The big idea: Natural selection is the process where individuals with helpful inherited features survive and reproduce more than those without them.
Over many generations, the helpful allele becomes more common in the population — the population becomes better adapted to its environment.
The key thing the exam tests: natural selection changes a population over generations, not a single individual within its lifetime.
Natural selection in three steps: a VARIED population → a selection pressure (a predator) removes the unfavoured (light) variant → the favoured (dark) variant survives, reproduces and becomes common.
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- Natural selection
- The process by which individuals with advantageous heritable variations survive and reproduce more than others, so the advantageous allele becomes more common over generations.
- Variation
- The differences between individuals in a population; only heritable (allele-based) variation can be acted on by natural selection.
- Adaptation
- A heritable feature that makes an organism better suited to survive and reproduce in its environment.
- Allele frequency
- How common a particular allele is in a population, measured as a proportion of all the alleles for that gene.
- Selection pressure
- An environmental factor (such as a predator, disease, climate or food shortage) that affects the survival of different variants.
'Survival of the fittest' — read it carefully: 'Fittest' does not mean strongest or fastest.
It means best able to survive AND reproduce in that environment — the individual that leaves the most offspring.
A well-camouflaged, average-strength animal can be 'fitter' than a strong one that gets eaten.
Examiners give up to 7 marks for explaining how a population changes by natural selection — so you need the full chain of cause and effect, not just 'the fittest survive'.
Learn it as an ordered sequence: each step causes the next.
The chain of natural selection
- Variation. A population shows heritable variation — individuals differ, and the differences are coded by alleles that can be inherited.
- Overproduction. More offspring are produced than the environment can support, so there is a struggle to survive (competition for food, space, mates).
- A selection pressure acts. A factor such as a predator, disease, climate or food shortage means not all individuals survive equally.
- Differential survival. Individuals with the advantageous allele are better adapted, so they survive the selection pressure more often.
- Differential reproduction. Survivors reproduce more, so they pass the advantageous allele to their offspring.
- Allele frequency changes. Over generations the frequency of the advantageous allele increases in the population.
- Outcome. The population becomes better adapted; given enough time and isolation it may even become a new variety or species.
The result is a change in ALLELE FREQUENCY: A 1-mark question may simply ask: what is the result of natural selection?
The answer is a change in allele frequency in the population over generations — the advantageous allele becomes more common.
It is not a change inside a single organism. An individual cannot evolve — it is born with its alleles and keeps them. The population is what changes.
| Does it change during natural selection? | Why | |
|---|---|---|
| An individual organism | No | An individual is born with its alleles and keeps them for life — it cannot 'evolve' to fit |
| The population | Yes | The proportion (frequency) of each allele shifts over generations as some variants out-breed others |
Worked example — peppered moths: On clean, pale tree bark, light moths are camouflaged and dark moths are eaten more (the predator is the selection pressure).
When pollution darkens the bark, the situation flips: now dark moths are camouflaged and light moths are eaten more.
The dark moths survive, reproduce, and pass on the dark allele — so the frequency of the dark allele rises in the polluted population. Same mechanism, new winner.
Step 2 is the engine: the selection pressure (here, predation) does NOT survive the worse-camouflaged light beetles equally — it removes more of them, so they leave fewer offspring.
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| Ingredient | What it means | Why the mechanism needs it |
|---|---|---|
| Heritable variation | Individuals differ, and the difference is coded in alleles that can be inherited | If everyone were identical, selection would have nothing to 'choose' between |
| Overproduction | More offspring are produced than the environment can support | It creates a struggle to survive — not all can live |
| Selection pressure | A factor (predator, disease, food shortage, climate) that affects survival | It decides which variants do better — it does the 'selecting' |
| Differential survival & reproduction | Better-adapted individuals survive more and leave more offspring | This is how the helpful allele is passed on more often than the harmful one |
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How this is tested: The signature 4.10.2 question is a 7-mark 'Explain' on Paper 2: explain how a population changes by natural selection. It comes up repeatedly.
To score high you must give the whole chain as separate points — variation, overproduction/competition, a selection pressure, differential survival, differential reproduction, inheritance of the allele, and the rise in allele frequency over generations.
A 1-mark Paper 1 item may instead ask for the result of natural selection — answer a change in allele frequency in the population (it becomes better adapted).
IB-style question — explain how a population changes by natural selection
A species of snail lives on rocks where ground colour varies. Birds prey on the snails. Explain how natural selection could lead to most of the snail population matching the rock colour over many generations. [7]
How to score all seven marks
- Variation. The snail population shows heritable variation in shell colour, caused by different alleles that can be inherited.
- Overproduction / struggle. Snails produce more offspring than can survive, so there is competition and not all live to reproduce.
- Selection pressure. The birds (predation) act as a selection pressure — they eat the snails that are easiest to see against the rock.
- Differential survival. Snails whose colour matches the rock are better camouflaged, so they are eaten less and survive more — they are better adapted.
- Differential reproduction & inheritance. The surviving, matching snails reproduce more and pass the camouflage allele to their offspring.
- Change in allele frequency. Over generations the frequency of the matching-colour allele increases in the population.
- Outcome. After many generations most of the population matches the rock colour — the population has become better adapted to its environment. (Award 1 mark per distinct point in the chain, up to 7.)
Final answer
Heritable variation in shell colour exists; more offspring are produced than survive (struggle to survive); birds (the selection pressure) eat the most visible snails; better-camouflaged snails survive and reproduce more (differential survival and reproduction); they pass the camouflage allele to offspring; so the frequency of that allele rises over generations; the population becomes better adapted and most snails match the rock.
✓ Why this scores full marks: Every link in the causal chain is a separate scoring point — variation, overproduction, the named selection pressure, differential survival, differential reproduction, inheritance of the allele, and the rise in allele frequency over generations.
A 7-mark 'Explain' needs seven distinct ideas in order, not 'the fittest survive' said seven ways. Naming the selection pressure (the birds) and stating the result as a change in allele frequency are the marks students most often drop.
The RESULT is in the third panel: the helpful allele (dark) is now far more frequent than it was. Natural selection changes the population's allele frequencies, not any single beetle.
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