Genetic Drift: the random change in the frequency of an allele (gene) within a population (Masel, 2011).
Genetic Drift is one of the four basic mechanisms of evolution. Since genetic drift is entirely based on chance it does not lead to adaptations (University of California Museum of Paleontology, n.d.). Unlike natural selection that is differential reproductive success that happens for a reason; genetic drift is differential reproductive success that just happens (Freeman and Herron, 2007)
The high incidence of Ellis-Van Creveld Syndrome in the Old Order Amish in Pennsylvania is an example of genetic drift within a human population. The practice of marrying and reproducing within their small community, (which remained isolated from the general population) caused an increased frequency of this normally rare genetic disease to occur. This is a special type of population bottleneck called a "Founder Effect" because the inflated number of cases of EVC Syndrome is not representative of a normal population where gene flow occurs (pbs.org).
Freeman, S. & Herron, J. (2007). Evolutionary Analysis 4th Ed. Form and Function, pp 363-396.
Masel, J. (2011). Genetic Drift. Current Biology 21 (20).
University of California Museum of Paleontology. (n.d.) Genetic Drift. retrieved from: http://evolution.berkeley.edu/evosite/evo101/IIIDGeneticdrift.shtml