Dunker Example of Genetic Drift
The Dunkers are members of a pious religious sect also known as the German Baptist Brethren. They are referred to as Dunkers due to their tradition of fully immersing, or dunking, those being baptized. They share a traditional rural life style with other Anabaptist groups, such as the Mennonites and the Amish, but their beliefs distinguish them from these other more well known sects. The Dunkers began as a small pacifist community that emerged from German Reformed and Lutheran Churches in 1708 in West Central Germany. During a 10 year period from 1719 to 1729, all of the Dunkers fled Germany to avoid religious intolerance. They crossed the Atlantic and settled in the farmlands of eastern Pennsylvania, where they prospered. By the early 1880's the sect had grown from the original 50 families to 58,000 people. In 1882, the Dunkers officially split between progressive and conservative (Old Order) members. The conservatives rejected modern farm machinery and other late 19th century inventions. Likewise, they wanted to maintain their older customs, style of dress, and forms of worship.In the early 1950's, the Old Order Dunker community of Franklin County, Pennsylvania was the subject of a genetic drift study. Since they had been self-isolated by their strict marriage rules for more than two centuries, it had been assumed that they were a more or less closed breeding population. However, genealogical records showed that among the 90 parents of the community in the 1950's, 13% of their marriages were with people from other Old Order Dunker communities and 24% were with converts. The evolutionary effect of genetic drift would have to be large to overcome this influx of genes from other populations.
Researchers collected blood samples from the Franklin County Dunker community members for typing and compared them to those of people from the areas of West Germany from which the Dunkers had originated and also from their European American non-Dunker neighbors in Pennsylvania. Since there were only about 220 years separating the Dunkers from Germans by the 1950s, it was not likely that natural selection would have had time to cause significant differences in blood type frequencies between these populations since there do not seem to be major selective advantages for people with one blood type or another. However, if there were environmental factors that could have caused the Dunker gene pool to deviate significantly from the German one during that relatively short time period, then the non-Dunker neighbors in Pennsylvania should also have experienced similar changes in their gene pool frequencies.
The results of the study indicated that genetic drift has been responsible for random changes in the Franklin County Dunker community gene pool. Nearly 60% of the Dunkers had blood type A. The West German population was 45% A and the U.S. neighbor population was only 40%. In other words, the Dunker's type A frequency was extreme rather than intermediate between the two comparison groups. Without genetic drift, the Dunkers would be expected to be similar to the German population or possibly intermediate between them and the U.S. neighbors. Further evidence of Dunker gene pool frequencies being altered by genetic drift was provided by comparisons of the MN blood system types. Both of the comparison groups were 30% type M, 50% MN, and 20% N. In contrast, the Dunkers were 44.5% M, 42% MN, and 13.5% N.
In order to determine the rate of change caused by genetic drift, the researchers divided the Dunkers into three age groups for blood type comparisons. These age groups roughly corresponded to generations. The greatest difference between them was evidenced for the MN blood system alleles.
age group alleles M N 3-27 74% 26% 28-55 66% 34% 56+ 55% 45% There was a change in M and N allele frequencies of 8-11% between generations. This is far more rapid evolution for this trait than would be predicted in the absence of genetic drift. This example illustrates the point that genetic drift can explain differences between populations believed on other grounds to be closely related.
NOTE: The difference between the Dunker and West German blood type frequencies may in part be due to the founder principle. This additional small population size effect is described next in this tutorial.
SOURCE: Glass, H. Bentley, "The Genetics of the Dunkers", Scientific American, 1953.
Copyright © 2002-2012 by Dennis O'Neil. All rights reserved.
illustration credits