Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Phenotype = observable or measurable traits; genotype = alleles carried. Natural selection acts on phenotypes; genotypes are affected only through the phenotypes they produce.
- Variation is genetic (mutation, new combinations from meiosis and fertilisation) or environmental. Only the genetic part reaches the next generation.
- A selective pressure is a feature of the environment that makes some phenotypes survive or reproduce better. Changing environments create new pressures.
- The same phenotype can raise fitness in one environment and lower it, or make no difference, in another.
- Differences in the types and numbers of molecules in cells can give some individuals a better chance of surviving and reproducing in a given environment.
Key relationships
| Example | Variation | Environment where it is favoured |
|---|---|---|
| Sickle-cell trait (one Hb S allele) | one amino acid change in β-globin | regions where malaria is common |
| Insecticide resistance | altered target protein, or more detoxifying enzyme | where the insecticide is used |
| Earlier flowering | heritable flowering date | warmer, earlier springs |
| Unsaturated membrane lipids | kinked fatty acid tails | cold conditions |
| Enzyme variants | different temperature optima | each in its own temperature range |
Chain to state in answers: molecule → cell function → phenotype → survival and reproduction in this environment.
| Calculation | Method |
|---|---|
| Relative fitness | mean success of phenotype ÷ mean success of best phenotype |
| Frequency after selection | survivors (or offspring) of a phenotype ÷ total survivors (or offspring) |
Assumptions
- Using survival as fitness assumes survivors of all phenotypes have equal numbers of offspring.
- Frequency projections assume offspring share their parent’s phenotype.
- All numbers in the guide and practice are fictional model values.
Mistakes to avoid
- Saying the environment or a chemical causes the helpful variation.
- Calling a trait “advantageous” without naming the environment.
- Treating environmentally caused differences as heritable.
- Calling insecticide resistance artificial selection (humans do not choose the parents).
- Skipping links in the molecule-to-fitness chain.
Quick self-check
- A phenotype has survival 0.45; the best phenotype has 0.90. Relative fitness? (0.45 ÷ 0.90 = 0.50)
- Why is the Hb S allele common in some regions despite causing disease? (Carriers resist severe malaria, so they have the highest fitness where malaria is common.)
- A bacterium is moved from 37 °C to 10 °C. What membrane change would help? (More unsaturated fatty acid tails, which keep the membrane fluid.)
Next: practice questions.