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Natural selection, step by step
How populations change over generations without anything planning it.
Natural selection is how organisms with traits that help them survive and reproduce pass those traits on more often, so a population changes over generations. It requires heritable variation that affects reproductive success in a particular environment.
Covers GCSE, AP and A-level. Exam boards differ slightly, so check your course specification.
Five steps
- 01
Variation
Individuals in a population differ. The variation comes from mutations and from sexual reproduction.
- 02
Struggle to survive
More offspring are born than the environment can support, so individuals compete for food, mates and safety.
- 03
Survival of the best adapted
Individuals with helpful features are more likely to survive and reproduce.
- 04
Inheritance
They pass on the alleles for those features to their offspring.
- 05
Change over time
Over many generations the helpful alleles become more common.
The population adapts
Try it: peppered moths
A simplified model of inherited moth colour and bird predation. Each step replaces the population with offspring; individual moths do not change colour. Survival weights are chosen to illustrate selection, not measured field data. The model omits dominance, migration and mutation.
Generation 1 20 light · 20 dark
Both kinds start equally common. Which will the birds find easier to spot?
What happened to real peppered moths?
Industrial pollution darkened tree surfaces and reduced pale lichens in parts of Britain. Dark moths became better camouflaged and more common. As pollution declined, pale moths became more common again. Field experiments support bird predation as a major cause of these changes. The colour forms were inherited; soot did not turn individual pale moths into dark ones.
A real-time example: antibiotic resistance
Natural selection can be seen happening in bacteria.
- Some bacteria already carry resistance because of a mutation or a resistance gene acquired from another bacterium.
- When the antibiotic is used, it kills susceptible bacteria or stops them growing. The resistant ones survive and reproduce.
- Resistant bacteria pass resistance genes to descendants; some genes can also spread between bacteria. The resistant fraction can increase.
Use antibiotics only when prescribed, and take them exactly as directed by your prescriber or pharmacist.
Natural vs artificial selection
| Feature | Natural selection | Artificial selection |
|---|---|---|
| Who chooses? | The environment | Humans |
| What is favoured? | Traits that help survival and reproduction | Traits humans want |
| Examples | Peppered moths, antibiotic resistance | Dog breeds, high-yield crops |
Key terms
- Variation
- Differences between individuals of the same species.
- Adaptation
- A feature that helps an organism survive and reproduce in its environment.
- Mutation
- A random change in DNA. The source of new alleles.
- Selection pressure
- A factor, such as predation or disease, that affects which individuals survive.
- Fitness
- How well an organism survives and reproduces. It's about leaving offspring, not about strength.
- Species
- A group of organisms that can interbreed to produce fertile offspring.
Common mistakes
Populations evolve over generations. Individuals can acclimatise, and mutations can occur during life, but organisms do not deliberately make the inherited changes they need.
Mutations are random. They don't happen because an organism needs them. Selection acts on the variation that already exists.
"Fittest" doesn't mean strongest. It means best at surviving and reproducing in that environment.
Acquired traits aren't inherited. A giraffe stretching its neck doesn't give its offspring longer necks.
A worked example
This is the kind of answer SnapBio gives you when you snap a question: the answer first, then the steps.
Explain how antibiotic-resistant bacteria become common.
How to get there
- 1
Some bacteria already have resistance genes, from a mutation or gene transfer.
- 2
The antibiotic kills susceptible bacteria or stops their growth, so resistant bacteria reproduce more successfully.
- 3
They pass on resistance genes, so the resistant fraction of the population can increase.
Check yourself
Which of these is needed for natural selection?
Natural selection needs heritable variation that affects reproductive success. Nothing needs to "want" to change.
In biology, what does fitness mean?
Fitness is about reproductive success: how many offspring an organism leaves.
Which statement is correct?
Evolution is change in a population over generations. Individuals don't evolve, mutations are random, and acquired traits aren't inherited.
FAQ
What is natural selection in simple terms?
Organisms with traits that help them survive and reproduce leave more offspring, so those traits become more common in the population over time.
Does natural selection lead to new species?
It can contribute to speciation. If populations become isolated and diverge, they can eventually become reproductively isolated, forming new species. Selection does not always lead to a new species.
What is the evidence for evolution?
Fossils, similarities in DNA and body structure between species, and changes we can observe directly, such as antibiotic resistance in bacteria.
Where does the variation come from?
Mutations create new alleles, and sexual reproduction shuffles them. See mitosis vs meiosis and Punnett squares.
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