The big idea: Phenotypic plasticity is when one genotype can produce different phenotypes depending on the environment.
The key point: the genes do not change — the environment changes how those genes are expressed during the organism's life.
So two organisms with identical DNA can end up looking or behaving differently, just because they grew up in different conditions.
- Genotype
- The set of alleles (the DNA) an organism carries.
- Phenotype
- The observable characteristics of an organism — what it actually looks like, how it behaves or functions.
- Phenotypic plasticity
- The ability of one genotype to produce different phenotypes in response to different environmental conditions, within an individual's lifetime.
- Environmental trigger
- A factor such as temperature, light, day length, food or predators that causes the plastic change.
The phrase to learn: Same genotype → different phenotype, because of the environment.
If you can say that one line and give an example, you can answer almost any plasticity question.
A characteristic comes from two things: the genotype (the alleles you inherit) and the environment you grow up in.
Phenotypic plasticity is the part that comes from the environment — so when you change the environment but keep the genes the same, you can still see a different phenotype.
How to spot plasticity (not a genetic change)
- The organisms have the same genotype (e.g. clones, or one species reared two ways).
- They are grown in different environments (e.g. different temperatures).
- They develop different phenotypes (e.g. different wing colour).
- The difference is NOT inherited — it happens within the individual's own lifetime and is not passed on through DNA.
Why it is NOT natural selection or a mutation: It is tempting to say a different appearance must mean a different gene — but plasticity is the opposite.
No new allele appears and no mutation happens. The DNA is unchanged.
Natural selection changes which alleles are common in a population over generations. Plasticity changes the phenotype of one individual because of its environment — and that change is not passed on.
| Phenotypic plasticity | Heritable genetic change (e.g. natural selection) | |
|---|---|---|
| What changes | The phenotype (the visible trait) changes | The genotype (the alleles) changes across generations |
| Cause | The environment (e.g. temperature, light, food) | Different alleles / mutations selected over generations |
| Genotype | Stays the SAME — no new alleles | Allele frequencies shift in the population |
| Timescale | Within one individual's own lifetime | Across many generations of a population |
| Inherited? | NO — not passed to offspring | YES — passed on through DNA |
| Reversible? | Often yes (depends on the environment) | No — fixed in the DNA |
The moth-wing example (the tested one): Suppose a moth species is reared at a cool temperature and a separate batch of the same species at a warm temperature.
The cool-reared moths develop darker wings and the warm-reared moths develop lighter wings — even though their genotype is the same.
The temperature during development changed how the wing-colour genes were expressed. That is phenotypic plasticity: the cause is the environment, not a new gene.
| Organism | Environmental trigger | Plastic phenotype |
|---|---|---|
| A moth species | Temperature during pupal development | Wing colour: cooler temperatures give a darker wing, warmer temperatures a lighter wing — same genotype |
| Arctic hare / fox | Day length / season | Coat colour: white in winter, brown in summer — a reversible seasonal switch |
| A water flea (Daphnia) | Chemicals released by predators | Grows a protective helmet/spines when predators are present |
| A plant in shade vs sun | Light intensity | Larger, thinner leaves in shade; smaller, thicker leaves in bright light |
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How this is tested: On Paper 1A a 1-mark item gives an example — classically moth wing colour that depends on rearing temperature — and asks you to explain it using phenotypic plasticity.
The marked idea is always the same: the genotype is unchanged, and the environment (temperature) causes the different phenotype within the individual's lifetime.
The trap answer is to call it a mutation, a new gene, or natural selection — it is none of those, because the DNA does not change.
IB-style question — explain temperature-dependent moth wing colour
Moths of one species were reared at two temperatures. Those reared in cooler conditions developed darker wings, while those reared in warmer conditions developed lighter wings, even though all the moths had the same genotype. Explain these differences in wing colour using the idea of phenotypic plasticity. [2]
How to score both marks
- State that the genotype is the same. All the moths carry the same alleles, so the difference is not caused by different genes or a mutation.
- Link the environment to the phenotype. The rearing temperature (the environment) changes how the wing-colour genes are expressed, producing a different phenotype (darker or lighter wings) in each group. This is phenotypic plasticity — one genotype giving different phenotypes in different environments. (Mark 1: same genotype / not genetic. Mark 2: the environment/temperature causes the different phenotype.)
Final answer
All the moths share the same genotype, so the wing-colour difference is not genetic. The environment — the rearing temperature — changes how the wing-colour genes are expressed, giving a different phenotype (darker when cool, lighter when warm). This is phenotypic plasticity: one genotype producing different phenotypes in different environments.
✓ Why this scores full marks: It does two distinct things: it rules out a genetic cause (same genotype), and it names the environmental cause (temperature) of the different phenotype.
Saying only 'temperature changed the colour' would miss the mark for same genotype — the examiner wants you to show you understand the genes did not change.