gene flow: changes in the gene pool of a population because of the introduction of genes from another population by migration. Example: the gene pool in Southeast Asia was changed when U.S. soldiers had children with Vietnamese women during the Vietnam War in the 1960s and early 1970s.

genetic drift: changes that take place because of random fluctuations of gene frequencies due to small population size (e.g., sampling size). Example: The frequency of Ellis-van Creveld syndrome -- a rare form of dwarfism that includes extra digits -- in the Amish population of eastern Pennsylvania, which has intermarried over many generations.
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  Genetic Drift and Founder Effect

founder effect: changes in a population when a small population moves to a new location bringing only a small fraction of the genetic variation of the parent population. The population then will contain only those genes the initial individuals brought with them. Example: The finches of the Galapagos Islands have the gene pool the original founding population of finches brought with them.