Key Idea: Most marks in topic 3.1 come from speciation, measuring diversity with Simpson's index, and natural selection. At HL, also learn isolation, endemism and the Anthropocene.
1. Speciation (most marks)
- Populations are separated, for example when plate tectonics splits a land mass.
- Each population meets different selection pressures and changes by natural selection.
- Over time they can no longer interbreed: they are separate species.
- At HL: isolation can also be behavioural or temporal, and islands have many endemic species.
2. Measuring biodiversity
- Simpson's reciprocal index: D = N(N − 1) ÷ Σn(n − 1).
- N is all individuals; n is the number of one species.
- A higher D means higher diversity.
- Diversity combines richness (how many species) and evenness (how equal their numbers).
3. Natural selection
- Variation: individuals differ.
- Overproduction and competition for limited resources.
- The best adapted survive and reproduce.
- They pass on their genes, so the population changes over generations.
How it is examined:
| Question | How to score |
|---|---|
| [1] to [2] Calculate Simpson's reciprocal index | Formula first, then the numbers, then the answer. |
| [2] to [4] Outline how natural selection works | Variation, competition, survival, reproduction, inheritance. |
| [4] Outline the levels of biodiversity | Genetic, species and habitat diversity, each defined. |
| [7] Explain how plate tectonics leads to speciation | Separation, different pressures, selection, can't interbreed. |
| [2] to [4] Explain why diversity gives resilience | More species and genes: more ways to survive a disturbance. |
Key terms, one line each:
| Term | In one line |
|---|---|
| Biodiversity | the variety of life at three levels: genetic, species and habitat |
| Reproductive isolation | populations can no longer interbreed, by place, timing or behaviour |
| Endemic species | a species found in only one place |
| Artificial selection | people breed for chosen traits, which reduces genetic diversity |
| Mass extinction | the loss of a large share of species in a short time |
| Anthropocene | a proposed epoch marked by human impacts on the Earth |
Last-minute check: answer, then tap to see.
Name one kind of reproductive isolation. Geographical, behavioural or temporal.
Why do islands have many endemic species? They are isolated, so species evolve on their own.
How does artificial selection reduce resilience? It reduces genetic diversity, so one disease can wipe out a crop.
What follows a mass extinction? Rapid speciation, as empty niches are filled.
Is the Anthropocene a formal epoch? No: it is proposed; a 2024 vote rejected it.
Almost all the bananas sold worldwide are one variety, the Cavendish, grown from cuttings. Explain why this makes the banana crop vulnerable.
Model answer plan
See the mark-by-mark plan — for / against / judgement, with marking guidance — in study mode.
Exam tips
- In a Simpson's calculation, write the formula first, then the numbers, then D.
- Name the steps of natural selection in order: variation, competition, survival, inheritance.
- For speciation, the key idea is that the groups can no longer interbreed.
- For isolation, name the kind: geographical, behavioural or temporal.