individuals reproduce is determined by whether they survive in that environment. In artificial selection, which individuals reproduce is determined by a human choosing them. Steps 1, 2, 3 and 5 — variation from mutation, many offspring, competition, alleles passed on — are identical.

The three steps Cambridge marks: (a) humans select the individuals with the desirable features; (b) these individuals are crossed; (c) the offspring showing the desirable features are selected and crossed again. And then the phrase that is worth a mark of its own: repeated over many generations. An answer describing a single cross has not described selective breeding.

Contexts you should be able to handle cold: maize with longer cobs, cattle with a higher milk yield, wheat with a higher grain yield, sheep with heavier fleeces, dogs bred for size, coat or temperament. The wording of the answer does not change; only the desirable feature does.

Natural Against Artificial: the Table to Have in Your Head ▶

Who selects: the environment | a human. This is the difference. Everything below is a consequence of it.

What is selected for: features that improve survival and reproduction in that environment | features humans find desirable, which may actually reduce survival in the wild — a wheat head too heavy to scatter its own seed, a dog whose face is too flat to breathe easily.

Timescale: usually very many generations, slow | still many generations, but usually much faster, because the selection is deliberate and intense. Effect on variation: variation is maintained | repeatedly crossing the same chosen type usually leaves less variation in the population. Source of variation: mutation — the same in both.

Say it out loud once: the difference is not speed alone, and it is not that one uses variation and the other does not. The difference is who or what does the selecting. Trap 12 exists because that sentence is skipped.

What Selective Breeding Costs, and Why It Slows Down ▶

Because the same extreme is chosen every generation, the population ends up with less variation. That has a price: a population with little variation contains fewer individuals that might survive a new disease or a change in conditions, so a single pathogen can affect the whole crop or herd. This is the same argument as the asexual reproduction discussion in the reproduction topic, arriving from a different direction.

It also explains why improvement slows. Selection can only choose among the variation that is present; once most of the population is already at the chosen extreme, there is little left to select from, and the mean stops moving. New variation can then only come from mutation, which is rare and random.

And the sentence to avoid, in every context: the breeder did not give the organism the feature. The longer cobs, the higher yield and the heavier fleece were already present as variation in the population. All the breeder controlled was which individuals were allowed to reproduce — which is exactly what the environment controls in framework 2.

❌ “Why Is This Wrong?” Exercises

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Six answers that read fluently and score badly. Four of them fail on the same fault. See whether you can hear it before you reveal it.

Exercise 1: “Explain how a strain of bacteria in a hospital became resistant to an antibiotic. [5]”

Student’s Answer“The antibiotic was used on the bacteria over and over again, so the bacteria got used to it and became resistant so they could survive. They built up a tolerance to the drug and learned to survive it, and then they passed this on to the next bacteria. Now the antibiotic does not work on them any more.”

The FlawIt is fluent, it reaches the right destination, and it scores zero. Every verb has the bacteria doing the changing: got used to, became resistant, built up, learned. That describes individual bacteria altering themselves in response to the drug, which is the giraffe-stretching-its-neck idea in a laboratory coat. There is no variation before the antibiotic, no mutation, nothing is killed, and no allele is mentioned. The last sentence — passing it on — is the only true one, and there is nothing in the answer for it to be passing.

Correct Answer“There was variation in the bacterial population: a few bacteria already carried an allele for resistance, produced by a random mutation, before the antibiotic was used [1]. Bacteria reproduce very rapidly and compete for nutrients and space [1]. When the antibiotic was used, the bacteria without the allele were killed [1]; the resistant bacteria were better adapted to this environment and were more likely to survive and reproduce [1]. They passed the allele to their offspring, so the proportion of the population that was resistant increased with each generation [1].”

Key RuleTest every sentence with: could this be true if no individual bacterium ever changed? The correct answer passes, because nothing in it depends on an individual changing — only on which individuals were killed and which reproduced.

Exercise 2: “A village has sprayed the same insecticide for nine years and it now kills far fewer mosquitoes. Explain why. [4]”

Student’s Answer“The mosquitoes that were sprayed and survived evolved during their lifetime and their bodies changed so that they can break the insecticide down. Because the spraying kept happening, a mutation happened in the mosquitoes to protect them, and each mosquito passed its new resistance to its babies until the whole population was immune.”

The FlawThree separate errors, and the middle one is the subtle one. First, an individual cannot evolve during its lifetime — only a population changes across generations. Second, “a mutation happened to protect them” makes the mutation a response to the spraying: mutation is random, and the resistance allele was already present before the programme began. Third, “immune” is the wrong word — immunity is what the body’s white blood cells provide against a pathogen, and it is not what is happening here. Nothing was killed anywhere in this answer, so no selection has taken place at all.

Correct Answer“There was already genetic variation in the mosquito population: some individuals carried an allele giving resistance to the insecticide, produced by a random mutation before spraying began [1]. Each spraying killed the mosquitoes that did not carry the allele [1]. The resistant mosquitoes were better adapted to this environment, so they were more likely to survive and reproduce [1], and they passed the allele to their offspring, so the proportion of resistant mosquitoes increased with every generation [1].”

Key RuleMutation is random and comes first; the insecticide is a selection pressure and comes second. Any answer in which the environmental factor causes the mutation has the two halves of the topic in the wrong order.

Exercise 3: “Explain how the leaf of a xerophyte is adapted to a dry environment. [4]”

Student’s Answer“Because the plant lives somewhere dry, it developed a thick waxy cuticle because it needed to save water, and it rolled up its leaves so that they can hide the stomata from the wind. It also grew hairs when it realised it was losing too much water. Over time the plant adapted itself perfectly to the desert.”

The FlawThe features are all correct and the answer still collects almost nothing, for two reasons. First, every feature is given a motive — because it needed to, when it realised, adapted itself. An adaptive feature is inherited, not developed on demand. Second, and just as costly, no mechanism is ever given: the answer never says how a thick cuticle or a rolled leaf reduces water loss. Adaptive-feature questions are marked feature plus explanation, so an answer with four features and no explanations is throwing away half the marks even before the teleology.

Correct Answer“The cuticle is thick and waxy, which gives a longer diffusion distance for water vapour, so less water is lost from the leaf surface [1]. The stomata are sunken in pits, so water vapour collects above them and the concentration gradient of water vapour is reduced, so less diffuses out [1]. The leaf is rolled, enclosing the stomata away from moving air, so the humid air is not blown away [1]. Hairs trap a layer of still, humid air next to the stomata, which has the same effect [1].”

Key RuleFeature → mechanism → benefit, in that order, with no motive anywhere. If you want to say why the plant has the feature at all, that is a natural selection answer, not an adaptation description — and it starts with variation.

Exercise 4: “Describe the process of natural selection. [5]”

Student’s Answer“Natural selection is survival of the fittest. Nature tries to improve the species, so the strongest and healthiest animals survive and the weak ones die out. Over time the species gets better and better until it is perfectly suited to its habitat, which is the aim of evolution.”

The FlawThis answer contains no biology at all, and it is the commonest five-mark disaster in the topic. “Survival of the fittest” explains nothing — it does not say what made those individuals more likely to survive. “Strongest and healthiest” is wrong as well as vague: the advantage is often a thicker shell or an enzyme, not strength. “Nature tries to improve” and “the aim of evolution” give the process a purpose it does not have. And there is no variation, no mutation, no reproduction and no mention of alleles — four of the five marking points are simply absent.

Correct Answer“There is genetic variation within the population [1]. Many offspring are produced — more than can survive [1]. There is a struggle for survival, including competition for resources such as food, water and space [1]. Individuals that are better adapted to their environment have a greater chance of surviving and reproducing than others [1]. These individuals pass on their alleles to the next generation, so the proportion of the population showing the advantageous feature increases over many generations [1].”

Key RuleFive marks, five sentences, in Cambridge’s order. Learn them as a fixed sequence and never substitute the phrase “survival of the fittest” for any of them — it is a summary of the answer, not the answer.

Exercise 5: “Explain how a breeder produced a variety of dog with a much shorter coat. [4]”

Student’s Answer“The breeder found a dog with a short coat and changed its genes so the coat stayed short. He then bred it with another dog and all the puppies had short coats, so the new variety was made in one generation. This is natural selection because the short-coated dogs were the ones that survived.”

The FlawFour faults, none of them teleological this time. The breeder does not change any genes — the alleles for a short coat were already present as variation in the population, and all the breeder controls is which dogs are crossed. One generation is not selective breeding: the process is repeated over many generations, and that phrase is a marking point on its own. “All the puppies had short coats” assumes an outcome the answer has not justified; some offspring will not show the feature, which is exactly why the third step exists. And it is not natural selection — the selecting was done by a human, not by the environment, and no dog was surviving or dying because of its coat.

Correct Answer“There was variation in coat length in the population, so some dogs already had shorter coats than others [1]. The breeder selected the individuals with the desirable feature — the shortest coats — and crossed them [1]. From the offspring, those showing the desirable feature were selected and crossed again [1]. This was repeated over many generations, so the proportion of the population with short coats increased until the variety was established [1]. This is artificial selection, because the selecting is done by humans and for a feature humans find desirable.”

Key RuleThree steps and a timescale: select, cross, select the offspring — repeated over many generations. And be careful with dogs specifically: many features chosen by breeders would be a disadvantage in the wild, which is a difference from natural selection worth a mark whenever the question invites a comparison.

Exercise 6: “Explain the difference between continuous and discontinuous variation, and state where new variation comes from. [5]”

Student’s Answer“Continuous variation is caused by the environment, like height, because it depends on what you eat. Discontinuous variation is caused by genes, like blood group. New variation comes from mutations, but mutations are always harmful so they only ever damage the organism, and radiation causes them by choosing which gene to change.”

The FlawThree faults. Continuous variation is caused by genes AND the environment, not by the environment alone — the answer drops half the cause and, with it, the mark. Mutations are not always harmful: most have no effect, some are harmful and a few are advantageous in a particular environment — and if none were ever advantageous, natural selection could never happen, so this claim contradicts the rest of the topic. And radiation does not choose which gene to change: ionising radiation and some chemicals increase the rate of mutation, but mutation itself is random. The definitions of the two kinds of variation are also missing: no “range between two extremes”, no “limited number with no intermediates”.

Correct Answer“Continuous variation gives a range of phenotypes between two extremes, for example body length or body mass, and is caused by genes and the environment [2]. Discontinuous variation gives a limited number of phenotypes with no intermediates, for example ABO blood groups or seed shape in peas, and is usually caused by genes only [2]. New variation comes from mutation, a random change in the base sequence of DNA, which is the way new alleles are formed; its rate is increased by ionising radiation and some chemicals [1].”

Key RuleDefinitions before examples, and both halves of every cause. “Genes and the environment” is one marking point that half an answer will miss, and “mutation forms new alleles” is the sentence that links 18.1 to everything in 18.3.

✍️ Ultra-Detailed Practice Questions

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Ten Cambridge-style challenge questions. Every one draws on more than one sub-topic, because that is how this topic is examined. Write your answer first, then reveal the model answer and the examiner’s notes.

Question 1

[7 marks]

(a) Define variation. [1] (b) A biologist measures the body mass of 200 mice from one population, and also records the coat colour of each mouse, which is either black or brown with no intermediate shades. State which characteristic shows continuous variation and which shows discontinuous variation, and give the cause of each. [4] (c) Explain why the existence of variation in this population is necessary for natural selection to take place. [2]

Model Answer(a) Differences between individuals of the same species [1].
(b) Body mass shows continuous variation — a range of phenotypes between two extremes [1] — caused by genes and the environment [1]. Coat colour shows discontinuous variation — a limited number of phenotypes with no intermediates [1] — usually caused by genes only [1].
(c) Because the individuals differ, some are better adapted to the environment than others and are more likely to survive and reproduce [1]; if every individual were identical there would be nothing for selection to act on, and the population could not become better adapted over the generations [1].

Examiner’s NotesPart (a) is one mark and the words same species earn it — “differences between organisms” is not enough, because two different species differing is not variation. In (b) the phrase “no intermediate shades” in the stem is the examiner telling you which one is discontinuous; read stems for that kind of gift. Part (c) is where 18.1 becomes 18.3, and it is the single idea the whole topic is built on.

Question 2

[8 marks]

(a) Suggest what W, X, Y and Z represent. [4] (b) Explain how X and Y are adaptive features of a plant whose leaves float on the surface of a pond. [3] (c) State one feature of this leaf that would be a disadvantage to a plant growing on a dry sand dune, and explain why. [1]

Model Answer(a) W = the lamina (leaf blade) floating at the water surface [1]; X = a stoma in the upper surface of the leaf [1]; Y = a large air space in the leaf tissue [1]; Z = the lower surface of the leaf, which is in contact with the water [1].
(b) X: the stomata are on the upper surface, which is in contact with the air, so gas exchange for photosynthesis and respiration can take place — a stoma on the lower surface would be under water and could exchange very little, because very little gas is dissolved in the water [1–2]. Y: the large air spaces make the leaf buoyant, so it floats at the surface where the light is, and they also store the gases needed for photosynthesis and respiration [1].
(c) The thin cuticle (or the stomata being exposed on the upper surface) [1] — on a dry dune it would allow far too much water to be lost by evaporation.

Examiner’s NotesThe position of the stomata is the answer examiners want most often, and the reason must be given in terms of what a stoma does: exchange gases with the air. Part (c) is the part that separates candidates — it forces you to notice that an adaptive feature is only advantageous in the environment the population lives in. The same leaf is superbly adapted in a pond and hopeless on a dune, and nothing about the leaf has changed. Here is the same figure with the features named:

Question 3

[7 marks]

In Topic 15 you learned that antibiotics kill bacteria but have no effect on viruses, and that strains such as MRSA reduce the effectiveness of antibiotics. (a) Explain why a doctor tells a patient to complete the whole course of antibiotics, even after the symptoms have gone. [3] (b) Explain why it is wrong to say that a patient who keeps getting infections has “become resistant to antibiotics”. [2] (c) The allele giving resistance in many bacteria is carried on a plasmid. State what a plasmid is and suggest one reason why this makes resistance spread quickly through a population of bacteria. [2]

Model Answer(a) The bacteria in the population vary in how easily they are killed, and the ones killed first are the least resistant [1]; stopping early leaves behind the bacteria that are hardest to kill, which are the most resistant ones [1]; these then survive, reproduce and pass on the resistance allele, so the proportion of resistant bacteria in the population increases [1].
(b) The patient is not resistant — the bacterium is [1]. Resistance is a feature of a population of bacteria, produced by an allele in the bacterial DNA; nothing about the patient’s own cells has changed [1].
(c) A plasmid is a small circle of DNA in a bacterial cell, separate from the main circular DNA [1]. Plasmids can be passed from one bacterium to another, so the resistance allele can spread between bacteria and not only from parent to offspring [1].

Examiner’s NotesPart (a) is the question Topic 15 asked you to accept and this topic finally lets you prove. Notice that the answer needs variation within the population of bacteria — not all bacteria of one species are equally sensitive — and that this variation is why stopping early is selective rather than just careless. Part (b) is worth two easy marks that are routinely lost; the language of resistance belongs to the bacterium every time. The plasmid in (c) comes from Topic 2.1, and it is the sort of cross-topic recall that a challenge paper adds precisely to separate the top grades.

Question 4

[8 marks]

In some regions where malaria is common, up to 20 % of people carry a particular allele. A person carrying one copy of this allele is much less likely to die from malaria, but a person carrying two copies develops a serious blood disorder. In regions where malaria has never been common, fewer than 1 % of people carry the allele. (a) Describe the relationship between the presence of malaria and the proportion of people carrying the allele. [1] (b) Explain, in terms of natural selection, why this allele is common in regions where malaria is common. [4] (c) Suggest why the allele has remained rare in regions where malaria has never been common. [2] (d) Use this example to explain what biologists mean when they say an individual is “better adapted”. [1]

Model Answer(a) The allele is much more common where malaria is common (up to 20 %) than where it is not (under 1 %) [1].
(b) The allele arose by random mutation and was present as variation in the population [1]. In a region where malaria is common, people carrying one copy are less likely to die from malaria, so they are better adapted to that environment [1] and are more likely to survive to adulthood and reproduce [1]. They therefore pass the allele on to their children, so over many generations the proportion of the population carrying it increases [1].
(c) Where there is no malaria the allele gives no survival advantage [1], but carrying two copies still causes a serious blood disorder, so it is a disadvantage and the people carrying it are less likely to survive and reproduce, so it stays rare [1].
(d) Being better adapted means being more likely to survive and reproduce in that particular environment — not being stronger, healthier or better in general [1].

Examiner’s NotesThis is the cleanest possible demonstration that “better adapted” is not the same as “better”. The identical allele is an advantage in one region and a disadvantage in another, and it has not changed at all — only the environment has. Part (b) is four marks, so write four separate sentences and do not let them collapse into “the people with the allele survived”. You are not expected to know anything about the inheritance of this allele beyond what the stem tells you; every question of this kind gives you what you need.

Question 5

[7 marks]

All modern breeds of dog belong to one species and can interbreed, yet they differ enormously in size, coat and shape. (a) Explain how breeds this different were produced from one original population. [4] (b) Some breeds have inherited health problems, such as breathing difficulty in dogs with very flat faces. Suggest why artificial selection can produce features that would be a disadvantage in the wild. [2] (c) Suggest one reason why a breed produced in this way may contain less variation than the original population. [1]

Model Answer(a) There was variation in the original population, caused by mutation, so individuals already differed in size, coat and shape [1]. Humans selected the individuals showing the desirable feature [1] and crossed them [1]. The offspring showing the desirable feature were then selected and crossed again, and this was repeated over many generations [1].
(b) In artificial selection the selecting is done by humans, and the feature chosen is the one humans find desirable, not the one that improves survival [1]; the dogs are protected, fed and bred by people, so a feature that would reduce survival in the wild is not selected against [1].
(c) The same type of individual is chosen repeatedly over many generations, so alleles for the features that were not chosen are gradually lost from the breed [1].

Examiner’s NotesPart (a) is three named steps plus “over many generations”, and the variation sentence at the start is what stops it turning into Trap 9. Part (b) is the sharpest illustration of the real difference between natural and artificial selection: in the wild the environment removes individuals that cannot breathe well, and a kennel does not. If you can say that, you have said what the mark scheme means by “who does the selecting”.

Question 6

[8 marks]

(a) Outline three differences between natural selection and artificial selection. [3] (b) State two things that are the same in both processes. [2] (c) A student writes: “Artificial selection is just natural selection speeded up by humans.” Explain why this statement is not accepted as a description of the difference. [2] (d) State the definition of adaptation. [1]

Model Answer(a) Any three of: the selecting is done by the environment in natural selection but by humans in artificial selection [1]; natural selection favours features that improve survival and reproduction in that environment, artificial selection favours features that humans find desirable, which may reduce survival in the wild [1]; artificial selection is usually much faster, because the selection is deliberate and intense [1]; artificial selection usually leaves the population with less variation [1].
(b) The variation is already present and arises by mutation in both [1]; in both, only some individuals reproduce and pass their alleles to the next generation, and the change takes many generations [1].
(c) Speed is a consequence, not the defining difference — the defining difference is who or what does the selecting [1]; and artificial selection is not simply faster in the same direction, because it selects for different things, features humans want rather than features that aid survival [1].
(d) The process, resulting from natural selection, by which populations become more suited to their environment over many generations [1].

Examiner’s NotesPart (b) is the part almost nobody prepares for, and it is the best evidence to an examiner that you have understood the topic rather than memorised a table. Both processes need variation, both need mutation as its source, both work by controlling which individuals reproduce, and both take many generations. In (d), every word of the definition earns its place: process, natural selection, populations, many generations.

Question 7

[7 marks]

A farmer selects maize plants for longer cobs. The mean cob length of his crop was 12.4 cm in generation 1, 16.1 cm in generation 5, 19.8 cm in generation 10, 20.1 cm in generation 15 and 20.2 cm in generation 20. (a) Calculate the percentage increase in mean cob length between generation 1 and generation 10. Show your working. [2] (b) Describe the three steps the farmer carried out in each generation. [3] (c) Suggest why the mean cob length increased very little after generation 10. [2]

Model Answer(a) 19.8 − 12.4 = 7.4; 7.4 ÷ 12.4 × 100 [1] = 59.7 % (accept 60 %) [1].
(b) He selected the plants with the longest cobs [1]; he crossed those selected plants to produce the next generation [1]; from the offspring he again selected those with the longest cobs and crossed them, repeating this in every generation [1].
(c) Selection can only act on the variation that is present, and after ten generations of choosing the longest cobs most of the population already carries those alleles, so there is little variation left to select from [1]; further increase would depend on a new mutation, which is random and rare [1]. (Accept also: the plant may be limited by other factors, such as the resources available to support a longer cob.)

Examiner’s NotesThe numbers are chosen to make part (c) possible: a steep rise for ten generations and then a plateau. Read a data set for its shape before you start writing, because the shape is usually the question. Note the wording in (b) — “in each generation” — which tells you that a description of a single cross cannot be worth three marks. And do not write that the farmer improved the maize or gave it longer cobs; the longer-cobbed plants were already in his field.

Question 8

[8 marks]

Village P has been sprayed with an insecticide for nine years. Village Q, 40 km away, has never been sprayed. A scientist wishes to find out whether the mosquitoes in the two villages differ in their resistance to the insecticide. (a) Describe how she could investigate this, including two variables she must keep the same. [4] (b) The mosquitoes from village P are found to be far more resistant. Explain why this result does not show that the insecticide caused the mosquitoes to change. [2] (c) Suggest why using two different insecticides in alternate years may slow the development of resistance. [2]

Model Answer(a) Collect a large sample of mosquitoes from each village [1]; expose each sample to the same standard dose of the same insecticide [1]; after a fixed time, count the percentage killed in each sample and compare [1]. Variables to keep the same, any two: the concentration and volume of insecticide, the time of exposure, the temperature, the species, age and sex of the mosquitoes, the number of mosquitoes in each sample [1]. Repeat and calculate a mean.
(b) The result shows only that the population in village P now contains a higher proportion of resistant individuals [1]; the resistance allele arose by random mutation and was present before spraying began — the insecticide acted as a selection pressure that killed the non-resistant mosquitoes, rather than changing any mosquito [1].
(c) A mosquito resistant to one insecticide is not necessarily resistant to the other [1], so in the alternate years those individuals are killed rather than favoured, and the proportion of the population resistant to either insecticide rises more slowly [1].

Examiner’s NotesPart (a) is a Paper 4 experimental-design mark scheme: a large sample, one variable changed, everything else controlled, and repeats. The single most common omission is sample size, and the second is stating what will be measured — “see which ones die” is not a measurement; “percentage killed” is. Part (b) is the whole topic in two marks: correlation in the data, selection in the explanation, and no organism changing anywhere.

Question 9

[9 marks]

A species of freshwater snail lives in a lake. The thickness of its shell shows continuous variation. A fish that crushes snail shells to eat them is introduced into the lake. Twenty years later, the mean shell thickness of the snail population is significantly greater than it was before the fish arrived. (a) Explain fully, using the principles of natural selection, how the mean shell thickness of the snail population increased. [6] (b) State one measurement the biologists could make that would provide evidence that shell thickness is inherited. [1] (c) Explain why the individual snails that were alive when the fish arrived did not themselves grow thicker shells. [2]

Model Answer(a) 1. There was genetic variation in the snail population: shell thickness varies, so some snails already had thicker shells than others, and the alleles for this were present before the fish arrived, having arisen by random mutation [1]. 2. The snails produce many offspring, more than can survive [1]. 3. There is a struggle for survival: the snails compete for resources such as food and space, and are now also subject to predation by the fish [1]. 4. The snails with thicker shells are less likely to be crushed and eaten, so they are better adapted to this environment and have a greater chance of surviving and reproducing than the thinner-shelled snails [1]. 5. These snails pass on their alleles for thicker shells to the next generation [1]. 6. Repeated over many generations, the proportion of the population with thicker shells increases, so the mean shell thickness of the population rises [1].
(b) Measure the mean shell thickness of the offspring of thick-shelled parents and compare it with that of the offspring of thin-shelled parents (or with the population mean) [1].
(c) An individual snail cannot change the alleles it was born with, and any change during its own life would not be passed on [1]; the population changed because which snails survived and reproduced changed, not because any snail altered itself [1].

Examiner’s NotesThis is the six-mark chain, marked step by step. Here is exactly where each mark goes and the phrase that loses it.

Mark 1 — variation, and it came first. Lost by writing “the snails developed thicker shells”. The mark needs variation already present and, ideally, mutation named as its source.
Mark 2 — many offspring. Lost by omission; almost everybody forgets it. It is one clause: more offspring are produced than can survive.
Mark 3 — struggle for survival and competition. Lost by writing only “the fish ate them”. Name competition for resources as well as the predation.
Mark 4 — better adapted, with the reason. Lost by writing “the strongest survived” or “survival of the fittest”. The mark needs the feature (thicker shell), the advantage (harder for the fish to crush) and the phrase more likely to survive and reproduce.
Mark 5 — alleles passed on. Lost by writing “they passed on their thick shells”. A shell is not inherited; the alleles are.
Mark 6 — over many generations, the proportion rises. Lost by writing “so the snails adapted” or “so the species evolved to have thicker shells”. The mark needs population, proportion and many generations.

Six sentences. Learn them once and they fit any organism a paper puts in front of you.

Question 10

[9 marks]

(a) Define adaptation. [2] (b) The same process explains both the appearance of antibiotic-resistant bacteria within a few weeks and the adaptive features of a xerophyte, which took an extremely long time to appear. Explain why the two take such different lengths of time. [3] (c) Explain why neither can be explained by an organism changing itself during its lifetime. [2] (d) A hospital stops using a particular antibiotic. Suggest why the proportion of bacteria resistant to it may fall. [2]

Model Answer(a) The process, resulting from natural selection [1], by which populations become more suited to their environment over many generations [1].
(b) Natural selection acts between generations, not within a lifetime, so the speed depends on how quickly generations follow one another [1]. Bacteria divide roughly every 20 minutes, so thousands of generations pass in a few weeks and the proportion of the population that is resistant can change very quickly [1]. A plant produces at most a few generations per year, so the same number of generations takes an extremely long time [1]. (Accept also: the very large bacterial population means more mutations occur, and the antibiotic is an intense selection pressure that kills almost everything without the allele.)
(c) A feature acquired during an organism’s lifetime does not change the alleles it passes to its offspring, so it cannot be inherited [1]; in both cases the variation was already present in the population before the environmental change, and what altered was which individuals survived and reproduced [1].
(d) Without the antibiotic, the resistance allele no longer gives a survival advantage [1]; producing the protein responsible for resistance uses resources, so non-resistant bacteria may grow and reproduce faster and compete successfully, and the proportion that is resistant falls slowly [1].

Examiner’s NotesPart (b) is the question that proves you understand what natural selection actually is: a change in the composition of a population between generations. Once that is clear, generation time explains the whole difference, and no other explanation is needed. Part (d) is the reverse of everything you have practised, and it catches candidates who have quietly decided that resistance is an improvement rather than an advantage in one particular environment. It is also the answer to why hospitals rotate their antibiotics — which is where this topic hands the argument back to Topic 15.