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

This guide covers 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. None of these statements is shown in bold type, so none is marked as Higher Tier only. Pair it with the revision notes and the practice questions. Course links: OCR GCSE Biology hub, checklist, free diagnostics and exam preparation.

What this topic covers

Outcome	You must be able to
B5.2a	State that there is usually extensive genetic variation within a population of a species
B5.2b	Describe how developments in biology have changed classification: natural and artificial systems, and molecular phylogenetics based on DNA sequencing
B5.2c	Explain how evolution happens through natural selection of variants whose phenotypes suit the environment, including the idea of mutation
B5.2d	Describe evolution as a change in the inherited characteristics of a population over time, which may form new species
B5.2e	Describe the evidence for evolution, including fossils and antibiotic resistance in bacteria
B5.2f	Describe the work of Darwin and Wallace and explain the impact of their ideas on modern biology, including seedbanks as a store of biodiversity

B5.2a Genetic variation within a population

A population is all the members of one species living in one area. Individuals differ. Some differences are genetic: they come from different alleles. Others are environmental, such as a plant growing taller in a sunny spot. Many features show both.

Genetic variation has three main sources.

• Mutation. A random change in the DNA base sequence. It is the only way a completely new allele appears.
• Meiosis. Gametes receive a random mix of alleles.
• Fertilisation. Which gamete meets which is random, so offspring have new combinations.

In most populations, extensive genetic variation exists before anything changes in the environment. That stored variation is what selection works on. A population with very little variation can have no variant that copes with a new threat.

B5.2b Classification and developments in biology

Classification sorts organisms into groups.

Type	Basis	Example
Artificial	A few chosen features that are convenient	Grouping animals as “can fly” or “cannot fly” puts a bat with a gull and a flightless beetle with a mouse
Natural	Many shared features and shared ancestry, so groups show how organisms are related	Putting bats with other mammals because of hair, milk and a shared ancestor

Natural classification is the aim of modern biology. Early natural systems used observable features such as body structure. Better microscopes revealed cell structures and biochemical features, which refined the groups.

The biggest change came from DNA sequencing. Molecular phylogenetics compares the base sequences of the same gene (or the amino acid sequences of the same protein) in different species. Two species that share a recent common ancestor have had little time to collect mutations, so their sequences differ in few places. Distantly related species differ in many places. The results are drawn as a branching diagram (a phylogenetic tree), where closely related species sit on neighbouring branches.

This method can reorganise groups that were based on appearance alone. Similar-looking organisms can turn out to be only distantly related, because similar environments can select for similar features. DNA sequences also work for organisms with few visible features, such as microbes. Sequencing of ribosomal RNA genes led to the Archaea being recognised as a domain separate from the Bacteria, in a three-domain system alongside the Eukaryota.

Worked example 1: reading sequence data

Part of one gene, 200 bases long, was sequenced in four species. The table gives the number of bases that differ.

	A	B	C	D
A	-	6	22	24
B	6	-	20	26
C	22	20	-	4
D	24	26	4	-

1. Percentage difference for A and B: 6 / 200 × 100 = 3.0%.
2. Percentage difference for C and D: 4 / 200 × 100 = 2.0%.
3. Percentage difference for A and D: 24 / 200 × 100 = 12.0%.
4. The smallest differences are A with B and C with D, so the tree has two pairs: (A, B) and (C, D).
5. The pairs differ from each other by 10 to 13%, so the two pairs separated long ago. A and B share a more recent common ancestor than A and C.

B5.2c Natural selection

Evolution by natural selection follows a chain of ideas. Learn it as a sequence, in order.

1. Individuals in a population show variation, because they carry different alleles.
2. More offspring are born than can survive. There is competition for food, mates, space or light, and pressure from predators and disease.
3. Individuals whose phenotype suits the environment better are more likely to survive and reproduce. This is the selection pressure.
4. Those survivors pass on the alleles for the useful feature to their offspring.
5. Over many generations, the allele becomes more common in the population.

Mutation supplies new alleles. Mutations are random: the environment does not cause a useful one. Most have no effect, some harm, and a very few help in a particular environment. If the environment changes, an allele that was harmless or rare may become an advantage.

Two common mistakes, both named in the specification’s notes on misconceptions, lose marks.

• Individuals do not evolve. A population changes because the proportion of alleles changes across generations.
• Organisms do not change “in order to” survive. Variants already exist, and the environment decides which survive.

Worked example 2: a selection investigation

A class laid 40 green and 40 brown paper “caterpillars” on a short lawn, and a partner removed those found in two minutes (acting as a bird). The partner found 9 green and 21 brown.

1. Green eaten: 9 / 40 × 100 = 22.5%; survived: 77.5%.
2. Brown eaten: 21 / 40 × 100 = 52.5%; survived: 47.5%.
3. Brown models were found 21 / 9 = 2.3 times as often as green ones.
4. Conclusion: on green grass, green caterpillars are camouflaged and survive better. If real caterpillars varied in colour through different alleles, the green allele would pass to more offspring.
5. Limits: paper models do not reproduce, one person searched, and one trial gives no estimate of spread. Repeating the trial several times and calculating a mean improves reliability.

B5.2d Evolution and new species

Evolution is a change in the inherited characteristics of a population over time, through natural selection. It does not happen within one lifetime.

A species is a group of organisms that can breed with each other to produce fertile offspring. A new species can form like this.

1. A population is split into two groups that no longer interbreed, for example by a sea, a mountain range or a river.
2. The two environments differ, so different phenotypes are selected in each. Different mutations also occur in each group.
3. After many generations the groups are so different that, if they meet, they cannot breed to produce fertile offspring. They are now separate species.

If a changing environment leaves a species with no suitable variants, it cannot compete or reproduce well enough and may become extinct.

B5.2e Evidence for evolution

Fossils

Fossils are the preserved remains or traces of organisms from millions of years ago, usually in rock. They can form from hard parts such as bones, shells and teeth that are slowly replaced by minerals. Layers of rock are laid down in order, so deeper fossils are generally older. Fossils show that:

• many past organisms differ from living ones, and many are extinct;
• features change gradually through a series of layers;
• some fossils show a mixture of features of two groups, such as Archaeopteryx, with feathers and also teeth and a bony tail.

The fossil record is incomplete. Many organisms had soft bodies that decayed, most did not die in places where fossils form, early fossils may have been destroyed by rock movement, and many fossils remain undiscovered.

Worked example 3: interpreting fossil data

The mean shell thickness of a snail species, measured in fossils from three rock layers, is 1.2 mm (10 million years old), 1.8 mm (6 million years old) and 2.7 mm (2 million years old).

1. Percentage change from oldest to youngest: (2.7 - 1.2) / 1.2 × 100 = 125%.
2. Oldest to middle: (1.8 - 1.2) / 1.2 × 100 = 50%. Middle to youngest: (2.7 - 1.8) / 1.8 × 100 = 50%.
3. Shells became thicker over time.
4. A suggestion: crabs that crush shells became more common, so thicker shells were selected.
5. Caution: three layers only show a trend. They cannot show the cause, which needs further evidence.

Antibiotic resistance in bacteria

Antibiotic resistance is evolution you can watch, because bacteria reproduce quickly.

1. A mutation gives one bacterium an allele that makes it resistant.
2. The antibiotic kills non-resistant bacteria, so the antibiotic is the selection pressure.
3. The resistant bacterium survives and divides, passing on the allele.
4. The population becomes mostly resistant. MRSA is a well-known example.

The same treatment then no longer works. Doctors reduce the spread by not prescribing antibiotics for viral infections or minor illness, and by asking patients to complete the course.

Worked example 4: how fast resistance spreads

A culture holds 5.0 × 10⁷ bacteria. One in 10⁷ carries a resistance allele. An antibiotic is added and kills every non-resistant cell. Resistant cells divide every 30 minutes.

1. Resistant cells at the start: 5.0 × 10⁷ × 1 / 10⁷ = 5.
2. Divisions in 6 hours: 6 × 60 / 30 = 12.
3. After 12 doublings: 5 × 2¹² = 5 × 4096 = 20 480 cells, assuming food stays plentiful.
4. Almost the whole population is now resistant, though it started with only a handful.

B5.2f Darwin, Wallace and their impact

Charles Darwin collected specimens on a long sea voyage, noticed that related organisms differ from place to place, and spent years gathering evidence. Alfred Russel Wallace worked independently, collecting specimens in south-east Asia, and reached the same idea of natural selection. Their ideas were presented together in 1858, and Darwin published On the Origin of Species in 1859.

The theory was controversial. It challenged religious views of the time, and Darwin could not explain how features were inherited, because genes were not yet understood. Mendel’s work on inheritance was largely ignored in his lifetime and became widely appreciated only later. The structure of DNA and the discovery of genes and mutation then explained both inheritance and the source of variation.

Impact on modern biology:

• it is the unifying idea for explaining biodiversity and why organisms are related to varying degrees;
• it underpins classification and molecular phylogenetics;
• it explains antibiotic resistance and so shapes how medicines are used;
• it supports conservation. Seedbanks store seeds of many species, kept cold and dry, as a store of biodiversity. They keep genetic variation that could be needed if wild populations are lost or the environment changes.

Common errors

• Writing “the moth changed colour to hide”. Write that moths with the allele for the better camouflage were more likely to survive and reproduce.
• Saying mutations happen “because” the organism needs them. They are random.
• Defining evolution as individuals adapting. It is a change in the population’s inherited features.
• Calling the fossil record complete, or ignoring its gaps.
• Describing artificial classification as “wrong”. It is simply based on a few convenient features.

Next steps

Condense this with the revision notes, then test yourself on the practice questions. Use the checklist to tick off B5.2a to B5.2f.

Official syllabus

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