Many empirical studies demonstrate some degree of genetic differentiat
ion among populations of the same species. Understanding the relative
importance of the processes causing this genetic differentiation has p
roven to be a difficult task. In particular, population differentiatio
n can be influenced primarily by selection, genetic drift, and migrati
on. We review the effect of drift and migration on patterns of genetic
variation, with special reference to the conditions necessary for pop
ulation differentiation. Conceptually, selection may be implicated in
cases of population differentiation if the effect of drift and migrati
on can be shown to be insufficient to cause the observed patterns. We
examine some of the pitfalls of this approach when used with allozyme
data, and revise a previous conclusion concerning the relative importa
nce of selection in poulations of scale insects.