Genetic drift.
This small wildflower population has a stable size of 10 plants.
Only 5 plants (those in white boxes) of generation 1 produce fertile offspring.
By chance, only 2 plants of generation 2 manage to leave fertile offspring.
The CW allele frequency (q) increases in generation 2,
then falls to zero
in generation 3. Genetic variation in this population has been reduced.
