Mixing groups? Weight each one by its share
- The idea
- Predict
- See it
- Why
- Check
The idea
“When two groups mix, I average their allele frequencies or add the newcomers' share to the old frequency.”
Averaging is the obvious move, and it works for groups of the same size. Adding the newcomers' share feels right too, since more newcomers do mean a bigger change.
The short version
The mistake: When two groups mix, I average their allele frequencies or add the newcomers' share to the old frequency.
What's actually true: Multiply each group's allele frequency by its share of the mixed group, then add the results.
- Allele frequency is the fraction of a gene's copies that are one allele. Each animal carries two copies, so a bigger group adds more copies to the pool.
- So multiply each group's frequency by its share of the mixed group, then add. For the bison, 0.75 x 0.2 + 0.25 x 0.6 = 0.30.
- This is gene flow. Migrants shift the frequency toward theirs by their share times the gap between the groups. More migrants, or a bigger gap, mean a bigger shift.
On the exam: Multiply each group's frequency by its share, then add. A plain average works only for equal groups.