By the end of this lesson
- Describe natural selection accurately.
- Link variation to allele-frequency change.
- Avoid teleological explanations of evolution.
BIG QUESTION
How can a population change without individuals choosing to evolve?
Variation already exists; differential reproductive success makes some alleles more common over generations.
Populations contain inherited variation
Mutation and sexual reproduction create genetic variation. Individuals with different alleles may show different phenotypes.
Environmental pressures such as predators, disease, climate or drugs affect which phenotypes survive and reproduce most successfully.
Differential reproduction changes frequencies
Individuals with an advantageous inherited phenotype are more likely to survive to reproduce or leave more offspring. They pass the associated alleles to more descendants.
Across many generations, those alleles become more frequent in the population: this is evolution by natural selectionThe process in which individuals with alleles that make them better adapted are more likely to survive and reproduce, passing those alleles on..
Individuals do not evolve because they need to
Selection does not create a useful mutation in response to need. Mutations occur before selection and are not directed by what an organism requires.
Selection acts on phenotype, but evolution is measured as change in population allele frequencies.
Natural-selection logic
Keep the cause-and-effect sequence in order.
| Stage | Population event | What does not happen |
|---|---|---|
| Variation | Individuals differ in inherited traits. | Need does not create tailored alleles. |
| Selection pressure | Environment affects survival/reproduction. | Environment does not choose intentionally. |
| Differential success | Some leave more offspring. | Individuals do not change their genes to adapt. |
| Inheritance | Advantage alleles pass to descendants. | Acquired traits are not automatically inherited. |
| Generations | Allele frequency changes. | One individual does not evolve. |
Language that earns precision
- natural selection
- Differential survival and reproduction causing heritable variants to change frequency over generations.
- selection pressure
- Environmental factor affecting survival or reproductive success.
- allele frequency
- Proportion of a particular allele in a population.
- adaptation
- Inherited feature increasing fitness in a stated environment.
Common misconceptions
Claim: Animals mutate because they need to adapt.
Correction: Mutations are not produced to meet need; selection favours existing variants.
Claim: The strongest individual always evolves.
Correction: Evolution is population allele-frequency change, not individual transformation.
Claim: An adaptation is always beneficial.
Correction: Its advantage depends on a particular environment.
Match the answer to the command word
Variation → pressure → differential reproduction → inheritance → frequency change.
Do not write “mutated to survive” or “evolved because it needed to”.
Recall, apply, then justify
Describe evolution by natural selection.
Hint
Variation, pressure, success, inheritance, generations.
Suggested marking points
- There is inherited variation in a population.
- A selection pressure affects survival or reproduction.
- Individuals with advantageous phenotypes leave more offspring.
- They pass associated alleles to descendants.
- The advantageous allele becomes more frequent over generations.
Explain why natural selection does not create new useful alleles.
Hint
Mutation first, selection later.
Suggested marking points
- New alleles arise by mutation before selection.
- Natural selection changes frequencies of existing variants through differential reproduction.
A drought favours plants with deeper roots. Explain what data would show evolution rather than only plastic response.
Planning hint
Inherited difference and allele-frequency change across generations.
What a strong design includes
- Plants must differ in inherited root-depth traits or associated alleles.
- Deeper-rooted plants must leave more offspring during drought.
- Offspring must inherit the advantage alleles.
- The alleles must become more frequent in later generations.
- A temporary deeper-root response in the same individuals alone is plasticity, not necessarily evolution.
- Genetic sampling before and after drought could test allele-frequency change.