For any locus, fitness is measured by the relative contribution of a genotype to the next generation. Can multiply the HW estimates by a fitness variable to include selection in the equation. Selection can be due to differential mortality, fertility, or reproduction. No known examples of this in humans. The bigger the population size the smaller the effect of drift on allele frequencies.
Small populations are very prone to drift and alleles can go to fixation quickly.
Population Bottlenecks can cause the same effect, i.e. a bottleneck is a reduction in genetic variation. Migration is another term used for this, but only covers some aspects of gene flow. People need not permanently move, only their genes. Visitors can leave their genes behind in the form of children. Gene Flow Gene flow between two populations, over time, will make them more similar in allele frequencies.