Drift over time
Drift increases among-population variance over time

Beginning with a set of populations where the allele frequency q is normally distriburted about a mean q = 0.5, drift will cause the frequency distribution to flatten.  Note that the 0.0 and 1.0 classes act as absorbing barriers: the distribution is "U" shaped because alleles frequencies cannot fall outside the interval 0.0 < p < 1.0.  Eventually all populations will either lose or fix the allele, at which point among-populations genetic variance will be maximized.

Text material © 2004 by Steven M. Carr