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

This guide covers 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: the evidence for evolution by natural selection, variation, adaptation, the mechanism of natural selection, and the implications of evolution for human populations. Classification, in 4.2.2(a) to (d), is a separate unit. Use the guide with the revision notes and the practice questions. The OCR A Level Biology hub, the checklist and the free diagnostics show the rest of the course.

What this unit covers

Section	What you must be able to do
4.2.2(e)	Describe the evidence for evolution by natural selection, including the contributions of Darwin and Wallace, and fossil, DNA and molecular evidence
4.2.2(f)	Describe intraspecific and interspecific variation; continuous and discontinuous variation with examples from plants, animals and microorganisms; genetic and environmental causes
4.2.2(g)	Describe anatomical, physiological and behavioural adaptations, and explain why unrelated groups can show similar anatomical features
4.2.2(h)	Explain how natural selection changes the characteristics of a population over time
4.2.2(i)	Explain the implications for humans of evolution, including pesticide resistance in insects and drug resistance in microorganisms

Evidence for evolution (4.2.2(e))

Evolution is the change in the inherited characteristics of a population over many generations. The theory of evolution by natural selection says that populations change because individuals with advantageous inherited features survive and reproduce more than others.

Darwin and Wallace

Charles Darwin collected observations on a long voyage in the 1830s, including finches and tortoises from the Galapagos Islands. He worked on his ideas for about twenty years. Alfred Russel Wallace, working independently in the Malay Archipelago, reached the same explanation. In 1858 papers by both men were presented together to the Linnean Society, and in 1859 Darwin published his book On the Origin of Species.

Both men noticed the same things:

• Organisms produce more offspring than can survive.
• Offspring of the same species differ from one another.
• Some of the differences are inherited.
• Individuals whose features suit the environment are more likely to survive and breed.

Neither man knew about genes or DNA. Their theory was accepted because later evidence supported it, and it has been tested repeatedly since.

Fossil evidence

Fossils are preserved remains or traces of organisms in rock. Older rock layers contain simpler or different forms, and the sequence of fossils shows gradual change. Fossil series, such as those of horse limbs or of early whales, show intermediate forms and changes in structure over time. Limits: the fossil record is incomplete, because most organisms do not fossilise, and soft-bodied organisms rarely do.

DNA and molecular evidence

All living organisms use DNA, the same genetic code and many of the same proteins, which supports common ancestry. Closely related species have more similar DNA base sequences, and their proteins have more similar amino acid sequences. The number of differences is a measure of how long ago two species shared a common ancestor, since differences build up by mutation over time. The specification adds that only genomic DNA, the whole DNA of an organism, is needed at AS level, and it names molecular evidence such as the amino acid sequences of proteins.

Worked example 1. Part of a gene of 1,200 bases was sequenced in four species. The table shows the number of base differences from species P.

Species	Base differences from P	Percentage difference
Q	6	6 ÷ 1,200 × 100 = 0.5%
R	18	18 ÷ 1,200 × 100 = 1.5%
S	60	60 ÷ 1,200 × 100 = 5.0%

Species Q is the most closely related to P, because it has the fewest differences and so shared a common ancestor with P most recently. Species S is the most distantly related. The data show relationships only if the same gene is compared in each species.

Variation (4.2.2(f))

Variation is the differences between individuals. Intraspecific variation is variation between members of the same species, such as height in humans. Interspecific variation is variation between different species, such as the difference between the leaves of an oak and a beech.

Continuous and discontinuous variation

	Continuous	Discontinuous
Values	Any value in a range	A small number of distinct categories
Graph	Bell-shaped (normal) curve, line graph or histogram	Bar chart
Genes	Many genes, each with a small effect (polygenic)	One or few genes
Environment	Large effect	Little or no effect
Plant example	Leaf length	Pea seed shape: round or wrinkled
Animal example	Body mass, human height	ABO blood group, sex
Microorganism example	Length of bacterial cells, colony diameter	Presence or absence of a gene for antibiotic resistance

Genetic causes

• Mutation creates new alleles. It is the only source of completely new alleles.
• Meiosis shuffles alleles: crossing over between homologous chromosomes, and independent assortment of homologous pairs.
• Random fertilisation of gametes combines alleles in new ways.
• In sexual reproduction, two parents give offspring a mixture of alleles. Asexual reproduction gives clones, which differ only through mutation and the environment.

Environmental causes

Environment affects the expression of the genotype. Examples are mineral supply and light on plant growth, diet on animal body mass, and soil pH on the flower colour of hydrangeas. Most characteristics are affected by both genes and environment. Identical twins share the same genotype, so differences between them are environmental.

Worked example 2. Shell widths (mm) of ten periwinkles from a sheltered shore (site A) and ten from an exposed shore (site B) were measured.

Site A: 21, 22, 22, 23, 23, 24, 24, 25, 25, 26. Site B: 26, 27, 27, 28, 28, 29, 29, 31, 32, 33.

Mean of A = 235 ÷ 10 = 23.5 mm. Mean of B = 290 ÷ 10 = 29.0 mm.

Standard deviation, s = √[Σ(x − mean)² ÷ (n − 1)]. For site A, Σ(x − mean)² = 22.5, so s = √(22.5 ÷ 9) = √2.5 = 1.58 mm. For site B the same method gives s = 2.31 mm.

The means differ by 5.5 mm. The standard deviations show a larger spread at site B. The ranges (21 to 26 and 26 to 33) barely overlap, but a difference in means alone cannot say whether the cause is genetic or environmental. That needs a controlled experiment, such as growing offspring of both groups in the same conditions.

Adaptations (4.2.2(g))

An adaptation is a feature, passed on in the genes, that gives an organism a better chance of surviving and breeding in its habitat.

• Anatomical adaptations are structural features. Marram grass has leaves that roll up, with hairs and sunken stomata, which trap humid air and reduce water loss.
• Physiological adaptations are processes inside the body. The kangaroo rat produces very concentrated urine, and seals have muscle rich in myoglobin, which stores oxygen for long dives.
• Behavioural adaptations are the ways an organism acts. Examples are migration, courtship displays, and huddling in emperor penguins.

Similar features in unrelated groups

Organisms from different taxonomic groups can show similar anatomical features because they face similar selection pressures. Natural selection favours similar structures, independently, in each group. The streamlined body and fins of a shark (a fish) and a dolphin (a mammal) are an example. Cacti in the Americas and some Euphorbia species in Africa both have fleshy, water-storing stems and reduced leaves. The groups are not closely related, so the similarity does not come from recent common ancestry. This is convergent evolution.

Natural selection (4.2.2(h))

Natural selection is the process by which alleles that help survival and breeding become more common in a population, because their carriers leave more offspring.

1. Genetic variation. A population contains different alleles, originally from mutation.
2. Selection pressure. An environmental factor, such as a predator, a pathogen, competition for food or a pesticide, reduces survival of some individuals more than others.
3. Differential survival and reproductive success. Carriers of the advantageous allele survive and breed more often.
4. Inheritance. They pass the allele to their offspring.
5. Change over generations. The proportion of the population with the advantageous characteristic increases. If the environment changes, a different characteristic may be favoured.

Worked example 3. A population of 1,000 insects contains 2% with an allele for resistance to a pesticide. A spray kills 90% of the non-resistant insects and 10% of the resistant insects. The survivors breed and the population returns to 1,000, with the same proportions of resistant and non-resistant offspring as in the survivors.

Resistant insects: 20 × 0.9 = 18 survive. Non-resistant insects: 980 × 0.1 = 98 survive. Total survivors = 116.

New percentage resistant = 18 ÷ 116 × 100 = 15.5%.

If the same spray is used each generation, the percentage resistant rises to 62.3% after the second spray and 93.7% after the third. The spray did not cause resistance. It selected the resistant insects that were already present.

Say “the proportion of the population with the allele increased” rather than “the insects adapted”. Individuals do not change their genotype to suit the environment.

Evolution and human populations (4.2.2(i))

Pesticide resistance in insects

Resistant insects survive spraying, reproduce and pass on the allele. Over several generations most of the population is resistant. The consequences are lower crop yields, higher costs, and in disease vectors such as mosquitoes, poorer control of diseases such as malaria. Ways to slow resistance include rotating pesticides, using different types of pest control together, and leaving some areas unsprayed so that non-resistant insects survive.

Drug resistance in microorganisms

Bacteria reproduce quickly and mutations occur at random. A mutation may give resistance to an antibiotic. Treatment with the antibiotic removes the non-resistant bacteria, while the resistant ones survive and multiply. Resistant bacteria such as MRSA (meticillin-resistant Staphylococcus aureus) are hard to treat, particularly in hospitals.

Worked example 4. A culture has 5.0 × 10⁸ bacteria. One in 10⁷ has a mutation for resistance to an antibiotic. The bacteria divide every 30 minutes.

Resistant bacteria present before treatment = 5.0 × 10⁸ × 10⁻⁷ = 50.

After treatment the 50 survive and divide. To reach the original number the population must multiply by 10⁷, which is log₂(10⁷) = 23.3 doublings, so 24 whole divisions. Time = 24 × 30 minutes = 12 hours, and now all the bacteria are resistant.

Reducing resistance depends on human actions:

• Prescribe antibiotics only when needed. They do not work against viruses.
• Complete the course, so that bacteria with partial resistance are killed.
• Use combinations of drugs for some infections, such as tuberculosis, because it is less likely that a bacterium is resistant to all of them.
• Limit antibiotic use in farming, and develop new antibiotics.
• Hospital hygiene and screening reduce the spread of resistant bacteria.

Common errors

• Writing that animals “need” a feature, so they “develop” it. Mutations are random and happen before selection.
• Saying a pesticide or antibiotic “causes” the mutation.
• Describing natural selection as individuals adapting. Populations change, individuals do not.
• Saying variation is only genetic. Most characteristics are affected by both genes and environment.
• Using “survival of the fittest” without mentioning reproduction and inheritance.
• Claiming similar features prove close relationship. Unrelated groups may converge.

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.