"The Amish population in Lancaster County, Pennsylvania has a high rate of genetic dwarfism because of low genetic diversity. This high rate of dwarfism can be traced back to the small number of European immigrants that started the population in North America. This is an example of one of the processes that allows genetic drift, which is known as"

Genetic drift is the process of change in allele frequency in a population because of chance increase or decrease in number of particular alleles in that population. It can be caused due to may reasons like founder effect, bottleneck effect etc.

Founder effect is a phenomena where a small group of individuals migrate from the original population to some other area and start a new population. The new population will all result from this small founder population. So if by chance some alleles were present more in the founder population, the new population will also show high frequency for it. As a result the allele frequencies in the new population can differ from the original first population.

Here, the Amish population in North America was settled by a small number of immigrants from Europe. Among those immigrants many had alleles for genetic dwarfism. So when this small population grew in America, genetic dwarfism also became prevalent in them.