The big idea: Evolution is the change in the heritable characteristics of a population over generations.
Three words do the heavy lifting:
- Heritable — the feature must be passed on in the genes, not learned or caused by lifestyle. - Population — evolution happens to a whole group, never to a single individual. - Over generations — it builds up across time, not within one lifetime.
Underneath, this is a change in allele frequency — how common each version of a gene is in the group.
Evolution IS…
- A change in a population over time
- A change in heritable (genetic) features
- A shift in how common each allele is
- Something that takes many generations
Evolution is NOT…
- A single organism changing during its life
- A learned skill or a tan from the sun
- An individual 'trying' to adapt
- Something that finishes in one generation
Evolution at the genetic level: the dark allele is rare in generation 0 but common many generations later. That rise in allele frequency in the gene pool IS evolution — no single individual changed colour.
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One-line definition to memorise: Evolution = a change in the heritable characteristics of a population over generations (a change in allele frequency).
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Within any population, individuals vary — some are taller, darker, faster or more resistant to a disease. A lot of this variation is heritable: it comes from the alleles (gene versions) an organism carries and can pass to its offspring.
If individuals carrying one allele leave more offspring than others, that allele becomes more common in the next generation. Over many generations the make-up of the whole population shifts. That shift is evolution.
- Evolution
- The change in the heritable characteristics of a population over generations.
- Heritable characteristic
- A feature controlled by genes that can be passed from parents to offspring.
- Population
- All the members of one species living in the same area that can interbreed.
- Allele
- One particular version of a gene (e.g. a gene for fur colour can have a dark allele and a light allele).
- Gene pool
- All the alleles present in a whole population.
- Allele frequency
- How common a particular allele is in the gene pool (its proportion).
So the deepest, most precise way to state what is changing is the allele frequency in the gene pool. If dark-fur alleles make up 30% of a population now and 70% several generations later, the population has evolved — even if no single animal ever changed colour.
The classic exam trap: An individual organism does not evolve. A mouse that grows a thicker coat in a cold winter has not evolved — that is just a response within its lifetime, and a thicker coat grown from the cold is not heritable.
Evolution is something that happens to the population across generations, through a change in allele frequency.
| Statement | Evolution? | Why |
|---|---|---|
| A beetle population becomes mostly green over 50 generations | Yes | Heritable change in the whole population over generations |
| A bodybuilder builds big muscles by training | No | Not heritable; one individual, one lifetime |
| A bacterial population becomes resistant to an antibiotic | Yes | Resistant alleles become more frequent across generations |
| A plant grows taller in richer soil | No | An environmental response, not a genetic change passed on |
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How this is tested: On Paper 1A (multiple choice) you are asked to pick the correct definition of evolution — the right answer always mentions a population (not an individual) and a heritable / genetic change over generations.
On Paper 2 a short Define / State question can ask you to write the definition out in your own words for 1–2 marks.
IB-style question — define evolution
Define the term evolution. [1]
How to earn the mark
- Identify the command term. 'Define' means give the precise meaning — a short, exact sentence, no example needed.
- Include the non-negotiable words. A correct definition must say it is a change in heritable characteristics, of a population, over generations.
- Answer the command term. Evolution is the change in the heritable characteristics of a population over generations (a change in allele frequency).
Final answer
Evolution is the change in the heritable characteristics of a population over generations (a change in allele frequency in the gene pool).
What earns the mark: Any answer that contains all three ideas — heritable change, in a population, over generations — scores. Phrasing it as a change in allele frequency is the gold-standard version.
Answers that only describe one individual changing, or that leave out heritable, do not earn the mark.