OCR A Level Biology A: Variation, adaptation and natural selection (H420) – Revision Notes

These notes condense section 4.2.2(e) to (i) of the OCR A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025. Read the study guide for full explanations, then test yourself with the practice questions. The OCR A Level Biology hub, the checklist and the free diagnostics help you plan.

Key definitions

Term	Meaning
Evolution	Change in the inherited characteristics of a population over generations
Natural selection	Alleles that help survival and breeding become more common because their carriers leave more offspring
Variation	Differences between individuals
Intraspecific variation	Variation within one species
Interspecific variation	Variation between different species
Adaptation	A feature, passed on in the genes, that gives an organism a better chance of surviving and breeding in its habitat
Selection pressure	An environmental factor that reduces survival or reproduction of some individuals more than others
Convergent evolution	Unrelated groups evolve similar features under similar selection pressures

4.2.2(e) Evidence for evolution

• Darwin: observations on a long voyage, including the Galapagos Islands. His book On the Origin of Species appeared in 1859.
• Wallace: reached the same explanation independently in the Malay Archipelago. Papers from both men were presented together in 1858.
• Their key points: overproduction of offspring, variation, inheritance of some differences, survival of those suited to the environment.
• Fossils: sequences in rock layers show change over time and intermediate forms. Limit: the record is incomplete.
• DNA (genomic DNA only is needed at AS level): closely related species have more similar base sequences. All organisms use DNA and the same genetic code.
• Molecular evidence: closely related species have more similar amino acid sequences in the same protein.
• Number of differences: more differences means a more distant common ancestor.

Method: percentage difference in a sequence

1. Count the differences between the two sequences of the same gene.
2. Divide by the total number of bases compared.
3. Multiply by 100.

Reminder: 18 differences in 1,200 bases is 1.5%.

4.2.2(f) Variation

	Continuous	Discontinuous
Values	Any value in a range	Distinct categories
Graph	Normal curve or histogram	Bar chart
Genes	Polygenic	One or few genes
Environment	Large effect	Little or no effect
Examples	Leaf length, body mass, bacterial colony diameter	Pea seed shape, ABO blood group, antibiotic resistance gene present or absent

Genetic causes: mutation, crossing over, independent assortment, random fertilisation, and the mixing of alleles from two parents.

Environmental causes: light, minerals, soil pH, temperature, diet, disease.

Must-know distinction: identical twins share a genotype, so their differences are mainly environmental. Asexually produced clones differ through environment and any new mutations.

Method: mean and standard deviation

1. Mean = Σx ÷ n.
2. Subtract the mean from each value and square it.
3. Add the squares and divide by (n − 1).
4. Take the square root.

Reminder: values 21, 22, 22, 23, 23, 24, 24, 25, 25, 26 give mean 23.5 and s = 1.58.

A larger standard deviation means more spread. Overlapping ranges mean the groups are not completely different.

4.2.2(g) Adaptations

Type	What it is	Example
Anatomical	Structure	Rolled leaves with hairs in marram grass
Physiological	Internal process	Concentrated urine in the kangaroo rat
Behavioural	Action	Migration, courtship display, penguin huddling

Similar features in unrelated groups: similar selection pressures favour similar structures in each group independently, for example streamlined bodies in sharks and dolphins, and water-storing stems in cacti and some Euphorbia species. This is convergent evolution. It does not show close relationship.

4.2.2(h) Mechanism of natural selection

1. Variation exists, from mutation and sexual reproduction.
2. A selection pressure acts.
3. Carriers of advantageous alleles survive and breed more often.
4. They pass the alleles to their offspring.
5. The proportion with the advantageous characteristic increases over generations.

Method: change in proportion after selection

1. Multiply each group’s number by its survival rate.
2. Add the survivors.
3. Divide the survivors of the advantageous group by the total, then multiply by 100.

Reminder: 1,000 insects, 2% resistant. Survival 0.9 for resistant and 0.1 for the rest. Survivors 18 and 98, so 18 ÷ 116 = 15.5% resistant.

Must-know distinctions:

• Mutation produces the allele before selection. Selection does not produce it.
• Selection acts on phenotypes, but the change in the population is in allele frequency.
• Individuals do not change. The population does.

4.2.2(i) Implications for humans

	Pesticide resistance	Antibiotic resistance
Organism	Insects, such as mosquitoes	Bacteria, such as MRSA
Cause	Chance mutation, then selection by the chemical	Chance mutation, then selection by the drug
Consequence	Crop loss, less control of disease vectors	Infections harder to treat, especially in hospitals
Management	Rotate chemicals, use other control methods, unsprayed areas	Prescribe only when needed, complete courses, drug combinations, hygiene, limit farm use, new antibiotics

Bacteria divide quickly, so resistance spreads fast. Antibiotics do not work on viruses.

Method: resistance in a bacterial culture

1. Multiply the number of bacteria by the proportion with the resistance mutation. This gives the number present before treatment.
2. After treatment only these survive.
3. Work out how many doublings are needed to return to the original number: log₂ of the factor needed.
4. Round up to whole divisions and multiply by the generation time.

Reminder: 5.0 × 10⁸ bacteria, 1 in 10⁷ resistant, so 50 resistant. Factor 10⁷ is 23.3 doublings, so 24 divisions. At 30 minutes each, that is 12 hours.

Command words and what to write

Command	What the answer needs
State, Name	A short answer with no explanation
Describe	What happens or what the data show, using figures from the table or graph
Explain	A reason linked to the biology, using “because” or “so that”
Suggest	Apply the ideas to an unfamiliar situation, with a sensible reason
Calculate	Show the working, give units or a percentage sign, and use the requested number of significant figures
Evaluate	Give points for and against, then a judgement

Writing a natural selection explanation

A full answer follows the sequence below, whatever the organism.

• A mutation produced the allele, or it was already present in the population.
• There is a selection pressure, which you must name.
• Individuals with the allele are more likely to survive, and so reproduce.
• They pass the allele to offspring.
• The allele frequency increases over generations.

Aim to include all five points in a full explanation. In a data question, add a figure from the data to show the change.

Interpreting evidence

• Fossil series: describe the change between older and younger rock, and state that the series suggests gradual change. Add one limit.
• Sequence tables: the smaller the difference, the more recent the common ancestor. Compare like with like: the same gene or protein in each species.
• Graphs of variation: a bell-shaped curve shows continuous variation. Separate bars show discontinuous variation. A mean with a large standard deviation shows wide variation in the sample.
• Resistance data: a rise in the percentage of resistant isolates over time is evidence of selection. It does not show how the resistance arose.

Convergent features to recall

Feature	Group 1	Group 2
Streamlined body and fins	Shark (fish)	Dolphin (mammal)
Fleshy water-storing stems, reduced leaves	Cacti in the Americas	Some Euphorbia species in Africa
Wings for flight	Birds	Bats

Quick self-test

1. State two ideas that Darwin and Wallace both reached.
2. Why is the fossil record incomplete evidence?
3. A gene of 800 bases differs in 12 places between two species. Calculate the percentage difference.
4. Give one example each of continuous and discontinuous variation in a plant.
5. State two genetic causes of variation.
6. Name the type of adaptation shown by migration.
7. Why might a dolphin and a shark have similar body shapes?
8. In a population of 500 beetles, 4% have a resistance allele. A pesticide leaves 90% of those with the allele alive and 10% of the others. What percentage of survivors are resistant? Give 3 significant figures.
9. Explain why finishing a course of antibiotics is advised.
10. Why does a pesticide not cause resistance?

Answers

1. Any two of: organisms overproduce offspring, offspring vary, some variation is inherited, those suited to the environment survive and reproduce more.
2. Most organisms do not fossilise, so many forms are missing, and soft-bodied organisms rarely fossilise.
3. 12 ÷ 800 × 100 = 1.5%.
4. For example leaf length (continuous) and pea seed shape (discontinuous).
5. Any two of: mutation, crossing over, independent assortment, random fertilisation.
6. Behavioural.
7. Similar selection pressures for fast movement through water favour a streamlined shape in each group.
8. Resistant: 20 × 0.9 = 18. Others: 480 × 0.1 = 48. Total 66. 18 ÷ 66 × 100 = 27.3%.
9. It kills all the bacteria, including those with some resistance, so fewer survive to reproduce.
10. The resistance allele arises by chance mutation before spraying. The pesticide only kills the non-resistant individuals, so the allele becomes more common.

Where marks are usually lost

• Writing that organisms “develop” a feature because they need it.
• Saying the antibiotic or pesticide causes the mutation.
• Describing individual organisms adapting rather than a population changing.
• Omitting reproduction and inheritance from an explanation of selection.
• Dividing by the wrong total in a percentage difference or percentage resistant.
• Calling continuous variation “caused by environment” only. Both genes and environment contribute.
• Using the term “genetic” for a difference that is clearly environmental, such as hydrangea colour.
• Saying similar features show close relationship.
• Giving a behavioural example when asked for a physiological one.

Official syllabus

OCR, A Level Biology A (H420) specification, Version 4.1 (April 2026), for first assessment in 2025: section 4.2.2(e) to (i), Classification and evolution.