Natural selection β Darwin's big idea
Variation, competition, selection, inheritance. Repeat for many generations. Population evolves.
The four-step argument Cambridge expects:
1. Variation.
- Individuals in a population vary in their traits (genetic variation).
- Comes from meiosis, fertilisation, mutation.
2. Competition.
- Organisms produce more offspring than environment can support.
- Limited food, water, mates, space β competition.
3. Selection (survival of the fittest).
- Some individuals are better suited to the environment than others.
- They SURVIVE longer and have MORE offspring.
- Their FAVOURABLE alleles increase in frequency.
4. Inheritance.
- Favourable alleles passed to offspring.
- Over many generations, the population shifts.
Result: EVOLUTION β change in the population's genetic makeup over time.
Famous example: peppered moths.
- In 1800s English forests, moths were mostly LIGHT (good camouflage on lichen-covered trees).
- Industrial revolution β soot blackened trees.
- DARK MOTHS now camouflaged better; light moths eaten.
- Within decades, dark moths became dominant.
- Anti-pollution laws in 20th century cleaned trees β light moths returned to dominance.
- Selection pressure shifted; allele frequencies shifted.
Worked qualitative. A geneticist suggests that giraffes' long necks evolved because their ancestors 'stretched their necks to reach high leaves'. Why is this WRONG?
- Lamarck's discredited theory.
- Stretching during life doesn't change DNA.
- Correct: ancestors with mutations for slightly longer necks reached more food β survived/reproduced more β passed on alleles β over many generations, average neck length increased.
Cambridge tip. Memorise the four steps. Cambridge often gives a scenario (peppered moths, antibiotic resistance) and asks 'explain in terms of natural selection'.
- Variation β competition β selection β inheritance.
- Favourable alleles spread.
- Population evolves over generations.
- Peppered moth: classic example.