Natural selection relies on five key observations and inferences first outlined by Darwin and Wallace:
Populations produce more offspring than can survive to reproduce, leading to competition for resources.
There is genetic variation between individuals in a population for most traits.
Certain variations (adaptations) make individuals more likely to survive and reproduce in a given environment.
Adaptive traits are heritable, so are passed on to offspring.
Over generations, advantageous alleles increase in frequency, while harmful alleles decrease in frequency.
Exam tip: Always link selection pressure to survival, reproduction and change in allele frequency in 3+ mark questions; CIE awards separate marks for each of these linked steps.
2. Three Main Types of Natural Selection★★★☆☆⏱ 6 min
Natural selection acts on polygenic traits (controlled by multiple genes) in three distinct ways, depending on which phenotype is favoured by the environment. Each type produces a different change to the distribution of phenotypes:
3. Allele Frequency Change from Selection★★★☆☆⏱ 5 min
Natural selection is measured by the change in allele frequency in a population over generations. A selection pressure that favours one allele will consistently increase its frequency from generation to generation.
Exam tip: Always remember: natural selection acts on individual organisms, but the evolutionary change occurs in populations, changing overall allele frequency. Individuals do not evolve.
4. Key Exam Examples★★☆☆☆⏱ 4 min
CIE examiners expect you to reference named, specific examples of natural selection in written answers, usually for 3-5 mark extended questions. Three of the most common examples are:
**Antibiotic resistance in bacteria**: Random mutation creates a resistance allele in one bacterium. Antibiotics act as a selection pressure, killing all non-resistant bacteria. Resistant bacteria survive and reproduce, passing on the resistance allele, leading to a fully resistant population.
**Peppered moth industrial melanism**: As described earlier, soot pollution changed camouflage and predation pressure, leading to increased frequency of the dark allele; clean air laws have since reversed this change.
**Beak size in Darwin's finches**: Drought reduces the supply of small soft seeds, leaving only large hard seeds. Finches with larger stronger beaks survive better, so average beak size increases in the population over a few generations.
Common Pitfalls
Why: Evolution acts on populations over generations, not individual organisms. Individuals cannot change their inherited DNA.
Why: Directional selection favours one extreme phenotype, while disruptive selection favours both extremes.
Why: Other mechanisms including genetic drift, gene flow and mutation also cause changes in allele frequency.
Why: CIE awards a specific marking point for explicitly identifying the environmental factor driving selection.
Why: Stabilizing selection selects against extreme phenotypes, so the population mean stays the same, while variation decreases.