IGCSE Biology Topic 18: Variation and Selection

18.1 Variation & Mutation 18.2 Adaptive Features 18.3 Natural Selection 18.4 Selective Breeding 18.5 Exam Technique

Hi Tara. Before anything else, the one thing that will settle your grade in this topic. Almost every mark lost in Topic 18 is lost to a single habit of writing: describing an organism as though it changed itself. Sentences like “the bacteria became resistant so they could survive”, “the giraffe stretched its neck and passed it on”, or “the plant developed a thick cuticle because it needed to save water” score nothing — not partial credit, nothing — because they describe the opposite of what actually happens. The cure is always the same one move: change the subject of the sentence from the individual to the population, and put the variation before the selection. We will practise that move until it is automatic.

Here is the shape of the topic. 18.1 is variation itself: what it means, the two shapes it comes in, and where new versions of genes come from. 18.2 is adaptive features, including the two plant types Cambridge names — xerophytes and hydrophytes. 18.3 is natural selection, and it is the centre of gravity of the whole topic; the worked example there is antibiotic resistance, which you met in Topic 15. 18.4 is selective breeding, which humans do on purpose, and the comparison table between the two kinds of selection. 18.5 is the vocabulary, the sentences that score, the sentences that do not, and the checklist for the night before.

One piece of housekeeping. Cambridge puts natural selection and selective breeding together in a single objective, 18.3 Selection. I have split them into two sections here on purpose, because they are examined separately and confusing them is the classic error in this topic — the same student who can recite the five steps of natural selection will then use them to describe a farmer choosing cattle. Two processes, two chains, two sections.

And one connection worth making now. You have already done Topic 15: Drugs, where you learned that some bacteria are resistant to antibiotics and that the antibiotic does not create the resistance. Those papers had to hand you that reasoning inside the question stem, because the topic that explains it — this one — had not been built yet. Topic 18 is where that logic is properly taught. When you reach 18.3 you are not meeting a new idea; you are finally being shown the machinery under one you already used.

18.1 Variation and Mutation ▼

▶ Watch: Variation and Mutation

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

What Variation Actually Means

Start with the definition, because it is narrower than the everyday word and the narrowness is where the mark is.

variation = differences between individuals
of the same species

Of the same species. A dog and a cat are not an example of variation — they are different species. Two dogs of different sizes are. If you write about “differences between organisms” and leave out “of the same species”, you have written a sentence that is true of the whole of biology and specific to nothing.

Why does Cambridge care so much about that phrase? Because variation is the raw material that everything later in this topic acts on. Natural selection cannot pick out the better-adapted individuals unless the individuals differ from one another first. Selective breeding cannot choose the highest-yielding wheat plant unless some plants already yield more than others. Hold on to that: variation comes first, selection comes second. Almost every wrong answer in this topic gets that order backwards.

The Two Shapes Variation Comes In

Measure a feature across a population, plot it, and the graph comes out in one of two shapes. Cambridge names both, and it wants specific wording for each.

Continuous variation on the left, discontinuous on the right. Notice the bars: touching on the left because every height in between exists; separate on the right because nothing exists in between.

	Continuous variation	Discontinuous variation
Definition to learn	a range of phenotypes between two extremes	a limited number of phenotypes with no intermediates
Cambridge’s examples	body length, body mass	ABO blood groups; seed shape in peas; seed colour in peas
Caused by	genes AND the environment	genes only (usually)
Graph shape	histogram, bars touching, often a hump in the middle	bar chart, bars separated, a few named categories
Test question to ask	can I find an individual halfway between two others? Yes.	can I find an individual halfway between two categories? No.

The last row of that table is the one to actually use. You will be handed an unfamiliar feature — snail shell banding, wing length in a beetle, tongue-rolling, seed mass — and asked which kind of variation it shows. Do not try to remember whether you were taught that particular example. Ask the halfway question. Is there such a thing as halfway between blood group A and blood group B? No, so it is discontinuous. Is there such a thing as halfway between 158 cm and 159 cm? Yes, so it is continuous.

The wording that gets rejected

“Continuous variation has lots of values and discontinuous has only a few” is the answer most students give, and it usually gets nothing. It is a description of the data, not of the biology. What the mark scheme wants is “a range between two extremes” against “no intermediates”. The phrase no intermediates is worth learning as a unit — it is the single most reliably credited phrase in 18.1.

The two shapes again, this time with numbered axes so you can practise reading values off: how many students sit in one 5 cm class, how many are in each blood group, how many more in one group than another. Left: heights of 84 students. Right: blood groups of a different sample of 30 students.

The same figure with the labels removed. Match each letter to what it marks.

Label it yourself

Every dropdown has the same list. Match each letter on the figure to what it marks, then press Check.

How to Investigate Variation (the Method Examiners Mark)

Choose one species, for example leaves from one species of tree, or students in one year group.
Take a large sample, at least 30, chosen at random: not just the biggest or the easiest to reach.
Continuous feature (leaf length, hand span, height): measure every individual the same way, with a ruler to the nearest millimetre. For a leaf, measure from the tip to where the blade meets the stalk, leaving out the stalk; for hand span, from the tip of the thumb to the tip of the little finger with the fingers fully spread.
Discontinuous feature (ABO blood group from records, seed colour in peas): put each individual in its category and count the number in each.
Record the results in a table. Continuous data: group the values into classes of equal width (for example 40–49 mm, 50–59 mm), use a tally, then total each class. Discontinuous data: one row per category.
Draw the right graph. Continuous data: a histogram, with the classes on the x-axis and the bars touching, because every value in between exists. Discontinuous data: a bar chart, with the bars separate, because there are no intermediates.

Worked example A student measured the lengths of 40 leaves from one laurel tree and grouped them: 40–49 mm, 3; 50–59 mm, 7; 60–69 mm, 14; 70–79 mm, 10; 80–89 mm, 6. She also counted 200 pea seeds: 152 yellow and 48 green. Analyse both sets of results and choose a graph for each.

Step 1: Check the total

3 + 7 + 14 + 10 + 6 = 40, so every leaf is in a class.

Step 2: Read the leaf data

The modal class is 60–69 mm. Percentage in the modal class = 14 ÷ 40 × 100 = 35 %.

Step 3: Name the type and the graph

Leaf length shows continuous variation, a range of phenotypes between two extremes, so she draws a histogram with touching bars. It is caused by genes and the environment: leaves on the same tree share the same genes, yet they still vary because of light, shade and position.

The two data sets drawn the right way. Bar heights are the frequencies in the table.

Step 4: The peas

Percentage green = 48 ÷ 200 × 100 = 24 %. Seed colour shows discontinuous variation: two categories with no intermediates, so she draws a bar chart with separate bars.

Why the Causes Differ — and Why That Is Not Arbitrary

It looks like a fact to memorise: continuous is genes plus environment, discontinuous is genes only. It is not arbitrary, and if you see why, you will never mix them up.

Think about your blood group. It is decided by which alleles you inherited and nothing else. Eating better, exercising more, moving to a colder country — none of these can nudge you from group O towards group A, because there is nowhere in between to be nudged to. The alleles you inherited settle the matter, and the outcome is a category rather than a value.

Now think about your height. Your alleles set a range within which you could end up, but where you land inside that range depends on your diet, your health, how much you slept during growth. The environment can push you a centimetre or two either way. And because it can push you a little way, all the in-between values exist. The environment is what fills in the gaps — that is the connection between “caused by genes and the environment” and “a continuous range”. One idea, not two.

Worked example A gardener grows 200 pea plants from seed collected from one plant. He records the mass of each mature plant and the shape of each seed the plants produce. The plant masses form a smooth range from 34 g to 91 g. The seeds are either round or wrinkled, with nothing in between. Explain what these two results show about the causes of each type of variation. [4]

Step 1 — name both types before explaining anything

Plant mass shows continuous variation: a range of phenotypes between two extremes. Seed shape shows discontinuous variation: a limited number of phenotypes with no intermediates. Naming them is often a mark each, and it costs you one line.

Step 2 — use the detail the question gave you

All 200 plants came from one parent plant, so the genetic differences between them are limited — yet the masses still spread across 57 g. Something other than the genes must be producing part of that spread.

Step 3 — say what that something is

The environment: differences in light, water, soil nutrients and space between one plant and the next. So plant mass is caused by genes and the environment together.

Step 4 — and contrast it

No amount of extra water or light turns a wrinkled seed into a round one, because there is no halfway shape to move towards. Seed shape is caused by genes only.

Plant mass = continuous variation, caused by genes and the environment; seed shape = discontinuous variation, caused by genes only. The 57 g spread among offspring of a single parent is the evidence that the environment contributes to the first and not the second.

Mutation: Where New Alleles Come From

Variation has to come from somewhere. Ultimately, all of it comes from mutation.

The two sentences Cambridge wants

Mutation = a genetic change.

Mutation is the way in which new alleles are formed.

That second sentence is the one that matters. Nothing else on the syllabus makes a new allele. Meiosis shuffles alleles, fertilisation combines them, selection changes how common they are — but only mutation creates one that was not there before.

Remember from Topic 17 that an allele is an alternative form of a gene, and a gene is a length of DNA that codes for a protein. So a new allele means a gene whose DNA now reads slightly differently — and therefore, usually, a protein with a slightly different shape, and therefore possibly a different feature in the organism.

Supplement

Gene mutation, defined precisely

A gene mutation is a random change in the base sequence of DNA. Three words in that sentence are each worth having: random, base sequence, and DNA.

A gene mutation is a random change in the base sequence of DNA. The word random means the change is not aimed at anything — it is not a response to the environment.

Random is the word that carries the whole topic. A mutation is not a reply to a problem. The bacterium does not produce a resistance allele because an antibiotic arrived; the mutation happened at some earlier point, for no reason connected to antibiotics at all, and it would have happened whether the antibiotic ever arrived or not. Hold on to this — in 18.3 it is the difference between a full-mark answer and a zero one.

The four sources of genetic variation in a population

Cambridge names four, and it is worth being able to say what each one does, because only the first makes anything new:

Mutation — makes new alleles. The only source of genuinely new variation.
Meiosis — the reduction division that makes gametes (Topic 17.3). It produces genetically different gametes, so no two are alike.
Random mating — which individual in the population happens to breed with which.
Random fertilisation — which particular gamete happens to fuse with which other one. Of the millions of sperm cells, one arrives.

Three of those four are shufflers. They deal the same pack into new hands. Only mutation prints a new card.

What Increases the Rate of Mutation

Mutations happen at a low rate all the time, as copying errors. Two things increase that rate, and Cambridge names both:

Ionising radiation — for example X-rays, gamma rays, ultraviolet in sunlight.
Some chemicals — for example those in tobacco smoke.

Two traps in one sentence

First: these factors increase the rate of mutation. They do not cause mutation in the sense of being necessary for it — mutations occur without them.

Second, and more important: they do not decide which mutation happens. Ionising radiation raises how often the dice are rolled. It does not load the dice. A common wrong answer says radiation “causes the mutations an organism needs” — that is the teleology trap arriving early, and it is wrong for the same reason it is always wrong.

And one more correction that is worth a mark whenever it comes up: mutations are not always harmful. Most have no noticeable effect at all. Some are harmful. A few happen to produce a feature that turns out to be an advantage in that particular environment — and those are the raw material for everything in 18.3. If mutations were always harmful, there would be no variation for selection to act on and no topic to study.

Check Yourself: 18.1 Variation and Mutation

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which is the Cambridge definition of variation?

A differences between individuals of the same species

B differences between organisms living in the same habitat

C differences between the alleles carried by one individual

D changes in an individual during its lifetime

The phrase of the same species is the definition. Option B swaps species for habitat, which lets in every other organism in the pond. Option C describes being heterozygous, not variation. Option D is the teleological trap in disguise — changes during a lifetime, such as getting a suntan, are not what this topic means by variation, because they are not inherited.

Question 2

Which pair of features are both examples of discontinuous variation?

A body mass and body length

B seed shape in peas and ABO blood group

C ABO blood group and body length

D seed mass in peas and seed colour in peas

Cambridge names three examples of discontinuous variation: ABO blood groups, seed shape in peas and seed colour in peas. Option D is the sharp one — seed colour is discontinuous but seed mass is a measurement with every value in between, so it is continuous. Read the noun, not just the organism.

Question 3

Why is continuous variation described as being caused by genes and the environment?

A because the environment can change which alleles an individual carries

B because features such as height are not controlled by genes at all

C because the alleles set a range and conditions such as diet decide where in that range an individual ends up

D because the environment causes mutations that produce intermediate phenotypes

Genes set the range; the environment positions you within it, which is exactly why the in-between values exist. Option A is a serious misconception — nothing in the environment edits the alleles you already have. Option B overcorrects. Option D confuses a rare random event with the everyday effect of diet and light.

Question 4

A student measures the shell diameter of 300 snails and finds values spread smoothly from 8 mm to 27 mm. She also records shell banding, which is either present or absent. Which statement is correct?

A Both features show continuous variation because both were measured in the same population.

B Diameter is discontinuous because it has an upper and a lower limit.

C Banding is continuous because the number of bands can vary.

D Diameter is continuous and banding is discontinuous, because there is no shell halfway between banded and unbanded.

Apply the halfway test to each feature separately. Option B is the commonest error here: having two extremes is part of the definition of continuous variation, not evidence against it. Option C answers a different question from the one asked — the feature recorded was present or absent, not how many.

Question 5

Which statement about mutation is correct?

A Mutation rearranges existing alleles into new combinations.

B Mutation is the way in which new alleles are formed.

C Mutation occurs only in gametes.

D Mutation always produces a harmful feature.

Option B is the syllabus sentence, and it is worth learning word for word. Option A describes what meiosis and fertilisation do — shuffling, not creating. Option D is the most widespread misconception in the whole topic; most mutations have no noticeable effect, and if all were harmful, natural selection would have nothing beneficial to act on.

Question 6

A gene mutation is best defined as

A a random change in the base sequence of DNA.

B a change in the number of chromosomes in a cell.

C a change in the shape of a protein caused by high temperature.

D a useful change in DNA produced in response to the environment.

Learn all three key words: random, base sequence, DNA. Option C is denaturing, from Topic 5. Option D contains two errors at once — it makes the change useful by definition, and it makes it a response to the environment. Both are the teleology trap, and both are fatal in 18.3.

Question 7

Which of these does not create a new allele?

A a copying error in DNA during cell division

B a chemical in tobacco smoke altering a base in a gene

C ionising radiation altering a base in a gene

D meiosis producing genetically different gametes

Meiosis is a shuffler, not a printer — it produces new combinations of existing alleles. A, B and C are all descriptions of mutation, which is the only process on the syllabus that makes an allele that did not exist before.

Question 8

Ionising radiation is described as increasing the rate of mutation. What does this mean?

A Mutations occur only where ionising radiation is present.

B Radiation selects which mutations will be beneficial.

C Random changes in the base sequence happen more often, but which change occurs is still random.

D Every cell exposed to radiation gains a new allele.

Radiation raises how often the dice are rolled; it does not load them. Option B is the trap — nothing about a mutagen chooses a useful outcome. Option A forgets that mutations occur as ordinary copying errors anyway, and option D turns a rate into a certainty.

Question 9

Which list gives the four sources of genetic variation in a population named by the syllabus?

A mutation, mitosis, random mating, natural selection

B mutation, meiosis, the environment, selective breeding

C mutation, meiosis, random mating, random fertilisation

D meiosis, fertilisation, diet, ionising radiation

Two words trip people here. Mitosis (option A) produces genetically identical cells, so it produces no variation at all. And natural selection is not a source of variation — it acts on variation that already exists. Option B smuggles in the environment, which produces differences that are not genetic and therefore not inherited.

Question 10

Two tomato plants grown from seeds of the same parent are placed in the same greenhouse. One is given twice as much fertiliser and grows 40 % taller. Which statement about this height difference is correct?

A It is an example of discontinuous variation.

B It shows that the fertiliser changed the alleles of the taller plant.

C It shows that height in tomatoes is not affected by genes.

D It is variation caused by the environment, and it will not be passed to the next generation.

Height shows continuous variation caused by genes and the environment; here the genes are near enough identical, so the difference is environmental — and environmental differences are not inherited. Option B is the same error as the giraffe stretching its neck: something that happens to an organism during its life does not rewrite its DNA.

Question 11

Look at the two charts below.

Which statement about Chart A and Chart B is correct?

A Chart A shows continuous variation and Chart B shows discontinuous variation, so Chart A is influenced by the environment as well as by genes.

B Chart A shows discontinuous variation because it has more than four bars.

C Chart B shows continuous variation because the percentages take a range of values.

D Neither chart shows variation, because each chart surveys only one species.

Chart A is a smooth range of heights with touching bars; Chart B has four separate named categories with nothing in between. Option C is the trap worth spotting: you read the x-axis to decide which type of variation it is, never the y-axis — the percentages on the vertical axis are just how common each category is. Option D reverses the definition; variation is within one species.

Question 12

Why does the fact that mutation is random matter so much later in this topic?

A because it means every mutation is equally likely to be beneficial or harmful

B because the useful allele exists before the environmental factor arrives, rather than appearing in response to it

C because it means selection must also be random

D because it means an organism can produce whichever allele its environment demands

This is the hinge of the whole topic: variation first, selection second. Option D states the teleological error outright — no organism produces the allele its surroundings call for. Option C gets the pairing backwards; the memorable line is mutation is random, selection is not. Option A misreads randomness as an even split of outcomes.

18.2 Adaptive Features, Xerophytes and Hydrophytes ▼

▶ Watch: Adaptive Features, Xerophytes and Hydrophytes

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

The Definition, and the Two Words People Leave Out

an adaptive feature = an inherited feature
that helps an organism to survive and reproduce
in its environment

Inherited — so a suntan is not an adaptive feature, and neither are the strong muscles of an athlete. If it is not passed on, it cannot be selected for. Survive AND reproduce — both halves. An organism that survives brilliantly and never breeds contributes nothing to the next generation, so from the point of view of this topic it might as well not have existed.

Most students write “a feature that helps an organism survive” and drop both of the italicised ideas. That answer usually gets one mark out of two. Write the full sentence — it takes four extra words.

The three-part sentence that scores on any organism

Cambridge will show you a species you have never met — a desert beetle, a fish from deep water, a plant from a salt marsh — and ask you to describe its adaptive features. You are not being tested on the organism. Use this shape, once per feature:

(1) name the feature you can actually see → (2) say what it does → (3) link that to survival or reproduction in that particular environment.

Part 3 is where the marks hide and where nearly everyone stops early. “It has thick fur” is part 1 only. “It has thick fur, which insulates the body” is parts 1 and 2. “It has thick fur, which insulates the body and reduces heat loss in a cold environment, so more individuals survive the winter” is the full answer.

Supplement Xerophytes: Plants Adapted to Very Dry Conditions

A xerophyte is a plant adapted to survive in very dry conditions. Cacti in deserts; marram grass on sand dunes, where the sand drains instantly and the wind never stops.

Before you learn any features, remember what the problem actually is. From Topic 8.3: transpiration is the loss of water vapour from a plant, and it happens because water evaporates from the mesophyll cell surfaces inside the leaf and then diffuses out through the stomata, down a concentration gradient. And you know the four factors that speed it up: high temperature, low humidity, wind, and high light intensity (which opens the stomata).

That gives you the whole of xerophyte biology in one line. A dune or a desert is hot, dry, bright and windy — every single factor that increases transpiration is at maximum. So every xerophyte feature does one of exactly two jobs: keep the humid air close to the stomata, or keep the moving air away from them. If you can remember those two jobs you can work out the features rather than reciting them.

A xerophyte leaf in cross section. Read each label as an answer to one question: does this feature trap humid air near the stoma, or does it keep the wind off?

Feature	What it does	Why it helps in a dry place
Thick waxy cuticle	a waterproof layer over the epidermis	a longer diffusion distance, and the cuticle itself is not permeable to water — almost all loss is forced through the stomata where it can be controlled
Sunken stomata	each stoma sits at the bottom of a pit	water vapour collects in the pit, so the concentration gradient of water vapour out of the leaf is smaller and diffusion is slower
Hairs on the epidermis	trap a layer of still, humid air over the surface	same mechanism — the air next to the stoma stays humid instead of being replaced by dry air
Rolled leaf	the leaf curls so the stomata are enclosed inside a tube	the enclosed air becomes humid and the wind cannot sweep it away
Small surface area (spines, needles, thick fleshy leaves)	less leaf surface exposed to the air	less area for evaporation; in a cactus the leaves are reduced to spines and the stem does the photosynthesis
Extensive shallow roots (cactus)	spread widely just below the surface	absorb rain water quickly over a large area, before it evaporates or drains away
Deep roots (many desert shrubs, marram grass)	grow down a long way	reach water deep in the ground
Water-storage tissue (swollen stem of a cactus, fleshy leaves)	stores water taken up after rain	the plant can use it during long dry periods
Fewer stomata	fewer pores in the surface	less area through which water vapour can diffuse out

One mechanism, five costumes

Sunken stomata, hairs and a rolled leaf are the same idea three times over: keep humid air sitting next to the stoma, so the concentration gradient for water vapour leaving the leaf stays small. If you understand one, you can explain all three — and an explanation that mentions the water vapour concentration gradient will out-score one that only says “it stops water escaping”.

Worked example Fig. 4.1 shows a cross section through the leaf of a plant found on a coastal sand dune. Identify the features labelled A and B, and explain how each reduces the rate of water loss from the leaf. [4]

Step 1 — identify from position, not from memory

A labels the outermost layer covering the epidermis: the thick waxy cuticle. B labels a stoma lying at the bottom of a pit in the surface: a sunken stoma. Notice that you can name both from the drawing alone, without recognising the species.

Step 2 — explain A properly

The cuticle is waterproof and is not permeable to water, so very little water vapour is lost straight through the upper surface. It also increases the distance any water would have to diffuse across.

Step 3 — explain B using the gradient

Water vapour that diffuses out of the stoma accumulates in the pit above it. The air immediately outside the stoma therefore stays humid, the concentration gradient of water vapour between the inside of the leaf and the air outside is reduced, and so the rate of diffusion out of the leaf falls.

Step 4 — check your sentences for the trap

Read what you wrote. If any sentence says the plant developed these features because it needed water, cross it out. These features are inherited; the plants on the dune that happened to have them survived and reproduced more than those that did not.

A = thick waxy cuticle: waterproof and not permeable to water, so water vapour is not lost through the upper surface. B = sunken stoma: water vapour collects in the pit, keeping the air outside the stoma humid, which reduces the water vapour concentration gradient and slows diffusion out of the leaf.

The same figure with the labels filled in. A = thick waxy cuticle, B = sunken stoma, C = hairs, D = rolled leaf, E = thick leaf with a small surface area.

A whole cactus. Every feature either gets and stores more water (roots, storage tissue) or loses less (spines, cuticle, sunken stomata on the stem).

Two jobs to remember. A xerophyte feature either reduces water loss or gets and stores more water. A hydrophyte feature either keeps the leaf in the light and the air, or holds enough gas.

Supplement Hydrophytes: Plants Adapted to Living in Water

A hydrophyte is a plant adapted to living in water — the water lily is the standard example, rooted in the mud of a pond with its leaves floating on the surface.

Here is the useful way to think about it. The hydrophyte has the opposite problem to the xerophyte. Losing water is not an issue at all; the plant is standing in it. What is scarce is dissolved gas — there is far less carbon dioxide and oxygen available in water than in air — and what is awkward is staying in the light, because light does not travel far down through pond water.

A hydrophyte. Every feature answers one of two problems: reach the air and the light, or hold enough gas inside the plant.

nothing for a thick waterproof layer to do.

Feature	How it helps in water
Stomata on the upper surface of the leaf only	The lower surface is in contact with water, where a stoma could exchange almost nothing. Putting them on top means gas exchange happens with the air. This is the one feature that reverses the ordinary land-plant arrangement, and it is the one examiners ask for most often.
Large air spaces in the leaf and stalk	They store the gases used in photosynthesis and respiration, and they make the leaf buoyant, so the lamina stays at the surface in the light and the air.
Broad, flat lamina floating on the surface	A large surface area in full light for photosynthesis, and in contact with the air rather than the water.
Flexible stalk, little supporting tissue	The water supports the plant, so rigid tissue is unnecessary; a flexible stalk bends with moving water instead of breaking.
Thin or absent cuticle	Reducing water loss is not a problem in a pond, so there is
Small roots, mainly for anchorage	Water and mineral ions can be absorbed through the surface of the parts under water, so large roots are not needed.
Little xylem	There is no need to move large volumes of water up the plant, and the water supports the plant.

Compare the two by environment, never by effort

If a question asks you to compare a xerophyte and a hydrophyte, resist writing that one “works harder” at saving water. Both are populations shaped by natural selection over many generations in two different environments. The clean comparison is: the xerophyte’s features reduce the loss of water vapour; the hydrophyte’s features maintain gas exchange and keep the leaf in the light. Same process, opposite pressures.

And watch the stomata. Ordinary land plants have most of their stomata on the lower surface; the hydrophyte has them on the upper surface; in marram grass they are sunken in grooves on the inner (upper) surface of the leaf, surrounded by hairs. When the leaf rolls, that surface ends up inside the tube, so the stomata sit in humid, still air; the outer surface, facing the wind, has a thick waxy cuticle and no stomata. In a cactus they are on the green stem, often sunken. Three arrangements, three environments — a favourite comparison question.

Check Yourself: 18.2 Adaptive Features

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which is the full definition of an adaptive feature?

A any feature that helps an organism to survive

B a feature an organism develops during its lifetime in response to its surroundings

C an inherited feature that helps an organism to survive and reproduce in its environment

D a feature shared by all members of a species that makes them stronger than other species

Two words carry the marks: inherited and reproduce. Option A is the answer most people give and it drops both. Option B is the classic teleological error — features acquired during a lifetime are not inherited, so selection cannot act on them. Option D drifts into comparing species, which is not what adaptation means.

Question 2

Which of these is not an adaptive feature?

A the thickened skin on the hands of a gardener who works without gloves

B the white winter coat of an Arctic hare

C the sunken stomata of marram grass

D the long roots of a desert shrub

Apply the word inherited as a filter. Thickened skin is caused by use during one lifetime, is not coded for by an allele that the gardener passes on, and therefore cannot be selected for. Everything else in the list is a feature the offspring inherit.

Question 3

Why do sunken stomata reduce the rate of water loss?

A The pit blocks the stoma so no gases can pass through it.

B Water vapour collects in the pit, reducing the water vapour concentration gradient out of the leaf.

C The pit lowers the temperature of the leaf so that no evaporation occurs.

D The stoma closes permanently once it is sunken.

This is a diffusion answer, straight out of Topic 3: a smaller concentration gradient means a slower rate of diffusion. Options A and D would stop carbon dioxide entering too, which would end photosynthesis — a xerophyte still has to feed itself. Option C confuses reducing evaporation with abolishing it.

Question 4

Fig. 5.1 shows a plant rooted in the mud of a pond, with one of its leaves magnified.

Which statement about the labelled features is correct?

A X labels stomata on the lower surface, which is the usual arrangement in land plants.

B Z labels the surface through which most of the leaf gas exchange occurs.

C Y labels a thick waxy cuticle that reduces water loss from the floating leaf.

D X labels stomata on the upper surface, which is where gas exchange with the air is possible.

X is on the upper surface, in contact with the air — the reversal of the land-plant arrangement, and the feature examiners ask for. Y is a large air space, which stores gases and gives buoyancy, not a cuticle. Z is the lower epidermis, which is under water and has no stomata, so almost no gas exchange happens there. Option C is worth noticing as a trap: reducing water loss is not a problem for a plant standing in a pond.

Question 5

Large air spaces are found in the leaves and stalks of a water lily. Which pair of functions do they serve?

A holding gases used in photosynthesis and respiration, and making the leaf buoyant

B storing water, and supporting the stalk rigidly

C trapping humid air, and reducing transpiration

D absorbing mineral ions, and transporting them to the leaf

Buoyancy plus a gas store is the pair. Option C is the answer that costs most marks: it is a genuine explanation, but of xerophyte features, in the wrong plant. Ask what is actually scarce in the environment — in a pond that is dissolved gas, never water.

Question 6

Marram grass rolls its leaves in dry, windy weather. How does rolling reduce water loss?

A It stops water reaching the leaf from the xylem.

B It increases the surface area exposed to the sun so that water evaporates before it reaches the stomata.

C It encloses the stomata inside a humid tube of still air that the wind cannot sweep away.

D It closes every stoma permanently.

Rolling gives the same mechanism as hairs and sunken stomata: humid, still air next to the stoma, so a smaller gradient and slower diffusion. Option B is nonsense in the right vocabulary, which is exactly what a good distractor looks like. Option D would also stop carbon dioxide entering.

Question 7

In a cactus the leaves are reduced to spines and photosynthesis takes place in the stem. Which is the best explanation of the advantage of this in a desert?

A Spines carry out photosynthesis more efficiently than broad leaves.

B The cactus does not carry out transpiration at all.

C Spines absorb water directly from the air at night.

D A much smaller leaf surface area is exposed to the air, so less water vapour is lost by transpiration.

Surface area is the key variable, exactly as it was for diffusion in Topic 3. Option A is backwards — spines are poor at photosynthesis, which is precisely why the stem takes over the job. Option B overstates: every plant with stomata loses some water vapour.

Question 8

A student writes: “The marram grass rolled its leaves because it needed to save water on the dune.” Why does this sentence score no marks?

A because marram grass does not roll its leaves

B because it describes an inherited feature as something the plant produced in response to a need

C because dunes are not dry environments

D because rolling leaves does not reduce water loss

The biology in the sentence is right; the direction of causation is wrong, and examiners treat that as a wrong answer rather than an imprecise one. The corrected version: rolled leaves are an inherited feature, and the plants on the dune that had it survived and reproduced more than those that did not.

Question 9

Where are the stomata found in each of these three plants?

A ordinary land plant: upper surface; hydrophyte: lower surface; xerophyte: sunken

B ordinary land plant: mainly lower surface; hydrophyte: upper surface; xerophyte: sunken in pits

C all three have stomata mainly on the lower surface

D ordinary land plant: lower surface; hydrophyte: no stomata at all; xerophyte: upper surface

Option A is the trap, because it has all three of the right ideas in the wrong order. Work each one out from the environment instead of memorising the row: the underside of a floating leaf touches water, so a stoma there would be useless; the underside of a land leaf is shaded and sheltered, which is why stomata sit there.

Question 10

Which explanation of a thick waxy cuticle would score full marks?

A It stops the leaf from drying out.

B It protects the leaf from being eaten.

C It reflects sunlight so the stomata stay closed.

D It is waterproof and not permeable to water, so little water vapour is lost through the epidermis, and it increases the distance water would have to diffuse.

Option A is true but is a statement, not an explanation — it repeats the question back. The mark scheme wants a mechanism: waterproof, not permeable, longer diffusion distance. Notice how the good answer reuses vocabulary from Topic 3 rather than inventing new words.

Question 11

A fish that lives in deep, dark water has very large eyes, a light-producing organ on its head, and a slow, energy-saving swimming style. A student is asked to describe its adaptive features for 3 marks. Which answer scores best?

A It has big eyes, a light on its head, and swims slowly.

B It developed large eyes because it needed to see in the dark.

C Large eyes absorb more of the very little light available, so it can detect prey in the dark; the light organ attracts prey towards it; slow swimming uses less energy, which matters where food is scarce.

D It is well adapted to its environment and so survives better than other fish.

Option A is three named features with no explanation — part 1 of the three-part sentence, three times. Option D is explanation with no features, which is the mirror-image mistake. Option B states the teleological error explicitly. Option C does feature, function and link to survival for each of the three, which is what a 3-mark answer of this type asks for.

Question 12

Which statement correctly compares a xerophyte with a hydrophyte?

A The features of the xerophyte reduce loss of water vapour; those of the hydrophyte maintain gas exchange and keep the lamina in the light.

B Both have a thick waxy cuticle, but for opposite reasons.

C The xerophyte works harder at saving water than the hydrophyte does.

D The hydrophyte has more adaptive features because water is a more difficult environment.

Compare by environmental problem, never by effort or by count. Option C is teleological language dressed up as a comparison. Option B is factually wrong: the hydrophyte has a thin cuticle or none, because reducing water loss is not a problem in a pond.

18.3 Natural Selection ▼

▶ Watch: Natural Selection and Selective Breeding

Opens on YouTube in a new tab. Watch one, then come back and do the Check Yourself questions — watching without testing yourself feels like learning but is not.

The Five Steps, and Why They Are Always the Same Five

Cambridge marks natural selection as a chain. There are five links, they come in a fixed order, and each one is worth a mark. If you write a beautiful paragraph that misses link 1, you lose that mark no matter how good the rest is. So learn the chain as a chain.

The five steps — in this order, every time

1. Variation already exists in the population. It is caused by mutation, and the alleles are there before the environmental factor arrives. Selection does not create variation; it acts on it.

2. Many offspring are produced — more than can survive.

3. There is a struggle for survival — competition for resources such as food, water, light, space and mates, plus predation and disease.

4. The individuals with the advantageous feature are more likely to survive and reproduce. They are better adapted — not “stronger”, not “fitter” in the everyday sense.

5. They pass on their alleles to the next generation, so the proportion of the population with that feature increases over many generations.

Look at what each link is doing. Step 1 supplies the raw material. Step 2 and step 3 together supply the pressure — if every offspring survived there would be nothing to select. Step 4 is the selection itself. Step 5 is the inheritance, which is what turns a lucky individual into a changed population. Miss step 1 and the answer sounds like the organism invented the feature. Miss step 5 and nothing has actually changed — you have described one lucky animal, not evolution.

The single most important sentence in this topic

Individuals do not adapt. Populations become adapted, over generations.

No bacterium ever became resistant. No rat ever became resistant. Each individual was born either carrying the allele or not, and it died the same way it was born. What changed is the proportion of the population that carries it. Every time you are about to write “it became”, stop and write “the proportion of the population that was… increased”.

The Centrepiece: Antibiotic Resistance

You have met this before. In Topic 15: Drugs you learned that some bacteria are resistant to antibiotics, that this reduces the effectiveness of antibiotics, and that MRSA is the example Cambridge names. Those papers had to spell out the selection reasoning inside the question stems, because the topic that explains it — this one — had not been built yet. This is where that reasoning is properly taught. Everything you were asked to accept in Topic 15 is about to be derived.

One population of bacteria at three times, with the five steps written underneath. The antibiotic changes nothing about any individual cell — it removes the competition.

Take the panels slowly, because the whole topic is in them.

Before. There are 30 bacteria. Twenty-eight are killed by the antibiotic; two are not, because at some earlier point a random mutation produced an allele that gives resistance — often carried on a plasmid, the small circle of DNA you met in the bacterial cell in Topic 2.1. No antibiotic has been used yet. Nothing about the environment produced that allele. It was already there.

During. The antibiotic arrives. The 28 sensitive bacteria are killed. The two resistant ones survive — not because they did anything, but because the thing that killed the others could not kill them.

After. The survivors reproduce, and they pass the resistance allele to their offspring. Bacteria divide roughly every 20 minutes, so in a day the two become an enormous population, and essentially all of it is resistant. A shift that would take thousands of years in a large slow-breeding animal takes days.

Two sentences to keep from Topic 15

The antibiotic does not create resistance. It selects for it.

The bacterium is resistant. The person never is. “My body has become resistant to antibiotics” is a sentence people say and it is wrong in a way that destroys the whole answer. What is true is that the bacteria the person is carrying include resistant ones.

The chain drawn as one population of bacteria at four moments. Count the cells in each panel: no individual cell ever changes — what changes is the proportion of the population carrying the allele.

The same figure with the labels removed. Match each letter to what it marks.

Label it yourself

Every dropdown has the same list. Match each letter on the figure to what it marks, then press Check.

Reading the Data: Resistance Against Time

Samples of one species of bacterium taken from patients in one hospital, 1998 to 2026. Values: 2, 5, 11, 21, 34, 47, 58 and 63 per cent.

Two things to notice, because they are the two things examiners ask about.

The first point is not zero. In 1998, 2 % of samples were already resistant, before the heavy use of this antibiotic in the hospital. That 2 % is step 1 of the chain, drawn as data. If a question asks for evidence that the variation existed before selection acted, this is the evidence, and quoting the number is worth the mark.

The curve steepens and then flattens. It rises by 13 percentage points in each four-year period from 2010 to 2018 (21 % → 34 % → 47 %): that is its steepest part. After that the rise slows: 11 points from 2018 to 2022, then only 5 points from 2022 to 2026. Even in 2026, 37 % of samples are still sensitive, so the curve has not flattened because the sensitive bacteria have run out.

The graph does not tell you why the rate changed. A likely reason is that the antibiotic was used less often in later years, so there was less selection. That is a suggestion, not something the data prove: without figures for how much of the drug was used each year, it is a correlation at best. Describing the shape, with figures quoted from the axes, out-scores “it goes up” every time.

The Drill: Build the Five-Step Answer in Your Own Words

Reading the five steps does not make you able to write them under time pressure. What makes you able to write them is having written them, twice, in two different contexts, in your own sentences. So this is a drill, not a passage. Get a piece of paper.

Here is the scaffolding. You will use the same five headings every single time this question appears, whatever the organism:

Step	What your sentence has to establish
1. Variation	Some individuals already carried the allele, caused by a random mutation, before the new environmental factor arrived.
2. Many offspring	The species produces more offspring than can survive.
3. Struggle for survival	Name what they compete for, and name the thing that is killing them.
4. Better adapted survive and reproduce	The ones with the allele are more likely to survive and reproduce than the ones without.
5. Alleles passed on	The survivors pass the allele to their offspring, so the proportion of the population carrying it rises over many generations.

Drill 1 — worked in full A hospital finds that an antibiotic which used to cure a bacterial infection now fails in most patients. Explain, using natural selection, how this population of bacteria came to be resistant. [5]

Write your five sentences first. Then open this and compare — not word for word, but step for step. Every step you can find in your version is a mark.

Step 1 — variation already exists

Within the bacterial population there was genetic variation: a few bacteria already carried an allele for resistance, produced by a random mutation, before this antibiotic was used.

Step 2 — many offspring

Bacteria reproduce very rapidly, dividing about every 20 minutes, so a very large number of offspring is produced — far more than the available nutrients can support.

Step 3 — struggle for survival

There is a struggle for survival: the bacteria compete for nutrients and space, and the antibiotic kills those that do not carry the resistance allele.

Step 4 — the better adapted survive and reproduce

The resistant bacteria are better adapted to an environment containing the antibiotic, so they are more likely to survive and reproduce than the sensitive ones.

Step 5 — alleles are passed on

The survivors pass the resistance allele on to their offspring, so over many generations the proportion of the population that is resistant increases, until almost all of it is resistant and the antibiotic no longer cures the infection.

Five sentences, five marks. Notice that not one of them has a bacterium as an agent doing something on purpose. Every sentence is about which individuals happened to survive and what they passed on.

Drill 2 — your turn Warfarin is a poison used to control rats; it prevents the blood from clotting. A farm began using warfarin in 1965 and used it continuously. By 1990 most of the rats on that farm survived doses of warfarin that would have killed a rat in 1965. Explain, using natural selection, how this happened. [5]

The scaffolding is below. The sentences are yours — write all five on paper before you open the solution. If you find yourself writing that the rats got used to the poison, stop and start that sentence again with the words “some rats already…”.

Variation — what was already true of the rat population in 1965?
Many offspring — what do you know about how rats breed?
Struggle for survival — what are they competing for, and what is killing them?
Better adapted survive and reproduce — which rats, and why more likely rather than certain?
Alleles passed on — what has changed about the population by 1990? Use the word proportion.

Step 1 — variation already exists

In 1965 there was already genetic variation in the rat population: a small number of rats carried an allele, produced by a random mutation, that made them resistant to warfarin. That allele was present before the farm ever used warfarin — the poison did not produce it.

Step 2 — many offspring

Rats produce many offspring, far more than the food and shelter on the farm can support, so not all of them can survive.

Step 3 — struggle for survival

There is a struggle for survival: the rats compete for resources such as food, water and shelter, and in addition the warfarin kills the rats that do not carry the resistance allele.

Step 4 — the better adapted survive and reproduce

The rats carrying the resistance allele are better adapted to a farm where warfarin is present, so they have a greater chance of surviving and reproducing than rats without it. Not a certainty — a resistant rat can still be caught by an owl or starve. The word is more likely.

Step 5 — alleles passed on

The surviving rats pass the resistance allele on to their offspring. Repeated over many generations, the proportion of the farm population carrying the allele rises, until by 1990 most rats on the farm are resistant.

Compare your five sentences with these five. Mark yourself out of 5, one mark per step you established. If you scored 3, the two you missed are almost certainly steps 1 and 5 — they are the two everyone leaves out, and they are the two that carry the whole argument.

The Five Traps, One Per Step

Every step of the chain has a specific wrong sentence attached to it. These are not careless slips — they are the sentences that feel most natural to write, which is exactly why they are dangerous. Learn the pair, not just the correct half.

Step	Sentence that loses the mark — do not write these	Sentence that earns it
1. Variation	✗ “The warfarin caused a mutation that made the rats resistant.” / “The rats changed because they needed to survive the poison.”	“Some rats already carried a resistance allele, produced by a random mutation, before warfarin was used.”
2. Many offspring	✗ Leaving the step out altogether — jumping straight from variation to survival.	“Rats produce many offspring, more than the environment can support.”
3. Struggle for survival	✗ “The rats got used to the poison.” / “The population learns to tolerate warfarin.”	“There is a struggle for survival: they compete for resources such as food and shelter, and warfarin kills those without the allele.”
4. Better adapted survive	✗ “The strongest rats survived.” / “Only the fittest survive.” / “The resistant rats definitely survive.”	“The resistant rats are better adapted to that environment, so they are more likely to survive and reproduce.”
5. Alleles passed on	✗ “So now all rats are resistant.” — a conclusion with no inheritance step and no mention of generations.	“The survivors pass the allele to their offspring, so the proportion of the population carrying it increases over many generations.”

Seven sentences that score nothing — every one of these is a wrong answer

✗ “The bacteria became resistant so that they can survive the antibiotic.”
✗ “The rats changed in order to survive the poison.”
✗ “The plant developed a thick cuticle because it needed to save water.”
✗ “The species adapted itself to the desert.”
✗ “The bacterium learns to survive the antibiotic.”
✗ “Natural selection tries to improve the species.”
✗ “Nature decides which animals get the useful features they wanted to have.”

Read them again and notice what they share: in every one, an organism or nature itself is the subject of an active verb — became, changed, developed, adapted, learns, tries, decides. That grammatical pattern is the error. Natural selection has no goal, no plan and no preference. It is simply the arithmetic of who left more offspring.

There is also one phrase to retire even though people use it constantly: “survival of the fittest”. It is circular as an explanation, and in everyday English “fittest” suggests physically strong, which is not what is meant. Write better adapted.

And the famous wrong answer, which is worth meeting once so that you recognise it: the giraffe stretched its neck reaching for high leaves, and its offspring inherited the longer neck. This is wrong at step 1. Stretching is something that happens to an individual during its life; it does not change the alleles in that giraffe’s gametes, so there is nothing to inherit. The correct account starts one step earlier: giraffes already varied in neck length, because of mutation; in a drought the ones with longer necks reached food others could not, so they were more likely to survive and reproduce; and they passed those alleles on, so the proportion of long-necked giraffes rose over many generations.

Supplement

Adaptation, defined as a process

adaptation = the process, resulting from natural selection,
by which populations become more suited to their environment
over many generations

Read the subject of that sentence: populations. And the timescale: over many generations. This definition is doing one job — making it impossible to describe adaptation as something an individual does to itself.

Be careful with the two related words, because a question will sometimes use one where you expect the other. An adaptive feature (18.2) is a thing — a rolled leaf, a resistance allele. Adaptation is the process that produced it. If a question asks you to define adaptation and you describe a rolled leaf, you have answered a different question.

Check Yourself: 18.3 Natural Selection

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which of the five steps of natural selection is the one most often left out of exam answers, and which idea does it establish?

A the struggle for survival, which establishes that resources are limited

B the greater chance of reproduction, which establishes that the strongest survive

C production of many offspring, which establishes that the species is common

D variation already existing, which establishes that the allele was present before the environmental factor arrived

Step 1 is the one that determines whether the rest of the answer means anything, and it is the one most commonly missing. Option C misreads step 2: many offspring matters because more are produced than can survive, not because the species is numerous. Option B slips in “strongest”, which is not what better adapted means.

Question 2

What is the effect of an antibiotic on a population of bacteria in which a few cells already carry a resistance allele?

A It causes the sensitive bacteria to mutate into resistant ones.

B It makes all the bacteria slightly more tolerant of the drug.

C It kills the sensitive bacteria, so the resistant ones face less competition for nutrients and space.

D It has no effect until a mutation occurs.

The antibiotic acts as the selection pressure: it removes the competition. Options A and B are the same misconception in two forms — that the drug changes the cells rather than filtering them. Option D forgets that the mutation had already happened before the drug arrived, which is the whole point of step 1.

Question 3

A hospital graph shows resistance rising from 2 % of samples in 1998 to 63 % in 2026. Which single figure is the best evidence that variation existed before the antibiotic was used heavily?

A the 2 % at the start, because it is not zero

B the 63 % at the end, because it shows the population changed

C the 61-percentage-point rise, because it is large

D the steepest part of the curve, because that is where selection was strongest

A starting value above zero is step 1 of the chain drawn as data: some resistant bacteria were already present. Option D is a true statement about the graph but answers a different question — it is evidence about the strength of selection, not about variation existing beforehand. Learn to match the figure to the claim.

Question 4

Fig. 2.1 shows one population of bacteria at three times, before, during and after treatment with an antibiotic. Type P cells are killed by the antibiotic; type Q cells are not.

Which statement best explains the change from A to C?

A The antibiotic converted the type P cells into type Q cells.

B Two type Q cells were present in A before treatment; the antibiotic killed the type P cells, and the two survivors reproduced and passed the allele on.

C The type P cells reproduced more slowly than the type Q cells because they were weaker.

D The population in C is a different species from the population in A.

Count the cells in panel A: 28 of one type and 2 of the other. Those two are the pre-existing variation, and everything after follows from them. Option A is the conversion misconception. Option D is worth naming as wrong: a change in the proportions within a population does not make it a new species.

Question 5

Which sentence would score a mark for step 4 of the chain?

A The strongest individuals survive and the weakest die out.

B Only the fittest survive, which is the survival of the fittest.

C Individuals carrying the allele are better adapted to that environment, so they are more likely to survive and reproduce.

D Every individual carrying the allele survives.

Two words do the work: better adapted and more likely. Option A substitutes strength for adaptation. Option B is circular, which is why examiners do not credit it. Option D turns a probability into a certainty — a resistant rat can still be eaten by an owl.

Question 6

Why does antibiotic resistance spread through a bacterial population in days, when a comparable change in a mammal population would take thousands of years?

A Bacteria mutate on purpose whereas mammals do not.

B Antibiotics are a stronger selection pressure than anything a mammal ever meets.

C Bacteria are not affected by competition for resources.

D Bacteria divide roughly every 20 minutes, so an enormous number of generations pass in a short time.

Natural selection is counted in generations, not in years — and a bacterial generation is 20 minutes. That single fact explains the difference in timescale. Option A is teleological and also false; option B is a guess that the question gives you no evidence for.

Question 7

A patient says “I have become resistant to antibiotics.” What is the biologically correct version of this statement?

A The patient has built up a tolerance to the drug over many courses of treatment.

B The bacteria the patient is carrying include resistant ones, so the antibiotic no longer clears the infection.

C The patient has mutated so that the antibiotic no longer works on her cells.

D The patient has developed antibodies against the antibiotic.

Resistance is a property of the bacterial cell, never of the person — this is straight from Topic 15 and it determines whether the rest of an answer points the right way. Option D borrows correct vocabulary from Topic 10 and applies it to the wrong thing; antibodies act on pathogens, not on drugs.

Question 8

Which statement about mutation and selection is correct?

A Mutation is random; selection is not, because it depends on the environment.

B Both mutation and selection are random.

C Mutation is directed by the environment; selection is random.

D Neither is random; both follow the needs of the species.

This pairing is worth memorising as a single line: mutation is random, selection is not. Which allele appears is a matter of chance; which allele then becomes common is decided by the environment. Options C and D both make the environment or the species reach back and produce the mutation, which is the central error of the topic.

Question 9

A student writes: “Over time the giraffes stretched their necks to reach higher leaves, and their offspring were born with longer necks.” At which step of the chain does this answer first go wrong?

A step 1, because the variation is presented as being produced by the giraffes rather than existing beforehand

B step 3, because no competition is mentioned

C step 5, because it does not use the word allele

D nowhere — it is a valid alternative explanation

It fails at the first link, and everything after inherits the failure. Stretching happens to an individual during its life and does not alter the alleles in its gametes, so there is nothing to pass on. The correct account begins “giraffes already varied in neck length”. Options B and C describe real omissions, but they are downstream of the fatal one.

Question 10

Which is the Supplement definition of adaptation?

A an inherited feature that helps an organism to survive and reproduce

B the process, resulting from natural selection, by which populations become more suited to their environment over many generations

C the way an individual organism adjusts to a change in its surroundings

D the increase in the number of adaptive features a species has over time

Option A is the definition of an adaptive feature — a thing, not a process — and swapping the two is a common way to lose an easy mark. Option C is the error the definition exists to rule out: the subject must be populations, and the timescale many generations.

Question 11

In a wet year, dark-shelled snails are eaten more often by thrushes than pale-shelled snails on a pale background. Shell colour is inherited. What will happen to the population over many generations if the pale background persists?

A Individual dark snails will gradually become paler during their lives.

B The snails will produce more offspring in order to survive predation.

C Dark snails will disappear completely within one generation.

D The proportion of snails carrying alleles for pale shells will increase.

Notice the shape of the correct answer: the proportion of the population carrying an allele changes. That is what natural selection produces, and it is the phrasing to reach for. Option A is the individual-changes error, option C ignores that selection acts over many generations and that dark snails still sometimes escape, and option B has the snails responding on purpose to a threat.

Question 12

Doctors are advised to prescribe antibiotics only when they are essential. Which explanation of this advice uses natural selection correctly?

A Frequent use gives bacteria more chances to develop the resistance they require.

B Antibiotics become weaker each time they are used.

C Every course of antibiotics kills sensitive bacteria and leaves any resistant ones to reproduce, so frequent use increases the proportion of resistant bacteria.

D Overuse makes patients resistant to their own antibiotics.

Each course is a round of selection, which is why reducing unnecessary courses slows the spread. Option A sounds close but has the bacteria acquiring what they require, which is the wrong direction of causation. Option B misplaces the change in the drug rather than the population, and option D puts the resistance in the patient.

18.4 Selective Breeding, and Natural vs Artificial Selection ▼

The Three Steps, and the Words That Cambridge Marks

Selective breeding — also called artificial selection — has its own chain, and it is shorter. Three steps, and each one has a verb the mark scheme is looking for.

The three steps — select, cross, select again

1. Humans select the individuals with the desirable feature.

2. Those individuals are crossed — bred together.

3. The offspring showing the desirable feature are selected and crossed again — and this is repeated over many generations.

Three verbs: select, cross, select. The third step is the one people leave out, and it is the step that makes it breeding rather than a single lucky pairing. If your answer describes one cross and stops, it has described nothing that would change a crop.

Four generations of one wheat crop. Head size stands for grain yield; the mean climbs from 14.0 to 25.7. The dashed rings are a human decision, made again in every generation.

Look carefully at what does and does not change in that diagram, because both matter.

What changes is the mean. Generation 1 has a mean yield of 14.0; by generation 4 it is 25.7. Nothing dramatic happened in any single step — each generation moves the average a few units, because only the highest-yielding plants were allowed to breed.

What does not change is the existence of variation. Every generation still contains a spread of plants: some better, some worse. That is essential, because if the offspring were all identical there would be nothing left to select in the next round. Selective breeding, exactly like natural selection, needs variation to already exist. It shifts the population; it does not create the differences it works with.

The Two Contexts Cambridge Names

The syllabus asks you to outline selective breeding to improve crop plants and domesticated animals, and to apply it to a context you are given. The two safest examples to have ready:

Context	The desirable feature	How the three steps run
Wheat (crop plant)	high grain yield — but also short stems that do not fall over, and resistance to fungal disease	the farmer keeps seed only from the highest-yielding plants; those plants are crossed; the highest-yielding offspring are kept and crossed again, season after season
Dairy cattle (domesticated animal)	high milk yield	the cows with the highest milk yield are chosen and bred with a bull whose mother and daughters had high yields; their daughters are measured, the best are kept for breeding, and this is repeated over many generations

A detail worth a mark in animal questions

Milk yield can only be measured in a female. So how does a farmer select a bull? By the yields of his female relatives — his mother, and later his daughters. It is a nice reminder that the feature being selected and the individual being selected are not always the same organism, and examiners like it because it cannot be answered from a memorised list.

Worked example Fig. 6.1 shows four generations of a wheat crop grown on one farm. Head size is drawn in proportion to the grain yield of each plant. In each generation only the two plants inside the dashed rings were used to produce the next generation. Describe how the farmer produced generation 4, and explain why the mean yield increased. [5]

Step 1 — start with the variation, exactly as in 18.3

Generation 1 shows variation in grain yield: the plants are not all the same, and that variation was there before the farmer did anything.

Step 2 — who selects, and on what basis

The farmer selects the two plants with the highest grain yield — the ringed plants. The selecting is done by a person, on the basis of a feature the person finds desirable.

Step 3 — cross them

Those two selected plants are crossed, and their seed is grown to produce generation 2.

Step 4 — and repeat, which is the step people miss

The offspring with the highest yield are selected again and crossed again, and this is repeated to give generations 3 and 4 — that is, over many generations.

Step 5 — explain the rise using alleles

Only the highest-yielding plants passed on their alleles, so in each generation the proportion of the population carrying alleles for high yield increased. The mean therefore rose from 14.0 to 25.7. Note that each generation still shows a spread — the variation has not disappeared, the average has moved.

Variation existed; the farmer selected the two highest-yielding plants; they were crossed; the highest-yielding offspring were selected and crossed again over several generations; so the proportion of plants carrying alleles for high yield rose, and the mean yield rose with it.

The same figure with the reasoning shown. The wheat did not grow bigger heads on purpose — the farmer chose which plants were allowed to reproduce.

One cycle of selective breeding, drawn for dairy cattle. The dashed arrow — going round again, generation after generation — is the step that exam answers most often leave out.

The same figure with the labels removed. Match each letter to what it marks.

Label it yourself

Every dropdown has the same list. Match each letter on the figure to what it marks, then press Check.

Supplement Natural Selection Against Artificial Selection

This is the comparison the Supplement asks for, and it is the place where confident students throw marks away — because they answer it with the first difference that comes to mind, which is usually the wrong one.

	Natural selection	Artificial selection (selective breeding)
Who or what does the selecting	the environment	humans
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
How long it takes	usually many, many generations; slow	many generations, but usually much faster, because the selection pressure is intense and deliberate
Result	the population becomes better adapted to its environment	the population develops the feature humans wanted; often with less genetic variation left
Where the variation comes from	mutation; selection acts on variation that already exists	mutation; selection acts on variation that already exists — identical in both

Read this before you answer any comparison question

The difference is WHO OR WHAT DOES THE SELECTING. That is the answer, and it should be your first sentence.

It is not speed alone. Artificial selection is usually faster, but that is a consequence, not the definition — and antibiotic resistance shows natural selection running in days.

It is not that one uses variation and the other does not. Both act on variation that already exists and neither creates it. An answer that says selective breeding “creates new features” while natural selection “uses existing ones” has the biology wrong in both halves.

One consequence in the table is worth understanding rather than memorising: artificially selected populations often end up with less genetic variation. That follows directly from the method. Every generation, only a small number of individuals are allowed to breed, so most of the alleles in the population are simply never passed on. Over many generations the population becomes more and more uniform — which is convenient for a farmer and risky for the crop, because a uniform population is one in which a new disease that affects one plant affects all of them. You met exactly this argument as the disadvantage of asexual reproduction back in 16.1; it is the same argument arriving from a different direction.

And the other consequence: a feature humans select for is chosen because humans want it, not because it helps the organism. A dairy cow producing forty litres of milk a day, or a wheat plant whose head is so heavy the stem struggles to hold it, would not last long without human care. Natural selection would not have produced either.

Check Yourself: 18.4 Selective Breeding

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

Which sequence describes selective breeding?

A humans select individuals with the desirable feature, then cross them, and keep the resulting crop

B humans select individuals with the desirable feature, cross them, then select the offspring showing the feature and cross those, over many generations

C humans expose individuals to conditions that produce the desirable feature, then breed from them

D humans remove the individuals without the desirable feature, so the rest change over time

Option A is the answer most people write and it stops after one cross — there is no repetition and no mention of generations, so it describes a single pairing rather than a breeding programme. Option C has the conditions producing the feature, which is the teleology trap in a farming costume.

Question 2

What is the single most important difference between natural selection and artificial selection?

A Artificial selection is faster.

B Artificial selection creates new alleles, whereas natural selection uses existing ones.

C Only natural selection depends on inheritance.

D In natural selection the environment does the selecting; in artificial selection humans do.

Who does the selecting is the definition; everything else follows from it. Option A is true in most cases but is a consequence, not the difference — and antibiotic resistance is natural selection running in days. Option B is wrong twice over: neither process creates alleles, and both act on variation produced by mutation.

Question 3

In the wheat diagram, each generation still shows a range of head sizes. Why does this matter?

A It shows the selective breeding has failed.

B It shows head size is caused by genes only.

C Variation must remain for the farmer to have something to select in the next generation.

D It proves the farmer selected the wrong plants.

Selection can only pick between things that differ. Option B gets it backwards — yield is a continuous feature affected by genes and by growing conditions, which is why each generation shows a spread rather than a few categories.

Question 4

A farmer wants to increase milk yield in her dairy herd. Milk yield can only be measured in females. How should she choose a bull?

A by using the milk yields of his mother and, later, his daughters

B by choosing the largest and strongest bull

C by feeding several bulls a high-quality diet and choosing the one that grows fastest

D she cannot select a bull at all, so she should only select cows

The bull carries alleles for milk yield even though he cannot express them, so his female relatives are the evidence. Option B substitutes an unrelated feature, and option C selects on something the environment largely produced — growth on a good diet is not evidence about alleles.

Question 5

Why do selectively bred populations often end up with reduced genetic variation?

A because breeding programmes prevent mutations from occurring

B because humans always choose genetically identical parents

C because the offspring are produced asexually

D because in every generation only a few individuals are bred from, so most alleles in the population are never passed on

It is a direct consequence of the method: a narrow set of parents means a narrow set of alleles reaching the next generation. Option A misunderstands mutation, which is random and continues regardless. Option C is wrong because selective breeding involves crossing, which is sexual reproduction.

Question 6

Why is reduced genetic variation in a crop a risk?

A The crop will stop responding to fertiliser.

B If a new disease affects one plant, it is likely to affect all of them, because few plants carry alleles that would give resistance.

C The plants will begin to reproduce asexually.

D Mutations will no longer occur in the crop.

This is the same argument you met as the disadvantage of asexual reproduction in 16.1, arriving from a different direction: a uniform population has no variation for selection to work with when conditions change.

Question 7

A breed of chicken produces very large breast muscles but cannot walk more than a few metres. What does this best illustrate?

A that artificial selection selects features humans find desirable, which may reduce the ability to survive without human care

B that natural selection produced this feature over many generations

C that mutations are always harmful

D that selective breeding always improves an organism

This is the sharpest way to see the difference between the two kinds of selection. The environment would never have favoured a bird that cannot walk; a human market did. Option D is the assumption the example is designed to break — “improved” here means improved for us.

Question 8

Which statement about variation in the two kinds of selection is correct?

A Natural selection acts on variation from mutation; artificial selection acts on variation produced by farmers.

B In both, the selection produces the variation it then acts on.

C In both, the variation arises by mutation and the selection acts on variation that already exists.

D Artificial selection requires no variation, because humans decide the outcome.

The source of variation is the same in both — this row of the comparison table is the one students most often get wrong, because it feels as though a deliberate human process ought to work differently. It does not. Only who does the selecting differs.

Question 9

A student writes about a herd of cattle: “Over many generations the cows became better at producing milk because the farmer needed more milk.” What is wrong with this sentence?

A Nothing — the farmer did want more milk.

B Cattle cannot be selectively bred.

C It makes the cows change in response to a need, instead of saying the farmer bred from the cows that already had the highest yields.

D Milk yield is not an inherited feature.

The farmer’s intention explains why he selected, but never how the herd changed. The change came from breeding only from the highest-yielding cows, so their alleles were passed on. The teleological trap appears in artificial selection too — it is not only a natural selection problem.

Question 10

Which of these is an example of natural selection rather than artificial selection?

A On an exposed cliff, the plants with short stems are less often damaged by wind and leave more offspring than tall ones.

B A breeder crosses two hens that lay the largest eggs.

C A grower keeps seed only from the tomato plants that ripen earliest.

D A gardener removes every weed from a flowerbed by hand.

Ask one question of each option: who or what decided which individuals reproduced? In A it was the wind — the environment. Option D is worth thinking about: a human is choosing what lives, but no breeding programme follows, so no feature is being selected for across generations.

Question 11

Wheat has been selectively bred for short, strong stems as well as for high yield. Why are short stems desirable?

A Short stems allow the plant to photosynthesise faster.

B A heavy head of grain on a tall stem is more likely to bend over, which makes the crop harder to harvest and the grain more likely to be lost.

C Short stems mean the plant does not need water.

D Short stems are produced by a mutation caused by fertiliser.

This is an application question, not a recall one — work it out from the physical situation. It also shows that a breeding programme usually selects for several features at once, which is why it takes many generations. Option D repeats the misconception that a chemical produces the specific mutation that would be useful.

Question 12

Which comparison would score marks in a question asking for the differences between natural and artificial selection?

A Natural selection is slow and artificial selection is fast; that is the whole difference.

B Natural selection happens outdoors and artificial selection happens on farms.

C Natural selection acts on phenotypes and artificial selection acts on genotypes.

D In natural selection the environment selects and the population becomes better adapted to that environment; in artificial selection humans select and the population gains a feature humans find desirable, often with less genetic variation remaining.

A full comparison names who selects, what is selected for, and the result — three pairs, so up to three marks. Option A gives the one difference that is a consequence rather than a cause. Option C is a sentence that sounds technical and is false: in both cases the selecting is done on the visible phenotype.

18.5 Exam Technique and the Vocabulary That Scores ▼

The Substitution Table

This is the most useful page in the guide. Almost every mark you will lose in Topic 18 is one of these left-hand phrases. Learn to hear yourself writing them.

Instead of this ✗	Write this ✓	Why
the bacteria became resistant	the proportion of bacteria carrying the resistance allele increased	No individual changes. The population changes.
the species adapted itself	the population became better adapted over many generations	Adaptation is a process acting on populations, not an action taken by an organism.
the strongest survive / survival of the fittest	the better adapted individuals are more likely to survive and reproduce	“Strongest” is wrong biology and “fittest” is circular.
they all survived	they were more likely to survive	Selection changes probabilities, not certainties.
the antibiotic made them resistant	the antibiotic killed the sensitive bacteria, leaving the resistant ones to reproduce	Selection removes, it does not create.
a mutation appeared because it was useful	a random mutation had already produced the allele	Mutation is random and comes first.
lots of values / only a few values	a range of phenotypes between two extremes / a limited number of phenotypes with no intermediates	The definitions are what is credited, not a description of the graph.
the plant is used to dry conditions	the plant has inherited features that reduce water loss	“Used to” describes an individual getting accustomed to something.
humans change the animal	humans select which individuals are allowed to breed	The three steps are select, cross, select again.
it evolved to have a long neck	individuals with alleles for longer necks left more offspring, so the proportion with long necks rose	“Evolved to” smuggles a purpose back in.

The 30-second proofreading pass

When you have written any answer in this topic, go back and look only at the verbs. If an organism, a species or nature is the subject of an active verb like became, developed, adapted, changed, chose or evolved to, that sentence is at risk. Rewrite it so the subject is either the proportion of the population or the individuals that carried the allele.

This one habit is worth more marks in Topic 18 than any amount of extra content.

The Command Words, and What Each One Buys You

Command word	What Cambridge expects	In this topic
State / Name	a short answer, no reasoning	“State one example of discontinuous variation.” One phrase. Do not explain.
Define	the exact wording of a definition	Variation, adaptive feature, adaptation, gene mutation. These are the four to have word-perfect.
Describe	say what happens, in order	“Describe natural selection” = the five steps. “Describe selective breeding” = the three steps.
Explain	say why — give a reason or mechanism	“Explain how the population became resistant” = the five steps plus the reason at each stage.
Outline	the main points only, briefly	“Outline how selective breeding is carried out” = select, cross, select again, over many generations.
Suggest	apply what you know to something unfamiliar; there may be more than one acceptable answer	Almost always attached to an organism you have never met. You are not expected to know it.

The Five Question Types, and the First Line of Each Answer

Topic 18 questions come in a small number of shapes. Knowing which shape you are looking at tells you the first sentence to write, which is most of the battle.

“Explain how this population came to be resistant / dark / short-stemmed” [4–6 marks]. First line: “There was already genetic variation in the population…” Then the other four steps. One sentence per step, and count them against the mark allocation before you move on.
“Describe how the farmer produced this variety” [3–4 marks]. First line: “The farmer selected the individuals with…” Then cross, then select the offspring and repeat over many generations.
“Describe the adaptive features of this organism” [3–4 marks]. One feature per sentence, and each sentence has three parts: name it, say what it does, link it to survival or reproduction in that environment.
“State two differences between natural and artificial selection” [2 marks]. First line: who does the selecting. Second: what is selected for. Never lead with speed.
Data questions — a graph, a table or a histogram. Quote figures with their units, describe the shape rather than just the direction, and if you are asked for evidence, name the specific number that supports the claim. “It increases” is worth about a quarter of what “it rises from 2 % in 1998 to 63 % in 2026, most steeply between 2010 and 2018” is worth.

Match the number of sentences to the number of marks

A 5-mark natural selection question is five marks because there are five steps. A 3-mark selective breeding question is three marks because there are three steps. This is the most predictable mark allocation on the whole syllabus — use it. Before you start writing, glance at the mark total and decide how many separate points you owe. If you have written four sentences for a 5-mark question, you know exactly what to go looking for, and it is almost always step 1 or step 5.

Six Things to Check the Night Before

Can you write the five steps of natural selection from memory, in order, without looking? Do it on paper. It should take ninety seconds.
Can you write the three steps of selective breeding, including “over many generations”?
Can you give the four definitions word for word — variation, adaptive feature, adaptation, gene mutation?
Can you name Cambridge’s examples: body length and body mass for continuous; ABO blood groups, seed shape in peas and seed colour in peas for discontinuous?
Can you list five xerophyte features with a mechanism for each, and the two key hydrophyte features (stomata on the upper surface, large air spaces)?
Can you say what the single difference between natural and artificial selection is, in one sentence, without mentioning speed?

If all six are yes, you are done with this topic. It really is that small a body of knowledge — which is exactly why the marks are decided by how you write rather than by how much you know.

Check Yourself: 18.5 Exam Technique

12 multiple choice questions. Click an option to check your answer.

Your Score 0 / 12

Question 1

A question is worth 5 marks and asks you to explain how a population of insects came to be resistant to an insecticide. How many separate points should your answer contain, and what are they?

A five: variation already present, many offspring, struggle for survival, better adapted are more likely to survive and reproduce, alleles passed on

B one long paragraph covering the idea thoroughly

C five: mutation, insecticide, death, survival, resistance

D three: select, cross, select again

The mark total is telling you the structure. Option D is the selective breeding chain used on a natural selection question — the classic confusion between the two halves of objective 18.3. Option C is five words rather than five statements; a list of nouns establishes nothing.

Question 2

Which rewrite fixes the sentence “the rats became resistant to warfarin”?

A the rats slowly became resistant to warfarin over many generations

B the rats evolved to be resistant to warfarin

C the proportion of rats carrying the resistance allele increased over many generations

D the rats developed resistance in response to the warfarin

Option A adds a timescale but leaves the rats as the thing that changed, which is the actual error. The fix is to change the subject of the sentence to the proportion of the population. Option B is the same error with a more scientific-sounding verb.

Question 3

A question says “Outline how selective breeding is used to improve a crop plant. [3]” Which answer fits the command word and the marks?

A a detailed account of the genetics of yield, with a Punnett square

B a description of the five steps of natural selection applied to wheat

C farmers choose the best plants and grow them

D humans select the plants with the desirable feature; those plants are crossed; the offspring showing the feature are selected and crossed again, over many generations

“Outline” asks for the main points briefly, and 3 marks tells you there are three of them. Option C collapses the three steps into one and never repeats the cycle. Option A gives detail the command word did not ask for and would cost time in the exam.

Question 4

Which of these is the best first sentence for a question asking you to state two differences between natural and artificial selection?

A Artificial selection is much quicker than natural selection.

B In natural selection the environment does the selecting, whereas in artificial selection humans do.

C Natural selection is a natural process and artificial selection is an artificial one.

D Artificial selection uses variation, whereas natural selection does not.

Lead with the defining difference and you have banked the safest mark before you write anything else. Option C restates the names of the two processes without saying anything. Option D is a serious error — both act on existing variation.

Question 5

Which phrase should you use instead of “survival of the fittest”?

A survival of the strongest

B only the healthiest individuals survive

C the species improves itself over time

D the better adapted individuals are more likely to survive and reproduce

Two phrases must appear: better adapted and more likely, and the word reproduce should not be dropped. Options A and B replace adaptation with physical condition. Option C gives the process a purpose it does not have.

Question 6

A graph shows the percentage of resistant bacteria rising from 2 % to 63 % over 28 years. A question says “Describe the change shown. [3]” Which answer scores best?

A It rises from 2 % in 1998 to 63 % in 2026; the rise is slow at first, steepest in the middle years, and begins to level off towards the end.

B The bacteria became more and more resistant as time went on.

C The percentage of resistant bacteria increases.

D The graph shows natural selection acting on the bacterial population.

Three marks means three things: the overall direction with figures, the changing steepness, and the levelling off. Option C gives one of the three. Option B is teleological and also confuses individual bacteria with the population. Option D explains rather than describes, which answers a question that was not asked.

Question 7

Which pair of definitions is word-perfect?

A variation = differences between organisms; adaptive feature = a feature that helps survival

B variation = differences caused by mutation; adaptive feature = a feature that an organism develops to suit its habitat

C variation = differences between individuals of the same species; adaptive feature = an inherited feature that helps an organism to survive and reproduce in its environment

D variation = differences between species; adaptive feature = a feature all members of a species share

Option A is the version most students write and it drops three creditable words: same species, inherited, reproduce. Option B narrows variation to one of its causes and then makes the adaptive feature something the organism develops, which is the teleological trap in a definition.

Question 8

You are shown an unfamiliar desert lizard and asked to suggest three adaptive features from a photograph. What should you do?

A Leave it blank, since you have not studied this species.

B For each of three visible features, name it, say what it does, and link it to survival or reproduction in a hot dry environment.

C Describe three features you can see, without saying what they do.

D Write about the adaptive features of a camel instead, since it lives in the same habitat.

The command word suggest is a signal that you are not expected to know the organism; you are expected to apply the three-part sentence. Option C stops after part one, which is the commonest way to score one mark out of three on a question you could have had in full.

Question 9

Which sentence would an examiner accept as a description of discontinuous variation?

A There are only a few possible values.

B There is a limited number of phenotypes with no intermediates.

C The bars on the graph do not touch each other.

D It is not affected by the environment in any way.

The phrase no intermediates is the one that is reliably credited. Option C describes how the data were drawn rather than the biology — a useful clue when you are reading a graph, but not a definition. Option A is the everyday version of the same idea and usually gets nothing.

Question 10

You have written this answer: “Some mosquitoes already carried an allele for resistance. The insecticide killed the others. So now most mosquitoes are resistant.” The question was worth 5 marks. Which two steps are missing?

A variation and the struggle for survival

B mutation and adaptation

C production of many offspring, and passing the alleles on to the next generation

D nothing is missing; the answer is complete

Check the answer against the chain: step 1 is present, step 3 is present in the killing, step 4 is implied. Missing are step 2 (many offspring, more than can survive) and step 5 (the survivors pass the allele on, so the proportion rises over generations). “So now most are resistant” states an outcome without the inheritance that produced it — a very common way to lose the final mark.

Question 11

Which is the most efficient use of the last four minutes of a paper, with two questions unanswered: a 1-mark “state one example of continuous variation” and a 5-mark “explain how a population became resistant”?

A Write “body mass” immediately, then spend the rest of the time on the five steps, one line each.

B Start the 5-mark question because it is worth more, and return to the 1-mark one if time allows.

C Write a plan for the 5-mark answer and leave the 1-mark one blank.

D Check the answers you have already written instead.

Marks per second is the only sensible measure under time pressure, and a two-word recall answer is the best-value thing on the page. Then the five steps are ideal in a hurry because they are five short separate sentences — even three of them score. Option B is what most people do and it regularly costs an easy mark.

Question 12

You proofread an answer and find the sentence “the plants adapted to the dry conditions”. What is the quickest safe fix?

A Change it to “the plants adapted themselves to the dry conditions”.

B Leave it — adapt is a syllabus word, so it must be acceptable.

C Change it to “the plants evolved for the dry conditions”.

D Change it to “the population became better adapted to the dry conditions over many generations”.

Two moves fix almost every sentence in this topic: make the subject the population, and add over many generations. Option B is a fair thought but the syllabus word is the noun adaptation, defined as a process acting on populations. Options A and C make the error more explicit rather than less.