Why is genetic diversity decreased when populations are split off from one another?

This question is all about genetic diversity and more specifically how difference in alleles is what creates diversity within a population of a species. (here I would ask about whether the student understands what an allele is and if not I would describe this further).When populations are split (e.g. by a geographic event/natural disaster) the FOUNDER EFFECT will come into play. This describes what happens when a few organisms from a population start a new colony. Only a limited number of organisms contribute their alleles to the gene pool therefore genetic diversity is lower than in the original population. (Can show in pictures and on the virtual whiteboards visually to a MyTutor student).
Example:The Amish population of the USA are all descendants from Swiss migrants. The population, due to cultural and religious beliefs, remain isolated and do not marry and/or breed with non-Amish people in the surrounding areas. Therefore no new alleles have been introduced into the population. The population suffers from a very high incidence of genetic disorders such as dwarfism and Angelman syndrome due to the lack of genetic diversity.