Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- An adaptation is a heritable variation that gives an advantage in a particular environment. Mutations arise at random; a changed environment favours variants that already exist.
- Heterozygote advantage: the heterozygote has a higher relative fitness than either homozygote, so both alleles persist (for example, sickle-cell trait where falciparum malaria is common).
- Invasive species spread by escaping their predators and diseases, using an open niche, or outcompeting natives (kudzu, zebra mussels).
- Human activity accelerates change: habitat loss (logging, urbanisation), monocropping (potato blight), introduced diseases (Dutch elm disease), climate change, biomagnification and eutrophication. Fast change can cause extinctions.
- Geological and meteorological events (El Niño, continental drift, the meteor impact about 66 million years ago) change habitats and species distributions.
Key relationships
| Disruption | Mechanism | Typical outcome |
|---|---|---|
| New selective pressure | existing variants survive and reproduce more | allele frequencies shift |
| Invasive species | few enemies, open niche or better competitor | natives decline; food web shifts |
| Biomagnification | persistent, fat-soluble toxin kept as biomass is passed up | top predators most contaminated |
| Eutrophication | nutrients → algal bloom → death → decomposers use O₂ | low-oxygen dead zone, fish deaths |
| Monocropping | one genetically uniform crop | whole crop vulnerable to one pathogen |
| El Niño | warm surface water, weak upwelling, fewer nutrients | plankton, fish and seabirds decline |
| Chi-square step | What to write |
|---|---|
| Null hypothesis | “No difference / no effect”, with the expected ratio |
| Expected counts | total × expected proportion |
| χ² = Σ (o − e)² ÷ e | one term per category |
| Degrees of freedom | categories − 1 |
| Compare (p = 0.05) | 3.84 (1 df), 5.99 (2), 7.81 (3) |
Assumptions behind the numbers
- Chi-square uses counts, not percentages or means, and each expected count should be reasonably large.
- Concentration factors in a food chain vary: transfer is not exactly 10% per level.
- Rejecting the null shows an association; a controlled comparison is needed to identify the cause.
Mistakes to avoid
- Saying the environment “causes” helpful mutations.
- Blaming living algae, not decomposers, for the oxygen crash.
- Calling every introduced species invasive.
- Using percentages instead of counts in χ².
- Writing “accept the null hypothesis” or “prove”. Use “fail to reject” and “support”.
- Forgetting to state the direction of the difference after rejecting the null.
Quick self-check
- A toxin at 0.02 ppm in algae increases 10 times at each of three steps. Concentration in the top consumer? (0.02 × 10 × 10 × 10 = 20 ppm)
- Equal numbers of seedlings are planted in soils A and B. Of the 80 that survive, 30 are in soil A and 50 in soil B. The null predicts 40 and 40. Conclusion? (χ² = 2.5 + 2.5 = 5.0 > 3.84 with 1 df, so reject the null.)
- Why does oxygen fall after an algal bloom ends? (Dead algae are broken down by aerobic decomposers, whose respiration uses oxygen.)
Next: practice questions.