posted by Anon
Imagine you start an experiment with two vials of flies. One fly vial has 20 flies, the other has 200 flies. After 20 generations of mating, which vial has lost the most alleles across the genome? Has genetic variation increased or decreased within each vial? Has genetic variation increased or decreased between the two vials?
The vial with 20 flies will experience greater sampling error and thus more genetic drift than the vial with 200 flies, so the vial with 20 flies has probably fixed/lost more alleles than the fly vial with 200 individuals. Genetic variation has thus decreased within each population because some alleles have been lost, others fixed.