SPEC 4.6.3.4 – 4.6.3.7 · PAPER 2 · ⭐ PRIORITY
Evidence For Evolution, Fossils, Extinction & Resistant Bacteria
The most content-dense lesson in Module 6. Four spec sub-topics, four exact lists you must memorise: 3 sources of evidence, 3 fossil formation methods, 5 extinction factors, 3 measures to slow antibiotic resistance.
Why This Lesson Matters
This lesson packs four spec sub-topics (4.6.3.4–4.6.3.7) into one. AQA loves it because each list has an exact spec-defined count — making it easy to design 1-mark MCQs and 4–6 mark "explain how" questions.
4
Spec Sub-Topics
In One Lesson
5+
Marks Typical For
Resistance Explains
⭐
Priority Topic
Module 6
The Strategy
The spec gives you exact lists — 3 evidence sources, 3 fossil methods, 5 extinction factors, 3 antibiotic measures. Memorise the counts alongside the items. If a question says "give two" or "give three", that's a deliberate hint about the spec list — pick from the named items, never invent.
Three Sources Of Evidence For Evolution
Spec 4.6.3.4. The theory of evolution by natural selection is now widely accepted. The spec lists exactly three reasons why.
Genetics
It has been shown that characteristics are passed on to offspring in genes. This is the inheritance mechanism Darwin couldn't explain (lesson 6.09).
Fossil Record
Fossils show how organisms have changed over millions of years. Shows new species emerging and old species going extinct.
Antibiotic Resistance
Bacteria evolving resistance — natural selection observable in real time. The strongest modern proof.
Mark scheme word
The spec uses the word "evidence" — not "proof". Science doesn't prove theories; it gathers evidence that supports or rejects them. Writing "fossils prove evolution" is technically wrong; "fossils provide evidence for evolution" is correct.
Fossils — Definition & Three Formation Methods
Fossil
The 'remains' of organisms from millions of years ago, which are found in rocks. (Spec wording, 4.6.3.5.)
Fossils may be formed in exactly three ways the spec lists. Each method covers a different type of fossil.
1
Parts that haven't decayed. One or more of the conditions needed for decay are absent — too cold, too dry, no oxygen. Examples: mammoths in permafrost, insects in amber.
2
Parts replaced by minerals. As the organism decays, its hard parts (bones, shells, wood) are gradually replaced by minerals. The classic stone fossil — e.g. dinosaur bones, petrified wood.
3
Preserved traces. Not the organism itself, but evidence it left behind — footprints, burrows, and rootlet traces.
How Fossils Show Evolution
Fossils are arranged in layers (strata) of sedimentary rock. The deeper the layer, the older the fossil.
What fossils tell us
How much (or how little) different organisms have changed as life developed on Earth.
Which species existed at which time periods.
Which species are extinct (no later fossils found).
How new species may have emerged from older ones.
Evolutionary trees (skills)
The spec asks you to extract and interpret information from charts, graphs, and tables — including evolutionary trees. These diagrams show how species are related: each branch point is a common ancestor; the longer the branches between two species, the more distantly related they are. You may be given an evolutionary tree in an exam and asked which two species share a recent common ancestor.
Why The Fossil Record Is Incomplete
The spec gives two main reasons. Students consistently lose marks here by writing "fossils are old" instead of naming specific mechanisms.
1
Many early forms of life were soft-bodied. Soft tissue decays before it can fossilise — so soft-bodied organisms leave few traces behind. This is why early life is poorly recorded.
2
Traces that existed have been mainly destroyed by geological activity. Tectonic movement, erosion, heat, and rock deformation have wiped out most fossils that did form.
The big consequence
This is why scientists cannot be certain how life began on Earth — the earliest organisms left almost no trace. Spec wording: "scientists cannot be certain about how life began on Earth."
Extinction — Definition & Five Factors
Extinction
Extinction occurs when there are no remaining individuals of a species still alive. (Spec wording, 4.6.3.6.)
Spec 4.6.3.6 asks you to describe factors which may contribute to extinction. AQA mark schemes consistently allow these five:
1
Changes to the environment
2
New predators
3
New diseases
4
New (more successful) competitors
5
Catastrophic event
Mark scheme nuance
For "new competitors", the mark scheme often credits "new more successful competitors" — the word "successful" matters because mere competition isn't enough; the species must be outcompeted. For "catastrophic event", you can either name one (asteroid, volcanic eruption) or use the spec phrase directly.
Why Bacteria Evolve Quickly
Spec 4.6.3.7. Bacteria can evolve rapidly because they reproduce at a fast rate. A single bacterium can divide every ~20 minutes — so in a single day, one bacterium can become billions.
The chain that drives resistance
Mutations of bacterial pathogens produce new strains.
Some strains might be resistant to antibiotics, and so are not killed.
They survive and reproduce, so the population of the resistant strain rises.
The resistant strain then spreads because people are not immune to it and there is no effective treatment.
The direction of causation matters
Mutations happen randomly and continuously — they are not caused by the antibiotic. The antibiotic selects for resistant bacteria that were already there. Writing "antibiotics make bacteria become resistant" is a common error that loses marks (see Common Errors slide).
The Antibiotic Resistance Chain
This is the canonical 5-mark "explain how" question on Paper 2. The chain must be in order, with mark-scheme vocabulary.
1
Variation (BEFORE antibiotic). Random mutations occur continuously in bacterial DNA. Occasionally — and by chance, not because of any antibiotic — a mutation produces a resistance allele. The mutation exists in the population before the antibiotic is used.
2
Selection. When the antibiotic is used, most bacteria are killed, but the resistant ones are not killed.
3
Survival. The resistant bacteria survive the antibiotic.
4
Reproduction. Survivors reproduce rapidly (binary fission), passing on the resistance allele to all offspring.
5
Spread. Over many generations, the resistant strain becomes dominant. It spreads further because people have no immunity and there is no effective treatment.
MRSA & The Three Measures To Slow Resistance
MRSA
The spec's named example of a resistant bacterium: MRSA is resistant to antibiotics. The acronym does not need to be expanded for the mark.
The spec gives exactly three measures to reduce the rate at which antibiotic-resistant strains develop. These three are the only ones that score — vague "use less" doesn't.
1
Doctors should not prescribe antibiotics inappropriately — for example, for non-serious or viral infections (antibiotics don't work on viruses anyway, and using them just selects for resistance).
2
Patients should complete their course of antibiotics so that all bacteria are killed and none survive to mutate and form resistant strains.
3
The agricultural use of antibiotics should be restricted — antibiotics are often added to livestock feed; widespread agricultural use accelerates resistance.
New Antibiotic Development Is Slow
Spec statement (4.6.3.7)
The development of new antibiotics is costly and slow. It is unlikely to keep up with the emergence of new resistant strains.
This is why the spec emphasises the three measures to slow resistance — we can't simply develop our way out of the problem. Bacterial evolution is faster than the drug pipeline.
Why this matters in exam answers
For evaluate-style questions about antibiotic resistance ("Discuss the impact of antibiotic resistance on healthcare"), the spec wants you to recognise that prevention is more effective than cure. The slow-and-costly drug development is the reason behavioural measures (the three above) are the focus.
Practice Question
Typical Paper 2 — "Explain how" antibiotic resistance question5 MARKS
A hospital reports that an infection caused by a strain of bacteria can no longer be treated by the antibiotic that worked five years ago.
Explain how a population of bacteria can become resistant to an antibiotic over time.
5-mark answer. Need all 5 stages of the resistance chain.
1
Random mutations occur in the bacterial DNA, producing a new strain with a resistance allele.
2
When the antibiotic is used, most bacteria are killed, but the resistant bacteria are not killed.
3
The resistant bacteria survive and reproduce rapidly by binary fission.
4
The resistance allele is passed on to offspring, so the population of the resistant strain rises.
5
The resistant strain spreads further because people have no immunity and there is no effective treatment.
Three Examiner Alerts
Alert 1 — Direction Of Causation
Bacteria are already resistant due to random mutation — the antibiotic doesn't cause resistance, it selects for bacteria that were already resistant. Writing "antibiotics make bacteria become resistant" or "bacteria adapt to antibiotics" is Lamarckism in disguise and caps the mark. The mutation comes first; the antibiotic acts as the selection pressure.
Alert 2 — Name The Three Measures
For "How can resistance be slowed?" the mark scheme wants the three spec-listed measures: don't prescribe inappropriately, complete the course, restrict agricultural use. Vague answers like "use fewer antibiotics" or "develop new drugs" don't score — the spec gives an exact list, and AQA examines against it.
Alert 3 — Evidence, Not Proof
Write that fossils, genetics, and antibiotic resistance provide evidence for evolution — not that they "prove" it. Science doesn't prove theories; it gathers supporting evidence. The mark scheme uses the word "evidence" deliberately, and so should you.
Common Errors To Avoid
Wrong
"Bacteria become resistant when exposed to antibiotics."
Right
Resistance arises by random mutation; antibiotics then select for the already-resistant bacteria.
Wrong
"Stop taking antibiotics when you feel better."
Right
Finish the full course — stopping early leaves the most resistant survivors to mutate further.
Wrong
"Soft-bodied animals don't fossilise."
Right
Soft-bodied organisms rarely fossilise (some do under exceptional conditions like amber). Use "few traces behind", not absolutes.
Wrong
"Extinct species might come back."
Right
Extinction = no remaining individuals. Once a species is gone, it is gone (current science).
Three Things To Remember
1
Three Sources Of Evidence (4.6.3.4)
Genetics (characteristics passed on in genes) + fossil record + antibiotic resistance (real-time natural selection). The theory of evolution is now widely accepted because of these three together.
2
The Exact Lists From The Spec
3 fossil formation methods: parts that haven't decayed · parts replaced by minerals · preserved traces (footprints, burrows, rootlet traces). 5 extinction factors: environmental change · new predators · new diseases · new more successful competitors · catastrophic event.
3
Antibiotic Resistance — Mechanism + 3 Measures
Chain: mutation → resistant strain → not killed by antibiotic → survives + reproduces → spreads (no immunity, no effective treatment). Three measures: don't prescribe inappropriately · complete the course · restrict agricultural use. New antibiotic development is too slow to keep up.
Summary
The Four Spec-Listed Counts
3 sources of evidence — genetics, fossils, antibiotic resistance
3 fossil formation methods — no decay, mineral replacement, traces
5 extinction factors — environment, predators, diseases, competitors, catastrophe
3 measures to slow resistance — appropriate prescribing, complete course, agricultural restriction
Mark Scheme Vocabulary
" Evidence for evolution" — not "proof"
" Random mutation " causes resistance — antibiotic selects for it
" No remaining individuals " — extinction definition
" Few traces behind " — soft-bodied organisms
Resistant strain spreads — " no immunity " + " no effective treatment "
Exam Tip
If a question says "give three" or "explain three reasons", that's the spec list signalling itself. For fossils → 3 formation methods. For extinction → pick 3 of the 5 factors. For resistance measures → all 3 are needed. Spec-listed counts are AQA's favourite question design — match them.
Next: Why This Lesson Matters