Key Stage 4 science · Biology – Evolution, inheritance and variation

GCSE evolution and natural selection: a guide for parents

GCSE biology uses one big idea to explain the diversity of life: populations vary, some individuals survive and breed more successfully than others, and over many generations the characteristics of the population change. Students learn where variation comes from, how natural selection works, what evidence supports evolution and how new discoveries have changed the way living things are classified.

Variation, natural selection and classification, explained

Variation starts with genes. Individuals of the same species differ because they carry different alleles, and because of the environment they grow up in. New alleles arise through mutations, which are random changes in DNA. Most mutations have no effect on the organism and a few are harmful, but occasionally one gives a survival advantage in a particular environment. In sexually reproducing species, the shuffling of alleles in gametes and the random joining of egg and sperm also produce new combinations, which is why siblings differ.

Natural selection is the process by which the individuals best suited to their environment are more likely to survive, breed and pass on their alleles. The steps are usually taught in order: there is variation within a population; there is competition for food, mates or space; individuals with advantageous characteristics are more likely to survive and reproduce; so their alleles become more common in the next generation; and over many generations the population changes. If populations of a species become so different that they can no longer interbreed to produce fertile offspring, they have become separate species. If a species cannot adapt as its environment changes, it can become extinct.

The evidence for evolution comes from several sources. Fossils show earlier forms of life and how organisms have changed, although the record is incomplete because many soft-bodied organisms rarely fossilise. Comparing DNA shows that species with a recent common ancestor have more similar genomes. Bacteria that become resistant to antibiotics can be seen evolving within a human lifetime, because a resistant mutant survives treatment and multiplies, which is why doctors try not to prescribe antibiotics unnecessarily. Darwin published his theory in the nineteenth century, and Alfred Russel Wallace reached similar ideas independently. Earlier thinkers, including Lamarck, suggested that characteristics gained during life were inherited, which later evidence did not support.

Finally, students learn how classification has developed. Linnaeus grouped organisms by their structure and characteristics and gave each species a two-part Latin name. The system that grew from his work, taught at GCSE as kingdom, phylum, class, order, family, genus and species, sorts living things into ever smaller groups. As microscopes improved and DNA could be sequenced, scientists found that some groups did not fit their relationships. A three-domain system now places bacteria, archaea and eukaryotes into separate domains, based on differences in their cell chemistry. Many courses also show evolutionary trees, which are drawn from this evidence. Your child may be asked to interpret one of these trees or to explain why classification changes as science develops.

Words your child will hear

Variation: Differences between individuals of the same species, caused by genes, the environment or both.
Mutation: A random change in DNA that may give a new allele.
Natural selection: The process by which individuals better suited to their environment survive and reproduce more, passing on their alleles.
Species: A group of organisms that can interbreed to produce fertile offspring.
Extinction: When no individuals of a species remain alive.
Binomial name: A two-part Latin name for a species, giving its genus and then its species, such as Homo sapiens.
Antibiotic resistance: When a strain of bacteria survives treatment because a mutation protects it from an antibiotic.

What the National Curriculum says

The programme of study for Key Stage 4 says:

Students should be taught about genetic variation in populations of a species
Students should be taught about the process of natural selection leading to evolution
Students should be taught about the evidence for evolution
Students should be taught about developments in biology affecting classification

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

A worked example

Before industry spread across Britain, most peppered moths were pale, which camouflaged them against light, lichen-covered tree bark. Later, in sooty areas, dark moths became common. Use natural selection to explain this change.

State the variation. In the population, some moths were pale and a few were dark, because of different alleles for wing colour. The dark allele arose originally by mutation.
Describe the change in the environment. Soot from factories killed the lichen and darkened tree trunks, so the bark was now dark.
Explain the selection pressure. Birds eat moths that they can see. On dark bark, pale moths stood out and were eaten more often, while dark moths were better camouflaged.
Link survival to reproduction and alleles. Dark moths were more likely to survive long enough to breed, so they passed the dark allele on to more offspring.
Describe the result over generations. The proportion of the dark allele in the population increased, so most moths became dark. Importantly, no individual moth changed colour. The population changed because of which moths survived.
Add a further point if the question allows. When cleaner air allowed lichens to return, pale moths were once again better camouflaged and became more common again.

Pale and dark moths existed through variation. Dark bark made pale moths easier for birds to see, so more dark moths survived and bred, and the dark allele became more common over many generations.

Common mistakes and how to help

Thinking individuals evolve or adapt during their own lifetimes

Everyday language says that a person “adapts” to a new job or climate. In biology, populations evolve over generations, and a giraffe stretching its neck does not give its offspring longer necks.

What helps: Always check that an answer says which alleles became more common in the population. A good test is to ask “Which animals bred more, and what did they pass on?”

Believing that mutations happen because an organism needs them

Phrases such as “the bacteria developed resistance to survive” suggest purpose. Mutations occur at random. The environment, such as an antibiotic, then selects which mutants survive.

What helps: Explain it as a lottery followed by a filter: random changes first, selection second. Ask your child to retell the antibiotic story in that order.

Reading “survival of the fittest” as the strongest or fastest winning

The phrase sounds like a contest of strength. Fitness in biology means how well adapted an organism is to its environment and how many offspring survive to breed. Camouflage, size, disease resistance and parenting can all matter more than strength.

What helps: Look at an animal such as a stick insect or a small bird, and ask what feature helps it leave more offspring. The answer is rarely brute strength.

Saying “evolution is only a theory” or that humans evolved from chimpanzees

In everyday life “theory” means a guess, but in science it means a well-tested explanation supported by lots of evidence. And humans and chimpanzees share a common ancestor that lived millions of years ago, instead of one descending from the other.

What helps: Compare to a family tree: you and your cousins share grandparents, but you are not descended from each other. Ask which pieces of evidence, such as fossils and DNA, support the theory.

How to help at home

Play a camouflage game. Scatter dozens of small paper dots in two or three colours on a patterned cloth or lawn and let family members pick them up against a 20-second timer. The colours that blend in survive longest. Repeat with the survivors to see the population change.
Talk about resistance. When your child is prescribed antibiotics, talk about why it matters to take them exactly as prescribed, and why they are not used for colds, which are caused by viruses. It makes a good real-life link to natural selection in bacteria.
Look at family and pet variation. Compare the heights, hair colours or markings of siblings, pets or garden plants grown from the same packet of seeds. Ask which differences are likely to be inherited, and which could be caused by the environment.
Visit a museum or look at fossils. A local museum can show fossils and how different life once was, and a family fossil hunt on a beach, with an adult, away from cliff faces and with the tide times checked, can make it real. Ask your teenager what a fossil can and cannot tell us, and why the record is incomplete.

Builds on

Key Stage 3: DNA, genes and variation
Key Stage 3: Natural selection and biodiversity