Loci with more than one allele are called polymorphic loci. Loci with only one allele are called monomorphic loci. Does not create new genetic variation, but does change the relative frequencies of alleles. 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.