Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- A population evolves when its allele frequencies change between generations. Selection is not the only cause.
- Mutation: random change in DNA; the only source of brand-new alleles. Supplies the variation selection acts on, but alone changes frequencies very slowly.
- Genetic drift: chance change in allele frequencies because each generation is a random sample of the last. Strongest in small populations. Can fix or lose alleles whatever their effect.
- Bottleneck effect: a population is reduced to a few individuals for at least one generation; survivors’ alleles are a chance sample, and lost alleles stay lost after regrowth.
- Founder effect: a few individuals start a separate population; its frequencies reflect the founders, not the source.
- Gene flow: migration adds or removes alleles; makes populations more alike and prevents them diverging into separate species.
Key relationships
| Process | Random? | Variation within a population | Difference between populations |
|---|---|---|---|
| Mutation | yes | increases (slowly) | can increase |
| Drift, bottleneck, founder | yes | decreases | increases |
| Gene flow | not selective | usually increases | decreases |
| Natural selection | no | often decreases | can increase |
| Calculation | How |
|---|---|
| total copies of a gene | 2 × number of diploid individuals |
| copies of A | 2 × (AA) + 1 × (Aa) |
| frequency of A | copies of A ÷ total copies |
| check | frequencies of all alleles add to 1 |
| mixing populations | add allele counts from each, divide by combined total |
Null hypotheses you may need
- “Survival (or reproduction) is independent of genotype”: if true, frequency changes are due to chance (drift).
- “Allele frequencies do not change between generations”: rejected if a clear, repeatable change is measured.
- Prediction for drift: replicate small populations spread out in random directions; large ones stay near the start.
Mistakes to avoid
- Saying mutations happen because an organism needs them.
- Saying drift happens only in small populations (it is just stronger there).
- Counting individuals or phenotypes instead of alleles.
- Forgetting to double homozygotes when counting alleles.
- Confusing bottleneck (whole population shrinks) with founder effect (small group separates).
- Saying individuals evolve.
Quick self-check
- Genotypes 30 BB, 50 Bb, 20 bb. Frequency of B? ((60 + 50) ÷ 200 = 0.55)
- Why can a harmful allele become common in a small population? (Drift is chance sampling; it ignores the allele’s effect.)
- What does gene flow do to two populations’ allele frequencies? (Pulls them toward each other.)
Next: practice questions.