Founder effect is a phrase that came up recently in our conversations about speciation. It refers to a situation where genetic diversity is somewhat limited due to a small initial "founding" population.
Some good examples can be found in Mark Ridley's Evolution A -Z. These include Ernst Mayr's observation that founder effect is "The establishment of a new population by a few original founders (in an extreme case a single fertilized female) which carry only a small fraction of the total genetic variation of the parental population."
The Pennsylvania Amish population is perhaps one of the most widely studied effects population of the Founder effect. The initial group of 200 German settlers carried with them mutations that resulted in Ellis-van Crevald syndrome which results in polydactyly (extra fingers), dwarfism, and other genetic abnormalities such as holes in the heart.
Evolution - A-Z - Founder effect. (n.d.). Evolution - A-Z - Founder effect. Retrieved October 23, 2013, from http://www.blackwellpublishing.com/ridley/a-z/Founder_effect.asp
Evolution: Library Genetic Drift and Founder Effect. Retrieved December 1, 2013, from http://www.pbs.org/wgbh/evolution/library/06/3/l_063_03.html.
Founder Effect, Bottle Necking, and Genetic Drift. (2010, December 27). YouTube. Retrieved October 28, 2013, from http://www.youtube.com/watch?v=Q6JEA2olNts
Founder effect. (n.d.). Founder effect. Retrieved October 28, 2013, from http://www.umbc.edu/bioclass/biol100/powerpoints/lecture10/sld032.htm