Theory of evolution, speciation, fossils, extinction and resistant bacteria – GCSE Biology exam questions with mark scheme

1Grade 6–75 marks

Some strains of bacteria are resistant to antibiotics.

(a) Explain how a population of antibiotic-resistant bacteria develops. [4]

(b) Give one way doctors can reduce the rate at which antibiotic-resistant strains develop. [1]

(a) mutation occurs (in a bacterium) giving resistance to the antibiotic(1)
(a) when the antibiotic is used, non-resistant bacteria are killed but resistant bacteria survive(1)
(a) resistant bacteria reproduce / divide (rapidly)
allow resistant bacteria have less competition so reproduce(1)
(a) passing on the (mutated) gene / allele for resistance, so the population of resistant bacteria increases / the resistant strain spreads(1)
(b) any one from: not prescribing antibiotics for non-serious or viral infections; (telling patients to) complete the full course; prescribe only the specific antibiotic for the specific bacteria(1)
Answer: (a) Mutation → resistant bacteria survive antibiotic → reproduce → pass on resistance gene (b) e.g. don't prescribe antibiotics for viral infections.
Examiner tip: Bacteria do not 'become immune' or 'get used to' antibiotics – the resistance comes from a random mutation.

2Grade 4–54 marks

Fossils provide evidence for evolution.

(a) Give two ways in which fossils can be formed. [2]

(b) Explain why scientists cannot use the fossil record to be certain about how life began on Earth. [2]

(a) any two from: from parts of organisms that have not decayed because one or more of the conditions needed for decay are absent; when parts of the organism are replaced by minerals as they decay; as preserved traces of organisms, e.g. footprints, burrows, rootlet traces(1)
(a) a second way(1)
(b) many early forms of life were soft-bodied so left few traces / no fossils(1)
(b) many traces / fossils that were formed have been destroyed by geological activity
allow not all fossils have been found / discovered
ignore 'the fossil record is incomplete' unless a reason is given(1)
Answer: (a) Lack of decay; replacement by minerals; preserved traces (b) Early life soft-bodied; fossils destroyed by geological activity.
Examiner tip: 'Fossils are old' is not a reason – explain why the evidence is missing.

3Grade 1–3Foundation5 marks

The golden toad lived in only one small area of mountain forest in Costa Rica. It was last seen in 1989 and is now extinct.

(a) What does extinct mean? [1]

(b) Which two factors can cause a species to become extinct? Tick (✓) two boxes. [2]

☐ a new disease
☐ a large increase in its food supply
☐ new, more successful competitors
☐ a decrease in the number of its predators
☐ more space to breed

(c) Suggest why a species that lives in only one small area is more likely to become extinct than a species that lives in many areas. [1]

(d) Scientists think the forest where the golden toad lived became warmer and drier over a few years.

Suggest why the toads could not survive this change. [1]

(a) there are no individuals of that species left alive (allow no members of the species remain / the species no longer exists anywhere; allow 'the species has (completely) died out'; do not accept 'endangered' or 'very few left')(1)
(b) a new disease ✓(1)
(b) new, more successful competitors ✓ (if more than two boxes ticked, 1 mark is lost for each extra tick)(1)
(c) one event / one change (e.g. a disease, a fire or a change in climate) could kill / affect the whole population (allow the population is small so all could be killed at once; allow they cannot move to another area)(1)
(d) any one from: the change was too fast for them to adapt / evolve; they did not have (enough) variation / alleles to survive the new conditions; they need moist conditions (e.g. their skin / eggs dried out); their food source / habitat was lost (ignore 'they were not adapted' unqualified)(1)
Answer: (a) no individuals of the species are left alive; (b) a new disease, new more successful competitors; (c) a single event could kill the whole (small) population; (d) the change happened too fast for them to adapt / their moist habitat was lost.
Examiner tip: 'Extinct' means none are left anywhere – answers like 'very rare' or 'few left' describe endangered species and score zero.

4Grade 4–55 marks

Fossils are the remains of organisms from millions of years ago, found in rocks.

(a) Draw one line from each fossil to the way it was formed. [3]

Fossil	Way it was formed
a whole insect found inside amber (hardened tree resin)	parts of the organism were gradually replaced by minerals
a stone ammonite shell found in limestone	traces of the organism were preserved
footprints of a dinosaur set in rock	parts of the organism did not decay

(b) Amber forms when sticky resin from a tree covers an insect and then hardens.

Explain why the insect in the amber did not decay. [2]

(a) insect in amber → parts of the organism did not decay(1)
(a) ammonite shell → parts were gradually replaced by minerals(1)
(a) dinosaur footprints → traces of the organism were preserved (more than one line from a fossil scores 0 for that fossil)(1)
(b) (the resin seals the insect so) there is no oxygen / air (allow no water / moisture; allow conditions needed for decay are absent)(1)
(b) so microorganisms / decomposers (bacteria / fungi) cannot survive / grow / get in (allow so decomposers cannot break the insect down; ignore 'germs')(1)
Answer: (a) amber insect – did not decay; ammonite – replaced by minerals; footprints – traces preserved. (b) No oxygen / moisture inside the resin, so microorganisms that cause decay cannot survive.
Examiner tip: For (b) you must link the missing condition (oxygen or moisture) to decomposers being unable to work – 'it was protected' alone scores nothing.

5Grade 6–76 marks

Scientists used the fossil record to estimate the number of families of marine animals alive at different times in the past.

The bar chart shows their estimates.

(a) Give the number of families of marine animals alive 300 million years ago. [1]

(b) Calculate the percentage decrease in the number of families between 260 million years ago and 250 million years ago.

Give your answer to 2 significant figures. [2]

(d) Explain why the number of families increased again between 250 million years ago and 150 million years ago. [2]

(a) 500(1)
(b) decrease = 550 − 200 = 350 and (350 ÷ 550) × 100 (allow correct method with values misread from the graph by ±10 for this mark)(1)
(b) 64 (%) (an answer of 63.6 or 63.64 scores 1 mark only – not given to 2 s.f.; do not accept 175 or 36)(1)
(c) any one from: (large) volcanic eruption(s); asteroid / meteorite collision; rapid / sudden climate change (allow global warming / ice age if described as sudden); (ignore 'disease' / 'predators' – not single catastrophic events)(1)
(d) (surviving) species evolved / changed by natural selection (into new species) (allow speciation occurred)(1)
(d) because there were empty habitats / niches / less competition (so new species could survive and reproduce) (allow 'over millions of years' linked to new species forming)(1)
Answer: (a) 500; (b) 350 ÷ 550 × 100 = 64%; (c) e.g. huge volcanic eruptions or an asteroid collision; (d) survivors evolved into new species by natural selection, filling empty niches with little competition.
Examiner tip: In (b) divide the decrease by the ORIGINAL value (550), not by 200 – dividing by the new value gives 175% and scores zero.

6Grade 4–5Triple only5 marks

(a) Complete the sentence. Choose the answer from the box. [1]

1809

1859

1909

Charles Darwin published On the Origin of Species in ____________ .

(b) Jean-Baptiste Lamarck had a different idea about evolution. He thought that changes an organism developed during its lifetime were passed on to its offspring.

A woman trains as a weightlifter for ten years. Her arm muscles become much larger.

Lamarck's idea would predict that her children would be born with large arm muscles.

Explain why scientists have rejected Lamarck's idea. Use the weightlifter in your answer. [2]

Give two reasons why. [2]

(a) 1859(1)
(b) the larger muscles are caused by training / use, not by a change in her genes / DNA (allow the change is acquired, not genetic; allow the change does not occur in her sex cells / gametes)(1)
(b) so the characteristic cannot be inherited / passed to her children (allow evidence: children of weightlifters are not born with larger muscles)(1)
(c) any two from: it challenged the (religious) idea that God made all animals and plants (allow it went against the Bible / creation story)(1)
(c) …there was not enough evidence (at the time) to convince scientists; the mechanism of inheritance and variation was not known (until 50 years later) (allow genes / DNA had not been discovered; ignore 'people did not believe him')(1)
Answer: (a) 1859; (b) her bigger muscles result from training, not from a change in her genes/gametes, so they cannot be passed on; (c) any two of: it challenged the idea that God made all organisms, there was not enough evidence, and the mechanism of inheritance was unknown.
Examiner tip: In (b) say WHY acquired changes are not inherited (they don't change the DNA in sex cells) – 'because it is wrong' scores nothing.

7Grade 8–9HigherTriple only6 marks

About 10 000 years ago a large lake in a desert contained one species of small fish called a pupfish.

As the climate became drier the lake shrank. It left several small, separate pools that are now kilometres apart.

Some pools are fed by hot springs and have very warm water.
Some pools are very salty.
The food available is different in each pool.

Today the pupfish in different pools look different. When fish from two of the pools are bred together they do not produce fertile offspring.

Explain how the pupfish in the separate pools could have evolved into different species. [6]

the original population is split into separate populations / the pools are separated (by dry land)(1)
(geographical) isolation: fish in different pools cannot meet / interbreed, so alleles cannot be exchanged between pools(1)
there is variation within each population (due to mutations / new alleles arising)(1)
the conditions are different in each pool (temperature, salt concentration, food)(1)
in each pool, the individuals with characteristics most suited to that pool's conditions are more likely to survive and breed(1)
so they pass on the alleles for those characteristics to their offspring(1)
this happens over many generations, so the populations become increasingly different (different alleles become common in each pool)(1)
eventually the populations are so different that they can no longer interbreed to produce fertile offspring – they are separate species(1)
ignore 'the fish adapt because they need to' / Lamarckian statements; do not credit 'the fish mutate to survive'(1)

Level	Marks	What it looks like
Level 3	5–6	A clear, logically ordered explanation that links isolation, variation, different conditions and natural selection over many generations to the populations becoming unable to interbreed to produce fertile offspring. Uses information about the pools.
Level 2	3–4	An explanation that includes isolation and natural selection with some linking, but the sequence is incomplete or the end point (unable to produce fertile offspring) is missing or vague.
Level 1	1–2	Simple, separate statements about isolation, adaptation or survival with little or no linking. May include some confusion, e.g. fish changing because they need to.
No relevant content	0	Nothing worthy of credit.

Answer: The pools isolate populations, so no interbreeding. Each population has variation from mutations. Different conditions in each pool mean natural selection favours different alleles: best-suited fish survive, breed and pass on alleles. Over many generations the populations become so different that they cannot interbreed to produce fertile offspring, so they are separate species.
Examiner tip: Top-level answers finish with the definition of a new species – 'can no longer interbreed to produce fertile offspring'. Stopping at 'they adapted' caps you at Level 2.

8Grade 6–7Triple only6 marks

Our understanding of inheritance and evolution developed over about 100 years.

(a) The events A, B, C and D are in the wrong order.

A The structure of DNA was worked out.
B Scientists showed that genes are found on chromosomes.
C Mendel carried out breeding experiments on pea plants.
D Scientists observed how chromosomes behave when cells divide.

Write the letters in the correct order, starting with the earliest. [2]

(b) Mendel concluded that characteristics are inherited as separate 'units' that are passed on unchanged. His ideas were not accepted by other scientists until after he died.

Suggest two reasons why. [2]

Describe two ways in which Wallace contributed to the theory of evolution. [2]

(a) C, D, B, A all correct (2 marks)(1)
(a) if not fully correct, allow 1 mark for any two letters in the correct position (e.g. C, B, D, A)(1)
(b) any two from: chromosomes / genes / DNA had not been discovered (so there was no explanation for his 'units') (allow the 'units' could not be seen); his work was not widely published / read (allow published in a little-known journal)(1)
(b) …he was a monk / not a well-known scientist; other scientists did not understand / value his mathematical (statistical) approach; his results were only on pea plants so scientists did not think they applied to other organisms (ignore 'no one believed him' unqualified)(1)
(c) any two from: he independently proposed the theory of evolution by natural selection (allow he came up with the same idea as Darwin at the same time); he published joint writings with Darwin (in 1858) (allow this prompted Darwin to publish On the Origin of Species)(1)
(c) …he gathered / provided evidence, e.g. his studies of warning colouration in animals (allow worked on how speciation happens; allow collected thousands of specimens as evidence)(1)
Answer: (a) C, D, B, A; (b) e.g. genes and chromosomes had not been discovered, and his work was not widely read / he was not a known scientist; (c) e.g. he independently proposed natural selection and published jointly with Darwin in 1858, and his work on warning colouration gave evidence.
Examiner tip: For Wallace, 'he helped Darwin' is too vague – name a specific contribution such as the joint 1858 publication or his work on warning colouration.

9Grade 6–7Higher8 marks

MRSA is a strain of bacteria that is resistant to several antibiotics.

(a) In good conditions, a population of MRSA bacteria divides once every 20 minutes. Each division produces a new generation.

Calculate how many generations of MRSA bacteria can be produced in 24 hours. [2]

(b) Rabbits produce about three generations each year.

Use your answer to (a) to explain why resistance to a poison evolves much faster in bacteria than in rabbits. [2]

Explain why. [2]

(d) Explain why the development of antibiotic resistance in bacteria provides evidence for evolution. [2]

(a) 24 × 60 = 1440 (minutes) or 3 generations per hour(1)
(a) 1440 ÷ 20 = 72 (generations) (an answer of 72 with no working scores 2 marks)(1)
(b) bacteria reproduce much more quickly / have many more generations in the same time (allow 72 per day compared with 3 per year)(1)
(b) so a mutation giving resistance / a resistant allele can be passed on to (and spread through) the population much faster (allow so natural selection can act over more generations in a short time)(1)
(c) antibiotics are used a lot in hospitals, so non-resistant bacteria are killed but resistant (MRSA) bacteria survive and reproduce (allow resistant bacteria have less competition)(1)
(c) patients are (often) ill / have weakened immune systems / have open wounds, and are in close contact so MRSA spreads easily (allow poor hygiene / contact with staff or equipment spreads it)(1)
(d) resistant bacteria are selected / survive and reproduce in the presence of antibiotics (natural selection) (allow resistant bacteria pass on the allele for resistance)(1)
(d) so the characteristics of the population change over time / we can observe evolution happening (within a short time) (allow the inherited characteristics of the population change)(1)
Answer: (a) 1440 ÷ 20 = 72 generations; (b) 72 generations a day v 3 a year, so a resistance allele spreads through the population far faster; (c) heavy antibiotic use selects for resistant bacteria and vulnerable patients in close contact spread it; (d) natural selection of resistant bacteria changes the population's inherited characteristics over time – evolution observed directly.
Examiner tip: In (d) don't just describe resistance – link it to natural selection changing the inherited characteristics of a population, which is the definition of evolution.

10Grade 6–76 marks

Modern whales do not have hind (back) legs. Scientists think whales evolved from animals that lived on land.

The diagram shows rock layers at one site. Each layer contains fossils of an animal that scientists think was an ancestor of modern whales. The diagram gives the age of each layer and the hind leg length of the fossil.

(a) Explain how the fossils provide evidence for evolution. [2]

(b) Calculate the mean rate of decrease in hind leg length between layer S and layer P.

Give your answer in cm per million years. [2]

Give two reasons why there could be gaps like this in the fossil record. [2]

(a) (older / deeper rock layers contain fossils with longer hind legs, so) the hind legs became shorter over time (allow the animals gradually changed over time)(1)
(a) this shows (a series of) organisms changing gradually over (millions of years / many generations) / shows how one species may have developed from an earlier one (allow shows a link between land animals and modern whales)(1)
(b) (45 − 3) ÷ (50 − 34) or 42 ÷ 16(1)
(b) 2.6 (cm per million years) (allow 2.63 / 2.625)(1)
(c) any two from: most organisms decay / the conditions for fossils to form are rare (allow the animals did not fossilise); fossils have been destroyed by geological activity (e.g. earthquakes / erosion / rock movement / rocks melted)(1)
(c) …fossils may be present but have not yet been found / dug up (ignore 'the animals were soft-bodied' – these animals had bones; ignore 'they were extinct')(1)
Answer: (a) Fossils in older (deeper) layers have longer hind legs, so the hind legs gradually shortened over time – showing species changing over millions of years; (b) 42 ÷ 16 = 2.6 cm per million years; (c) e.g. fossils rarely form because most remains decay, and fossils can be destroyed by geological activity.
Examiner tip: In (c) 'soft-bodied' doesn't fit here – these animals had bones. Read the context: use rare fossil formation, geological destruction or 'not yet found'.

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