Answer: Genetic Drift
Explanation: Genetic drift is an evolutionary mechanism whereby gene or allele frequency of a particular population change over generations due to random chance. The effect of genetic drift is most visible in small population due to reduction in population size resulting from natural disaster or splitting of a small group from the main population to formation a colony. However, it can occur in large population as well but it's effect is as strong as that occurring within a small population.
The genetic drift among the Dunkers could be attributed to the official split between conservatives and progressives members in 1882.