Key Idea: Most marks in topic 3.1 come from speciation, measuring diversity with Simpson's index, and natural selection. Learn those first.
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.
- Islands such as Madagascar have many endemic species, found nowhere else.
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 |
| Species richness | the number of different species in an area |
| Species evenness | how equal the numbers of each species are |
| Natural selection | the better adapted survive, reproduce and pass on their genes |
| Speciation | the formation of a new species through evolution |
| Endemic species | a species found in only one place |
Last-minute check: answer, then tap to see.
Name the three levels of biodiversity. Genetic, species and habitat diversity.
What does a higher Simpson's index mean? Higher diversity.
Why does separation lead to new species? Each group meets different pressures and changes until they cannot interbreed.
Why does higher diversity make an ecosystem more resilient? More species and genes, so some survive a disturbance.
Which comes first in natural selection? Variation between individuals.
About 3 million years ago, the Isthmus of Panama rose and separated the Caribbean Sea from the Pacific Ocean. Explain how this could have led to the formation of new species of snapping shrimp.
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.
- Name a real example: Madagascar, the Galápagos, the Isthmus of Panama.