Final answer:
The higher occurrence of Ellis-van Creveld syndrome among the Old Order Amish is an example of the founder effect, a genetic phenomenon that increases the frequency of rare alleles within a population started by a small group of founders.
Step-by-step explanation:
The elevated frequency of Ellis-van Creveld syndrome in Old Order Amish is an example of the founder effect. This genetic phenomenon occurs when a new population is established by a small number of individuals, and as a result, rare alleles that were present in the initial founders become more common in the descendants. The Old Order Amish are an example of this, with a higher prevalence of Ellis-van Creveld syndrome due to the gene for this condition being carried by one of the original 200 Amish founders. Over time, the syndrome has become more frequent in the Amish gene pool due to their limited genetic diversity and practice of endogamy.