OCR GCSE Biology A: Variation, natural selection, evolution and classification (J247) – Revision Notes

These are quick-recall notes for topic B5.2 Natural selection and evolution in OCR GCSE (9-1) Gateway Science Biology A (J247), Version 4.0 (August 2026), outcomes B5.2a to B5.2f. No B5.2 statement is shown in bold type, so none is Higher Tier only. For full explanations and worked examples, read the study guide. Then try the practice questions. Course links: OCR GCSE Biology hub, checklist and free diagnostics.

Key definitions

Term	Meaning
Population	All the members of one species living in one area
Species	Organisms that can breed together to produce fertile offspring
Genetic variation	Differences between individuals caused by different alleles
Mutation	A random alteration to the order of bases in DNA, which is how a new allele first appears
Selection pressure	A factor in the environment (predator, disease, climate, competition) that makes some variants more likely to survive
Natural selection	Variants best suited to the environment survive and reproduce more, so their alleles become more common
Evolution	A change in the inherited characteristics of a population over time
Extinction	No members of a species remain alive
Molecular phylogenetics	Comparing DNA (or protein) sequences to work out how related species are
Seedbank	A store of seeds kept to preserve biodiversity

B5.2a Variation

• Sources of genetic variation: mutation, meiosis (random mix of alleles in gametes) and random fertilisation.
• Environmental variation comes from conditions such as light, food and temperature.
• A population usually holds extensive genetic variation before the environment changes.
• No variation means no variants to select.

B5.2b Classification

	Artificial	Natural
Basis	A few chosen, convenient features	Many features and shared ancestry
Shows relationships?	No	Yes
Example	Sorting animals by “has wings”	Placing bats with mammals

• Developments that changed classification: better microscopes and biochemistry, and then DNA sequencing.
• Molecular phylogenetics: fewer differences in the sequence of a gene means a more recent common ancestor.
• Sequencing can regroup organisms that look alike but are not closely related, because similar environments can select similar features.
• Ribosomal RNA sequencing led to the three-domain system: Archaea, Bacteria, Eukaryota.

Method: build a tree from sequence differences

1. Convert each count of differences to a percentage of the bases compared.
2. Find the smallest difference and pair those two species.
3. Pair the next smallest, and join pairs by their larger differences.
4. State that the smallest differences mean the most recent common ancestor.

B5.2c Natural selection

Learn the chain in this order.

1. Variation exists (different alleles).
2. More offspring are born than survive.
3. Individuals with the better-suited phenotype are more likely to survive and reproduce.
4. They pass on the helpful allele.
5. The allele becomes more common over generations.

• Mutations are random. The environment does not make a useful mutation happen.
• A mutation that is harmless in one environment can be an advantage when the environment changes.
• Say “the proportion of the population with the allele rose”, not “the organism changed”.

Phrases that gain credit

• “Some individuals had an allele that gave them…”
• “…so they were more likely to survive, reproduce and pass on the allele”.
• “Over many generations the allele became more common.”

B5.2d Evolution and new species

1. A population is separated into groups that do not interbreed.
2. Each group faces different selection pressures and different mutations.
3. Over many generations the groups become so different that they cannot produce fertile offspring together.
4. Two species now exist.

• Evolution happens to populations over generations, never to a single organism.
• A species with no variant able to cope with a changed environment may become extinct.

B5.2e Evidence for evolution

Evidence	What it shows	Limitation
Fossils	Past organisms differed from today’s; features change through rock layers; intermediate forms exist	Record is incomplete: soft bodies decay, few remains fossilise, many fossils undiscovered
Antibiotic resistance	Evolution observed within human lifetimes	Applies only to fast-reproducing organisms in this form

Antibiotic resistance chain

1. Random mutation gives one bacterium a resistance allele.
2. The antibiotic kills non-resistant cells (selection pressure).
3. The resistant cell survives and divides.
4. The allele spreads through the population, and the antibiotic stops working.

Reducing resistance: do not use antibiotics for viral infections, finish prescribed courses, and avoid unnecessary use.

B5.2f Darwin and Wallace

• Both developed the idea of natural selection independently; their ideas were presented together in 1858.
• Darwin published On the Origin of Species in 1859.
• The ideas were challenged, partly because the mechanism of inheritance was unknown. Mendel’s work was not widely appreciated at first. DNA and genes later explained inheritance and mutation.
• Impact on modern biology: explains biodiversity and relatedness, underpins classification, explains antibiotic resistance, and informs conservation.
• Seedbanks keep seeds cold and dry so that genetic variation survives if wild populations are lost.

Must-know distinctions

• Genetic and environmental variation: alleles versus conditions. Only genetic variation can be inherited and selected.
• Mutation and selection: mutation creates a new allele at random; selection decides whether it spreads.
• Evolution and adaptation: a feature that helps survival is an adaptation. Evolution is the change in the population over generations.
• Evolution and extinction: if no variant can cope with a new selection pressure, the species may die out instead of changing.
• Fossil evidence and antibiotic resistance: fossils show change over long periods in the past; resistance shows selection happening now.

Small worked reminders

Seedbank sample. A seedbank holds 5000 seeds of a wild plant and a test shows 90% germinate. Viable seeds = 5000 × 0.90 = 4500. A lower germination result would mean the stored seeds need replacing, because dead seeds store no genetic variation.

Reading a table. Species X and Y differ in 8 of 160 bases (5.0%). X and Z differ in 48 of 160 (30.0%). Write: “X and Y have fewer differences, so they share a more recent common ancestor than X and Z.”

Maths and data reminders

Task	Method	Example
Percentage survived	survivors / total × 100	18 of 25 survive: 72%
Percentage change	(new - old) / old × 100	30 mm to 24 mm: -20%
Percentage sequence difference	differences / bases compared × 100	8 of 160 bases: 5.0%
Doublings	time / doubling time	4 h at 30 min: 8 doublings
Population after n doublings	start × 2 to the power n	1 cell after 8 doublings: 256

Quick self-test

1. Name two sources of genetic variation.
2. Give one difference between natural and artificial classification.
3. A gene of 160 bases differs in 8 places between species P and Q, and in 48 places between P and R. Give both as percentages and say which pair is more closely related.
4. Why is the statement “the beetles changed colour to survive” wrong?
5. Put in order: survive and reproduce; variation; allele becomes more common; selection pressure.
6. Of 25 dark owls released, 18 survive a winter; of 25 pale owls, 7 survive. Calculate each survival percentage.
7. State two reasons why the fossil record is incomplete.
8. One bacterium divides every 30 minutes. How many bacteria after 4 hours, assuming no deaths?
9. Fossil leaf length falls from 30 mm to 24 mm. Calculate the percentage change.
10. Why does a seedbank help biodiversity?
11. State what Darwin and Wallace had in common.
12. Define a species.

Answers

1. Mutation; meiosis; random fertilisation (any two).
2. Natural classification uses many features and shows ancestry; artificial uses a few convenient features and does not show relationships.
3. 8 / 160 × 100 = 5.0%; 48 / 160 × 100 = 30.0%. P and Q are more closely related.
4. Individuals do not change colour by selection. Variants with an allele for better colour already existed and survived and reproduced more.
5. Variation; selection pressure; survive and reproduce; allele becomes more common.
6. Dark 72%; pale 28%.
7. Soft-bodied organisms decay; few organisms are fossilised; fossils destroyed by rock movement; many not yet found (any two).
8. 4 × 60 / 30 = 8 doublings; 2⁸ = 256.
9. (24 - 30) / 30 × 100 = -20%, a decrease of 20%.
10. It stores seeds, and so genetic variation, that can restore species if wild populations are lost.
11. Each independently reached the idea of evolution by natural selection, and their work was presented together.
12. A group of organisms that can breed together to produce fertile offspring.

Where marks are usually lost

• Writing that an individual evolved or adapted, instead of describing a change in the population over generations.
• Saying organisms mutate “in order to” survive.
• Leaving out the final step: the allele is passed on and becomes more common.
• Writing “survive” without “and reproduce”.
• Describing artificial classification without saying what it is based on.
• Giving only “the fossil record is incomplete” without a reason.
• Not using figures from a table or graph when asked to describe a trend.
• Calculating percentage change against the new value instead of the original.
• Writing that antibiotics “cause” resistance, when the antibiotic selects for resistance that arises by mutation.

Official syllabus

OCR, GCSE (9-1) Gateway Science Biology A (J247) specification, Version 4.0 (August 2026), first assessment 2018, topic B5.2.