Maple syrup urine disease (MSUD) is a rare metabolic disease worldwide, however it is quite common in the Mennonite communities

Question

Maple syrup urine disease (MSUD) is a rare metabolic disease worldwide, however it is quite common in the Mennonite communities in Pennsylvania (US). One of the early members of the community had the mutation resulting in MSUD, and given the small, insular nature of the community, the gene was repeatedly passed on. Over time, the presence of the MSUD gene increased in the population. This is an example of: 
the principle of succession 
artificial selection 
microevolution 
macroevolution

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