Variation and selection

• One test that catches most of these. Read your answer back and underline every verb whose subject is a single organism. If any of them is became, developed, adapted or changed, the sentence is probably describing something that did not happen. Rewrite it with the population as the subject.
• The one-question test. Ask: can I imagine an individual sitting exactly between these two? If yes, the variation is continuous. If the idea is meaningless — half blood group A, a seed that is three-quarters round — it is discontinuous.
• Six words that carry a natural-selection answer. Variation — offspring — competition — adapted — reproduction — alleles. If all six appear in your answer, in that order, you have covered (a) to (e) of the statement.
• One sentence that ties the chapter together. Variation arrives first, by mutation and reproduction (18.1); some of it happens to suit the place the organism lives (18.2); and the environment, or a breeder, then decides which of it becomes common over generations (18.3). Everything in Topic 18 is a step of that sentence.

• Core 18.1.1 Describe variation as differences between individuals of the same species.
• Core 18.1.2 State that continuous variation results in a range of phenotypes between two extremes, and give body length and body mass as examples.
• Core 18.1.3 State that discontinuous variation results in a limited number of phenotypes with no intermediates, and give ABO blood groups, seed shape in peas and seed colour in peas as examples.
• Core 18.1.4 State that discontinuous variation is usually caused by genes only and continuous variation by both genes and the environment.
• Core 18.1.5 Investigate and describe examples of continuous and discontinuous variation.
• Core 18.1.6 Describe mutation as genetic change.
• Core 18.1.7 State that mutation is the way in which new alleles are formed.
• Core 18.1.8 State that ionising radiation and some chemicals increase the rate of mutation.
• Core 18.2.1 Describe an adaptive feature as an inherited feature that helps an organism to survive and reproduce in its environment.
• Core 18.2.2 Interpret images or other information about a species to describe its adaptive features.
• Core 18.3.1 Describe natural selection with reference to genetic variation within populations, production of many offspring, the struggle for survival including competition for resources, a greater chance of reproduction by better-adapted individuals, and the passing on of their alleles.
• Core 18.3.2 Describe selective breeding with reference to selection by humans of individuals with desirable features, crossing those individuals, and selection of the offspring that show the desirable features.
• Core 18.3.3 Outline how selective breeding by artificial selection is carried out over many generations to improve crop plants and domesticated animals, and apply this to given contexts.
• Supplement 18.1.9 Describe gene mutation as a random change in the base sequence of DNA.
• Supplement 18.1.10 State that mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations.
• Supplement 18.2.3 Explain the adaptive features of hydrophytes and xerophytes to their environments.
• Supplement 18.3.4 Describe adaptation as the process, resulting from natural selection, by which populations become more suited to their environment over many generations.
• Supplement 18.3.5 Describe the development of strains of antibiotic resistant bacteria as an example of natural selection.
• Supplement 18.3.6 Outline the differences between natural and artificial selection.

Why this is the example the syllabus chose. Antibiotic resistance is natural selection running fast enough to watch. Bacteria divide in minutes, so a change that would take thousands of years in a beetle population takes days in a bacterial one — and the consequence, an infection that no longer responds to treatment, is why it matters outside the examination.

• Saying that an organism changed, when the statement is about a population. "The beetles became darker." "The bacteria adapted." No individual in either account changes; what changes is how common each inherited form is. Fix Make the population the subject of the sentence: "the proportion of dark beetles increased".
• Reversing the direction of causation. "The antibiotic made the bacteria mutate." "The plant developed a thick cuticle because it needed to conserve water." Variation comes first, by chance; the environment then acts on what is already there. Fix Always put the variation before the pressure in your answer, in that order.
• Treating a mutagen as a cause rather than a rate. Statement 18.1.8 says ionising radiation and some chemicals increase the rate of mutation. Mutations happen anyway, at a low background rate, without either. Fix Use the syllabus's own phrase: increases the rate of mutation.
• Calling an acquired feature adaptive. A scar, a tan, muscle built by training, a plant kept small by shade — all real, none inherited, none an adaptive feature. 18.2.1 requires the feature to be inherited. Fix Ask: could an offspring be born with this? If not, it is not an adaptive feature.
• Stopping an adaptive-feature answer at survival. 18.2.1 says survive and reproduce. An answer that explains only how the organism stays alive has given half the definition. Fix Add the second clause: staying alive longer means more opportunities to reproduce and pass on the alleles.
• Writing "genes only" and "genes and environment" the wrong way round. Statement 18.1.4 pairs discontinuous with genes only and continuous with both genes and the environment. Under time pressure the pairing flips easily. Fix Anchor it to an example you cannot get wrong: diet changes your body mass (continuous, so environment is involved) but nothing you eat changes your blood group (discontinuous, genes only).
• “The beetles became darker to avoid predators.” This says individuals changed, and that they changed on purpose. Neither happened. Fix “Dark beetles were less often eaten, so more of them survived to reproduce, and the proportion of dark beetles increased.”
• “The environment caused a mutation for dark colour.” Mutation is random with respect to need; the environment selects among variants that already exist. Fix “Dark and pale forms were already present in the population through earlier mutation.”
• “Only the strongest survive.” Strength is usually irrelevant. What matters is which inherited features suit this environment, and point (d) measures the outcome in chance of reproduction, not in fights. Fix “Individuals better adapted to the environment have a greater chance of reproducing.”
• “Every pale beetle died.” Selection is a difference in average success, not a rule with no exceptions. Pale beetles are still present at generation 50. Fix “A smaller proportion of pale beetles survived to reproduce.”
• “The bacteria became immune to the antibiotic.” Immunity is a property of a host’s lymphocytes and antibodies (Topic 10). Bacteria are not immune to antibiotics; they are resistant. Fix Use the word resistant, every time.
• “The person became resistant to the antibiotic.” The population that changes is the bacterial one. The patient’s own cells are not involved. Fix “The bacteria infecting the patient became resistant.”
• “The antibiotic made the bacteria mutate.” This is the whole misconception the statement exists to correct. The mutation came first, at random; the antibiotic then killed everything that did not already carry the allele. Fix “A resistant bacterium was already present because of an earlier random mutation.”

• Where the tiering bites in Topic 18. It bites in two places and both are worth knowing before you read on. Mutation: Core 18.1.6 says genetic change — three words — while Supplement 18.1.9 says a random change in the base sequence of DNA. A Core candidate who writes the Supplement version has not made an error, but a Core candidate who thinks the Supplement version is required has taken on work the syllabus did not ask for. Selection: Core 18.3.1 and 18.3.2 describe the two processes separately; comparing them is Supplement 18.3.6. So a Core candidate needs both accounts but is not examined on the differences between them.
• The error that costs most across this whole topic. Writing about an individual when the statement is about a population. "The rabbit grew thicker fur to survive the cold" fails 18.3.1 no matter how much else is right, because no rabbit in the account ever changes. The population changes, because rabbits that already had thicker fur left more offspring.
• Keep the syllabus's hedge. The statement says discontinuous variation is usually caused by genes only. That word is in the official wording, so it is safe to write, and it protects you: it is the difference between a statement that is true and a generalisation about every categorical feature you will ever meet.
• Three ways to draw the graph wrongly. Leaving gaps between the bars (that makes it a bar chart, and says your data are categories). Omitting the unit on the x-axis — “body length” is not a label, “body length / cm” is. And using unequal class widths without saying so, which distorts the shape the whole graph exists to show.
• Say “usually caused by genes only”, and keep the hedge. For blood group and pea seed shape the environment really does have no effect worth mentioning, and the syllabus's own word is usually. Do not over-generalise the claim to every categorical feature you meet: some features that fall into categories are influenced by the environment as well, which is exactly why the statement is hedged. Note also that a discontinuous feature is not limited to two categories — ABO has four phenotypes, produced by three alleles of a single gene.
• Four, and the statement's verb is state. 18.1.10 does not ask you to explain meiosis, describe mating behaviour or calculate the number of possible gamete combinations. It asks you to state that these four processes are sources of genetic variation in populations. Name all four and you have the mark; name three and you have not.
• The last two rows are the same feature. That is the point of including them. “Is this an adaptive feature?” is never answerable from the feature alone; it is answerable only once you know where the organism lives. A question that gives you a habitat is giving you half the answer.
• The shape of a complete sentence here. “Structure — therefore physical consequence — which matters because the environment is like this — so the organism survives and reproduces more successfully.” Four clauses. The first and the last are the easy ones to write; the two in the middle are where the adaptation actually gets explained.
• The verb is explain, so answer in pairs. “A thick waxy cuticle” is a feature, not an explanation. “A thick waxy cuticle, so very little water evaporates through the leaf surface, which matters because a desert plant cannot replace what it loses” is an explanation. Two fully explained features answer the statement; six named ones do not, however many you list.
• This statement supplies its own structure. 18.3.1 lists five lettered points, (a) to (e), in a logical order: variation exists, offspring are over-produced, survival is a struggle, the better-adapted reproduce more, their alleles pass on. Number them as you write and it is very hard to leave one out; write a paragraph without them and it is very easy.
• Point (c) is the one that is easiest to leave out. A candidate who writes "choose the best plants and breed them together" has described (a) and (b) and stopped. Without (c) the process ends after one generation and achieves almost nothing, because the offspring of a single cross vary just as their parents' population did. Say what happens to the offspring.
• The phrase the statement itself contains. “Repeated over many generations.” 18.3.3 has those words in it, so an outline without them is not a complete answer to the statement. It costs four words to include, and without it what you have described is a single cross, which improves nothing.
• Read the verb before you write. 18.3.6 says outline the differences. In a three-mark question, three clear differences will score three; a paragraph that spends two of its sentences on what the processes share will not. The similarities above are here so that you recognise a wrong "difference" when one occurs to you — for example, "artificial selection produces new alleles", which is false of both.
• Magnification, if it is asked for. The formula is the same one as Topic 2: \[\text{magnification} = \frac{\text{image size}}{\text{actual size}}\] Measure the image in mm, convert both quantities to the same unit before dividing, and give the answer as a number with a multiplication sign, not a unit.

• This section sets out which components can ask about Topic 18, and what its statements themselves demand. It does not predict what any particular paper will contain — nobody can — so everything below is read off the syllabus: the assessment overview, the command words, and the list of practical contexts. Topic 18 supplies one of those named practical contexts, so it reaches Papers 5 and 6 directly rather than only as secondary data.
• A distribution or a table arrives and you name the kind of variation and give the feature of the definition that settles it. Statements 18.1.2 and 18.1.3 each say what the variation results in, so naming the type without that is an incomplete answer.
• Natural selection (18.3.1) or selective breeding (18.3.2). Both statements are written as lettered sub-points, so they have a built-in structure: write them as numbered steps in the syllabus's own order and nothing gets left out.
• Statement 18.2.2 is a skill, applied to a photograph or drawing of a species you have never seen. Name a visible structure, say what it does physically, and link it to surviving and reproducing in that environment.
• Paper 5 or Paper 6, on the practical context named at 18.1.5. Sampling method, sample size, the variable and its unit, precision, the graph. How the individuals were chosen is the element most easily left out, and the one everything else depends on.
• Command word watch. The definitions below are the syllabus's own, from its command-word list. State — "express in clear terms"; 18.1.2, 18.1.3, 18.1.4, 18.1.7, 18.1.8 and 18.1.10 are all state statements, so a mechanism you were not asked for costs time and can introduce an error. Describe — "state the points of a topic / give characteristics and main features"; 18.3.1 and 18.3.2 hand you those points already, as lettered sub-points. Explain — "set out purposes or reasons / make the relationships between things clear"; in this topic that is 18.2.3, where each feature needs its reason attached. Outline — "set out the main points"; 18.3.3 and 18.3.6, so a full essay on either is over-answering. Suggest — "apply knowledge and understanding to situations where there are a range of valid responses"; there is more than one acceptable answer, so give a reasoned one rather than hunting for the single right one.