Small populations are more subject to random loss of alleles - in small populations a single individual carries a greater portion of the populations alleles. Special cases which magnify drift by sample error . Further drift in small population by random loss. In both cases, the resorted populations are different from the original and have less genetic variation. Major points of drift.
Drift is principle evolutionary force for neutral genes.
NS is the major force altering the frequency of adaptive genes. The evolution by drift is by Random Changes. Evolution by natural selection is not random but requires specific interactions with phenotype and the environment. Random fixation of alleles. Alleles go to 1.0 frequency at random.