Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Evolution: a change in the heritable characteristics of a population over generations. Populations evolve; individuals do not.
- Natural selection is a major mechanism of evolution and the one that produces adaptations. Random processes (Topic 7.4) also cause evolution.
- Darwin’s argument (Darwin and Wallace, 1858; On the Origin of Species, 1859): more offspring are produced than can survive; resources are limited, so individuals compete; individuals vary; much variation is inherited; so individuals with favourable phenotypes survive and reproduce more, and their traits become more common.
- Fitness is measured by reproductive success: offspring that themselves survive to breed.
- Biotic (living) and abiotic (non-living) conditions fluctuate, so different variants can be favoured in different generations. This changes the rate and direction of evolution.
Key relationships
| Quantity | How to find it | Notes |
|---|---|---|
| Mean offspring per individual | total offspring of a phenotype ÷ number of individuals | or: fraction surviving × offspring per survivor |
| Relative fitness, w | mean offspring of phenotype ÷ mean offspring of best phenotype | best phenotype has w = 1; no units |
| Fraction of a phenotype next generation | its offspring ÷ total offspring | assumes the trait is inherited exactly |
| Percentage survival | survivors ÷ starting number × 100 | survival alone is not fitness |
| Missing ingredient | Why selection fails |
|---|---|
| No variation | nothing to choose between |
| Variation not heritable | survivors’ offspring do not share the trait |
| No difference in reproduction | every variant leaves the same share of offspring |
Assumptions
- Projections to the next generation assume the trait is fully heritable and ignore mutation, migration and chance.
- Survival data alone do not show fitness; you also need offspring numbers.
- Fictional data in the guide and practice are for illustration only.
Mistakes to avoid
- Saying an individual “evolved” or “adapted” by changing its genes.
- Saying a trait appeared “because the organism needed it”.
- Equating fitness with strength, size or lifespan.
- Forgetting that the favoured trait depends on the current environment.
- Dividing by the wrong phenotype: relative fitness divides by the most successful one.
- Describing a graph without naming its axes, groups and units.
Quick self-check
- Phenotype X averages 1.5 offspring; phenotype Y, the most successful, averages 2.5. Relative fitness of X? (1.5 ÷ 2.5 = 0.60)
- Name two biotic and two abiotic factors that could change selection on a lizard’s body colour. (Biotic: predators, prey, parasites, mates. Abiotic: rock colour, temperature, sunlight.)
- Why can a sterile animal that lives for 20 years have a fitness of zero? (Fitness counts offspring, and it leaves none.)
Next: practice questions.