📝 Exam Summary: Biodiversity & evolution
Key Idea: Biodiversity is the variety of life. It exists at three levels, arises through evolution, and gives ecosystems resilience.
🌍 Three levels of biodiversity
- Habitat diversity – variety of habitat types (forests, rivers, reefs)
- Species diversity – number AND evenness of species
- Genetic diversity – variation within a species
🔁 Natural selection (4 steps)
- Variation exists (from mutation)
- A selection pressure acts → some individuals survive better
- Survivors reproduce and pass on the helpful alleles
- Over many generations the trait spreads → if populations become isolated they can no longer interbreed = a new species (speciation)
🧮 Measuring biodiversity
- Richness = number of species; evenness = how balanced their numbers are
- Simpson's reciprocal index: D = N(N−1) / Σn(n−1)
- Higher D = more diverse (more species + more even)
Tip: Show Simpson's working in 3 lines: formula → substitution → answer. For speciation you MUST say the populations can no longer interbreed to produce fertile offspring.