Genetic drift has no direction
- The idea
- Predict
- See it
- Why
- Check
The idea
“Genetic drift pushes an allele's frequency steadily up or down over the generations.”
Drift graphs often show one allele climbing until it's fixed, so it looks like a trend. Each step of that climb was chance.
The short version
The mistake: Genetic drift pushes an allele's frequency steadily up or down over the generations.
What's actually true: Drift is change by chance in which alleles get passed on, so a frequency can wander up or down with no set direction.
- Genetic drift is change in allele frequency by chance. Which individuals happen to breed, and which alleles they pass on, varies each generation.
- Each change is random in direction. Over time drift tends to fix or lose alleles, but which way it goes is up to chance.
- Drift is strongest in small populations. In a large one the chance changes stay tiny, so a steady trend there points to selection or gene flow.
On the exam: Drift has no direction. A steady trend, especially in a big population, points to selection or gene flow.