← Evolutionary processes leading to speciation
4 Mechanisms that change allele frequencies
| Mechanism | What it does | Effect |
|---|---|---|
| Natural selection | Individuals with favourable alleles survive & reproduce more | Adaptive: increases beneficial alleles |
| Genetic drift | Random change in allele frequency (chance) | Non-adaptive; strong in small populations |
| Gene flow | Movement of alleles between populations | Makes populations more similar |
| Mutation | New alleles arise | Introduces new variation (raw material) |
Types of natural selection
- Directional: favours one extreme → shifts the mean (e.g. antibiotic resistance).
- Stabilising: favours the average, selects against extremes (e.g. human birth weight).
- Disruptive: favours both extremes over the middle → can split a population.
Genetic drift: bottleneck & founder effect
- Bottleneck: a catastrophe drastically cuts population size → surviving gene pool is a random subset (reduced diversity).
- Founder effect: a few individuals start a new population → carry only a sample of the original alleles.
- Drift matters most in small populations, where chance can fix or lose alleles.
A fast check before naming a mechanism in an answer
- Have I named the SPECIFIC mechanism (drift, gene flow, mutation, or a named type of selection), not just "evolution happened"?
- Have I linked it to a specific feature of the population described in the resource, not just the definition in the abstract?