Drift is strong in small populations and weak in big ones
- The idea
- Predict
- See it
- Why
- Check
The idea
“Drift is just chance, and chance acts everywhere, so it moves every population about equally.”
You probably pictured chance hitting every population the same. In a large one, though, lucky and unlucky events mostly cancel out.
The short version
The mistake: Drift is just chance, and chance acts everywhere, so it moves every population about equally.
What's actually true: Drift is random sampling error. It moves allele frequencies a lot in small populations and very little in large ones.
- Genetic drift is change in allele frequency by chance. Each generation is a random sample of the last one's alleles.
- Small samples miss the true proportion by a lot, big ones by very little. Flip 10 coins and 7 heads is common. Flip 10,000 and 70% heads almost never happens.
- Drift has no direction. In a small population it can lose or fix an allele. A steady change one way in a big one points to selection, or to gene flow.
- A recent bottleneck or founder event also makes a population small.
On the exam: Before you say drift, check for a small population and a frequency that jumps around instead of climbing.