Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- All species have evolved and are still evolving, humans included. Populations evolve; individuals do not.
- Evolution continues because new mutations arise each generation, environments change, other species (hosts, parasites, predators) evolve too, and drift acts in every population.
- Fastest change: large populations with short generations (bacteria, viruses, tumour cells, insects, weeds).
Key relationships
| Line of evidence | What you see |
|---|---|
| Genomic change over time | new mutations every generation; long-term lab populations of bacteria keep changing and can gain new abilities |
| Continuous fossil change | a measurable trait shifts gradually through successive rock layers |
| Resistance | antibiotics, pesticides, herbicides and chemotherapy drugs stop working as resistant variants spread |
| Emergent diseases | pathogens jump to new hosts, escape immunity, gain resistance or swap genes |
| Calculation | How |
|---|---|
| New mutations per generation in a population | genome size × rate per base pair × number of cells |
| Expected mutants for one trait | population size × mutation rate for that trait |
| Chance of two independent mutations in one cell | multiply the two rates |
| Frequency of resistant cells | resistant ÷ (resistant + susceptible) |
How resistance spreads
- Variation exists before the chemical is used.
- The chemical kills susceptible individuals.
- Resistant survivors reproduce and pass on resistance alleles (bacteria may also transfer them).
- Repeated use makes the resistant type common.
Mistakes to avoid
- Saying the chemical causes resistance mutations.
- Saying an individual organism evolves or adapts genetically during its life.
- Treating a single field comparison as proof; look for a baseline, replication and controls.
- Forgetting that wild-caught organisms may differ because of their environment; rear offspring under common conditions to test inheritance.
- Calling evolution “progress”: it is change that suits the current environment.
Quick self-check
- Why do combination treatments slow the spread of resistance? (A cell needs two independent mutations; the chance is the product of two small rates, so such cells are very unlikely to exist at the start.)
- A population of 3 × 10⁸ cells; resistance mutation rate 2 × 10⁻⁸ per cell. Expected resistant cells? (3 × 10⁸ × 2 × 10⁻⁸ = 6)
- Why do flu vaccines need updating? (The virus’s surface proteins keep changing, so antibodies to older forms bind less well.)
Next: practice questions.