Quick Summary
Evolution explains the diversity of life on Earth. This chapter is important for fossils, natural selection, variation, speciation, and evidence for evolution.
Key Definitions
- Evolution: Gradual change in organisms over generations.
- Fossil: Preserved remains or traces of ancient life.
- Natural selection: Survival and reproduction of organisms with favourable variations.
- Speciation: Formation of new species.
- Variation: Difference among individuals.
- Homologous organs: Same origin but different functions.
- Analogous organs: Different origin but similar functions.
Evidence for Evolution
| Evidence | What it shows |
|---|---|
| Fossils | Past life forms and gradual changes |
| Homologous organs | Common ancestry |
| Analogous organs | Similar adaptation |
| Embryological evidence | Developmental similarities |
Origin of Life — key ideas
- Oparin–Haldane theory (chemical evolution): life arose from simple molecules in the early ocean ("primordial soup") that combined into complex organic molecules.
- Miller–Urey experiment: passed sparks (lightning) through a mixture of methane, ammonia, hydrogen and water vapour and produced amino acids — supporting chemical evolution.
Natural Selection
Organisms show variations. If a variation helps survival in a particular environment, organisms with that variation reproduce more successfully. Over many generations, this changes populations.
Natural Selection — step by step
Classic example: the peppered moth — dark moths survived better on soot-darkened trees during industrialisation (industrial melanism).
Lamarck proposed inheritance of acquired characters (e.g. giraffe's neck); Darwin's natural selection is the accepted explanation.
Homologous and Analogous Organs
| Type | Meaning | Example idea |
|---|---|---|
| Homologous organs | Same basic origin, different function | Forelimbs of vertebrates |
| Analogous organs | Different origin, similar function | Wings of insects and birds |
Speciation
Speciation can occur when populations become separated, accumulate variations, and can no longer interbreed. Geographic isolation and natural selection can support this process.
Textbook Solution Strategy
1. Define evolution-related terms exactly. 2. Use examples for homologous and analogous organs. 3. Explain natural selection in steps. 4. For fossil questions, mention preserved remains and age evidence. 5. Connect variation to survival and reproduction.
Important Questions
One Mark
- Define evolution.
- What are fossils?
- What is speciation?
Two Marks
- Differentiate between homologous and analogous organs.
- What is natural selection?
- Why are fossils important?
Five Marks
- Explain evidence for evolution.
- Describe natural selection with an example.
- Explain how speciation may occur.
Common Mistakes
- Writing evolution as a sudden change.
- Confusing homologous and analogous organs.
- Forgetting variation in natural selection answers.
- Describing fossils only as bones.
Revision Checklist
- Evolution definitions revised
- Fossil importance memorised
- Homologous vs analogous table practised
- Natural selection steps revised
- Speciation concept understood