What you'll be able to do by the end
- Explain evolution by natural selection using the four-step sequence
- Describe the difference between inherited and acquired characteristics, and why only one is passed on
- Explain how antibiotic resistance arises, and evaluate strategies to slow it
- Describe how selective breeding works and evaluate its risks
- Explain speciation and the role of isolation
- Describe the evidence for evolution — fossils, resistant bacteria, comparative anatomy
- Explain why the fossil record is incomplete
- Use the binomial system and classify into the five kingdoms and three domains
- Interpret and construct simple evolutionary trees
Teach this unit after Adaptation and Competition — it answers the question that unit leaves open: if organisms don't develop adaptations because they need them, where do adaptations come from?
Part 1 — Variation: where it all starts
Nothing in this unit works without variation. Within any species, individuals differ. Some of that difference is inherited; some is not.
| Type | Cause | Passed on? |
|---|---|---|
| Genetic | Differences in genes, from mutation and sexual reproduction | Yes |
| Environmental | Surroundings — diet, injury, sunlight | No |
| Combined | Most characteristics — height, body mass | Only the genetic part |
Mutation — a random change in the DNA base sequence. Most have no effect; a few are harmful; very occasionally one produces a helpful characteristic. Sexual reproduction — each offspring gets a different combination of alleles, which is why siblings differ.
Mutations happen at random, not in response to need. A bacterium does not mutate because an antibiotic is present — mutations were already occurring; the antibiotic simply revealed which ones mattered. This is the most misunderstood idea in the whole topic, and it's worth a mark in almost every natural-selection question.
Part 2 — Natural selection
1. Variation — individuals vary, and some variation is genetic.
2. Selection pressure — more offspring are produced than can survive; individuals compete for limited resources or face a threat.
3. Survival of the advantaged — individuals suited to conditions are more likely to survive and reproduce.
4. Inheritance — survivors pass their alleles on; over many generations the advantageous allele becomes more common.
Worked example: the peppered moth
Before industrialisation, most peppered moths were pale and speckled — good camouflage on lichen-covered trees. A dark form existed but was rare. Industrial soot killed the lichen and blackened the bark.
| Step | What happened |
|---|---|
| Variation | Both pale and dark moths existed, from a genetic difference |
| Selection pressure | Birds hunting by sight; on blackened bark, pale moths were conspicuous |
| Survival | Dark moths were better camouflaged, survived and reproduced more |
| Inheritance | Dark moths passed the allele on; within decades they dominated industrial areas |
The control: in rural Wales and Scotland, where trees stayed pale, pale moths remained common. Same species, different selection pressure, different outcome.
A question in a novel context
Aldabra giant tortoises on predator-rich islands have thicker, heavier shells than those on islands with few predators. Explain how thicker shells could have evolved in the predator-rich population. (4 marks)
The four phrases that earn the marks: "There was variation…" · "…more likely to survive and reproduce…" · "…passed the alleles to their offspring…" · "…over many generations…" If all four appear, in that order, you've almost certainly scored full marks.
Inherited versus acquired characteristics
Lamarck's idea: characteristics acquired in a lifetime can be passed on — a giraffe stretches its neck, its offspring are born with longer necks. Why it's wrong: stretching a neck doesn't change the DNA in the gametes. Only changes to genes are inherited.
| Characteristic | Inherited or acquired? | Passed on? |
|---|---|---|
| Eye colour | Inherited | Yes |
| A scar | Acquired | No |
| Blood group | Inherited | Yes |
| Large muscles from training | Acquired | No |
| A hedgehog's spines | Inherited | Yes |
| A tortoise's cracked shell from injury | Acquired | No |
Questions frequently ask you to compare Darwin and Lamarck, or explain why a Lamarckian statement is wrong. He was right that organisms change over time and that it relates to their environment — he was wrong about the mechanism. A good answer says what he got right before what he got wrong.
Part 3 — Antibiotic resistance: natural selection you can watch
This is natural selection happening fast enough to observe, which is why it appears on every paper.
1. Variation. In a large bacterial population, random mutations occur; occasionally one gives resistance. 2. Selection pressure. The patient takes the antibiotic. 3. Survival. Non-resistant bacteria are killed; the resistant ones survive. 4. Reproduction. Bacteria divide rapidly — some every 20 minutes — so the resistant survivors multiply with no competition. Result: a population where most bacteria are resistant, and the antibiotic no longer works.
Do not write "the bacteria became resistant." Individual bacteria don't become resistant — the mutation was already there in some of them; the antibiotic selected for it. Write: "Some bacteria already had a mutation giving resistance. The antibiotic killed the others, so only the resistant ones survived and reproduced."
MRSA — Methicillin-Resistant Staphylococcus aureus. S. aureus lives harmlessly on skin; MRSA strains resist methicillin and many other antibiotics, making infections very hard to treat — a serious hospital problem, where patients may already be ill or have open wounds.
Slowing resistance — and evaluating the strategies
| Strategy | How it helps | Limitation |
|---|---|---|
| Don't prescribe antibiotics for viral infections | Antibiotics don't work on viruses; unnecessary use adds selection pressure | Patients expect a prescription; telling viral from bacterial takes time/testing |
| Complete the full course | Stopping early leaves the more resistant bacteria alive to reproduce | Relies on compliance; people stop when they feel better |
| Restrict agricultural use | Large quantities are used in livestock, driving resistance | Economic pressure on farming; enforcement varies internationally |
| Develop new antibiotics | Provides alternatives when existing ones fail | Slow and expensive; resistance eventually develops to new ones too |
Questions often ask you to evaluate rather than list. Evaluation means giving both the benefit and the limitation, then reaching a judgement. A list of four strategies with no limitations will not reach the top band.
Part 4 — Selective breeding
Humans choose which organisms to breed together, to develop a desired characteristic. The process: choose parents with the desired characteristic → breed them → from the offspring, select the best → breed those → repeat over many generations.
| Organism | Selected for |
|---|---|
| Crops | Disease resistance, high yield, uniform ripening |
| Cattle | Milk yield, meat quality, docility |
| Dogs | Body shape, temperament, working ability, appearance |
| Flowers | Colour, scent, size |
| African pygmy hedgehogs | Colour morphs, temperament |
Natural selection vs selective breeding
| Natural selection | Selective breeding | |
|---|---|---|
| Who selects? | The environment | Humans |
| Selected for | Survival and reproduction | Whatever humans want |
| Speed | Usually very slow | Much faster |
| Result | Organisms suited to their environment | Organisms suited to human purposes, sometimes at a cost to the animal |
The problem: inbreeding
Using a small number of related individuals as parents reduces the gene pool, causing: inherited disease (recessive harmful alleles become more common — pugs and bulldogs suffer breathing difficulties from flat faces; German shepherds have high rates of hip dysplasia); vulnerability to disease (a genetically uniform population has no resistant individuals, so one new pathogen can destroy it); and reduced fertility.
Ethical breeders of African pygmy hedgehogs keep pedigree records specifically to avoid mating related animals, because inbreeding in this species is linked to Wobbly Hedgehog Syndrome — a fatal neurological disease. That's selective-breeding theory as a real welfare decision, made by real breeders, for a reason a student can understand.
Part 5 — Speciation
A species is a group of organisms that can breed together to produce fertile offspring. The word fertile is essential: a horse and a donkey can breed to produce a mule, but mules are infertile, so horses and donkeys are separate species.
How new species form: 1. Isolation — a population is split into two groups that can no longer interbreed (a river changes course, a mountain rises, a group reaches an island). 2. Different conditions — the two environments differ. 3. Different selection pressures — different characteristics are advantageous in each place. 4. Genetic divergence — different alleles are selected for; mutations occur independently. 5. Speciation — eventually the two populations are so genetically different that, if they met again, they could no longer interbreed to produce fertile offspring. They are now separate species.
Finches from the South American mainland reached the Galápagos Islands. Each island offered different food — large hard seeds, small seeds, insects, cactus. On each, different beak shapes were advantageous; populations diverged. Today there are around 15 species, all descended from one ancestral population, distinguished largely by beak shape and size.
Part 6 — Evidence for evolution
Fossils are the remains or traces of organisms from millions of years ago, found in rock. They form when hard parts (bone) are gradually replaced by minerals; when parts don't decay because conditions for decay are absent (too little oxygen, too dry, too cold, too acidic); or as preserved traces (footprints, burrows, droppings). They show how organisms have changed over time.
Why the fossil record is incomplete
A common exam question. Three reasons: 1. many early life forms were soft-bodied, leaving few traces; 2. fossilisation is rare — most organisms decay completely; 3. many fossils have been destroyed by geological activity, or have not yet been found. Because of this, scientists cannot be certain how life began.
Other evidence
Antibiotic-resistant bacteria — evolution observed within a human lifetime. Comparative anatomy — the pentadactyl limb, a five-boned structure shared by humans, bats, whales and cats, used for entirely different purposes: same structure, different function, strongly suggesting a common ancestor. DNA evidence — species with more similar DNA are more closely related; now the strongest evidence available, and it has caused several classification changes.
Three reasons: it challenged the religious belief that God created all life; there was insufficient evidence at the time; and the mechanism of inheritance was unknown — genes weren't understood for another fifty years, so Darwin couldn't explain how characteristics were passed on.
Part 7 — Classification
The binomial system
Devised by Carl Linnaeus. Every organism gets a two-part Latin name — Genus species — with the genus capitalised, the species lower case, both italic (or underlined when handwritten).
| Common name | Binomial name |
|---|---|
| African pygmy hedgehog | Atelerix albiventris |
| Bearded dragon | Pogona vitticeps |
| Aldabra giant tortoise | Aldabrachelys gigantea |
| Panther chameleon | Furcifer pardalis |
| Sugar glider | Petaurus breviceps |
| Human | Homo sapiens |
A "robin" in Britain is a different bird from a "robin" in America; a "hedgehog" might mean any of seventeen species. Common names vary by country and language; binomial names are the same worldwide — worth a mark if asked the advantage of the binomial system.
Linnaeus's hierarchy
Kingdom → Phylum → Class → Order → Family → Genus → Species. Groups get smaller and more specific as you go down.
| Level | Bearded dragon |
|---|---|
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Reptilia |
| Order | Squamata |
| Family | Agamidae |
| Genus | Pogona |
| Species | vitticeps |
The three domains
Improvements in microscopes and DNA analysis revealed relationships Linnaeus could not see. Carl Woese proposed three domains above kingdom:
| Domain | What's in it |
|---|---|
| Archaea | Primitive bacteria, often in extreme environments |
| Bacteria | True bacteria |
| Eukaryota | Protists, fungi, plants and animals — all with a nucleus |
The key finding: archaea and bacteria look similar under a microscope but are genetically very different — as different from each other as either is from us. Only DNA analysis revealed this.
Evolutionary trees
They show how closely related organisms are, using fossil evidence and DNA. Each branch point is a common ancestor; the more recent the common ancestor, the more closely related. Do not judge relatedness by how close two species appear on the page — count back to where the branches meet.
A and B share a recent common ancestor; C branched off much earlier. So A and B are more closely related, even if C is drawn nearby or looks physically similar. Physical similarity is not the test — shared ancestry is.
Exam-style questions
Answer all questions. Marks are shown in brackets. Total: 45 marks.
Question 1
Rats in a city are treated with a poison called warfarin. Over several years, the proportion of rats that survive warfarin poisoning increases.
(Total 7 marks)
Question 2
MRSA is a strain of bacterium resistant to many antibiotics.
(Total 8 marks)
Question 3
Breeders of pedigree dogs select for particular features. Bulldogs have been bred for short, flat faces.
(Total 8 marks)
Question 4
The Aldabra giant tortoise is found on the Aldabra atoll. Fossil evidence suggests tortoises reached the islands from Madagascar millions of years ago, probably floating on vegetation.
(Total 8 marks)
Question 5
(Total 8 marks)
Question 6 — extended response
Linnaeus classified organisms by observable structure and characteristics. Modern classification uses Woese's three-domain system. Explain why classification systems have changed over time, and evaluate the advantages of the modern system. (6)
(Total 6 marks)
TOTAL FOR PAPER: 45 MARKS
Mark scheme
Question 1
| (a) 4. Genetic variation / a random mutation gave some rats resistance | 1 |
| Warfarin acted as a selection pressure — non-resistant rats were killed | 1 |
| Resistant rats survived and reproduced | 1 |
| The resistance allele passed to offspring and became more common over generations | 1 |
| (b) 2. Individual rats do not become resistant during their lifetime | 1 |
| The mutation was already present by chance before exposure; the poison only selected for it | 1 |
Also credit: "immune" is the wrong word — immunity relates to the immune system and pathogens, not poison resistance.
(c) 1 — Rats have a much shorter generation time / reproduce far more quickly, so many more generations pass in the same period.
Question 2
| (a) 4. Random mutation in some bacteria produces resistance | 1 |
| The antibiotic kills non-resistant bacteria | 1 |
| Resistant bacteria survive and reproduce, with reduced competition | 1 |
| Bacteria reproduce rapidly, so the resistant population increases quickly | 1 |
| (b) 2. Feeling better does not mean all bacteria are dead | 1 |
| The bacteria surviving longest are the most resistant; stopping early leaves them to reproduce | 1 |
| (c) 2. Handwashing reduces transmission between patients rather than killing bacteria | 1 |
| Fewer infected → fewer courses prescribed → less selection pressure and fewer chances for resistant strains to spread | 1 |
Candidates who only say "it stops germs spreading" get one mark.
Question 3
| (a) 3. Choose parents with the desired characteristic | 1 |
| Breed them together | 1 |
| Select the best offspring and breed those, repeating over many generations | 1 |
| (b) 3. Uses closely related individuals, reducing the gene pool | 1 |
| Harmful recessive alleles become more common / more likely inherited from both parents | 1 |
| The selected feature itself may be harmful — a flat face restricts the airway | 1 |
Also credit: reduced variation → vulnerable to a new disease.
| (c) 2. Similarity: both involve favourable characteristics passed on over generations / both change allele frequency | 1 |
| Difference: environment selects vs humans select (also accept: much faster; may produce organisms less suited to survival) | 1 |
Question 4
| (a) 5. The two populations became geographically isolated and could no longer interbreed | 1 |
| Conditions on the islands differed from the mainland | 1 |
| Different characteristics were advantageous, so different selection pressures acted | 1 |
| Different alleles were selected for; mutations occurred independently | 1 |
| Over many generations they became so genetically different they could no longer interbreed to produce fertile offspring | 1 |
The final mark requires "fertile" — it's the definition of a species.
(b) 1 — Aldabrachelys gigantea. Genus capitalised, species lower case, italicised/underlined. 0 for "aldabrachelys Gigantea" or unitalicised when "written correctly" is specified.
| (c) 2. Organisms can look similar without being closely related (convergent evolution), or look different while closely related | 1 |
| DNA shows the actual genetic relationship / shared ancestry, which appearance may not reflect | 1 |
Question 5
(a) 3 — one each: many early organisms were soft-bodied; fossilisation is rare (most decay completely); many fossils destroyed by geological activity or not yet found.
| (b) 3. The same basic bone structure appears in species with very different lifestyles | 1 |
| The structure has been adapted for different functions — flight, swimming, grasping | 1 |
| This suggests a common ancestor from which the limb was inherited | 1 |
(c) 2 — any two: challenged religious belief; insufficient evidence at the time; mechanism of inheritance not understood (genes unknown for another fifty years).
Question 6 — extended response (6 marks, levels-marked)
Level 3 (5–6): explains why classification changed (improved technology/evidence) and evaluates the modern system with advantages and a limitation, reaching a supported judgement. Level 2 (3–4): reasons for change + at least one advantage, limited development. Level 1 (1–2): relevant points as a list.
Indicative — why it changed: Linnaeus could only use observable characteristics; better microscopes revealed internal structures; biochemistry improved; DNA analysis revealed genetic relationships directly; archaea vs bacteria look similar but are genetically very different (only DNA showed this). Advantages: reflects actual evolutionary relationships, not surface similarity; avoids grouping by convergent evolution; allows evolutionary trees; internationally accepted. Limitations (top band): needs expensive technology; classifications keep changing; some organisms (e.g. asexual) hard to classify where the species definition doesn't apply cleanly.
Common mistakes in this topic
"The bacteria became resistant." They didn't — some already were, and the antibiotic killed the rest.
"Immune" instead of "resistant." Immunity is an immune-system response to a pathogen; resistance to a poison or antibiotic is different.
Forgetting "fertile." A species is defined by producing fertile offspring. Leave the word out and you've defined something else.
Missing "over many generations." Natural-selection questions almost always have a mark for the timescale — one sentence, one mark.
Judging evolutionary trees by page position. Count back to the branch point; two species drawn next to each other may be distantly related.
Wrong binomial capitalisation. Homo sapiens, never homo Sapiens or Homo Sapiens. A free mark, regularly dropped.
Explaining Lamarck as simply stupid. He was right that species change and that the environment is involved; wrong about the mechanism. Say both.
Teaching notes
This unit answers the question the adaptation unit leaves open. Students will have been told "individuals don't develop adaptations because they need them" without being told where adaptations come from. Open by asking them, and let the unit be the answer.
The four-step sequence is worth drilling to automaticity. Variation → selection pressure → survival and reproduction → inheritance over generations. Once students can write those four sentences about any organism, they can answer any natural-selection question on any paper.
Antibiotic resistance is the highest-value section — on almost every paper, links to the immune system and drug development, and students can see it in the news.
Question 2(c) is the discriminating question: handwashing reduces transmission → reduces prescriptions → reduces selection pressure. Linking three ideas separates a grade 7 from a grade 5.
The species definition catches everyone. Ask early: are a horse and a donkey the same species? They breed successfully — most say yes. The mule is the answer, and they remember it.
Selective breeding is where ethics enters naturally: bulldogs' breathing, hip dysplasia in German shepherds, Wobbly Hedgehog Syndrome in inbred hedgehogs. Students have strong opinions and it's genuinely relevant to anyone keeping animals.