The difference between genetic drift and gene flow lies in their mechanisms of affecting population genetics. Genetic drift refers to changes in allele frequencies within a population due to random sampling effects, which can lead to the loss of genetic variation, especially in small populations. In contrast, gene flow involves the transfer of genetic material between populations, typically through migration, which can introduce new alleles and increase genetic diversity. Both mechanisms impact evolution, but they do so through distinct processes that affect the genetic structure of populations over time.