ITD in Ashkenazi Jews--genetic drift or selection?

Dystonia Musculorum Deformans/ethnology,genetics,history Europe, Eastern/epidemiology Female Fertility Gene Frequency Haplotypes/genetics Heterozygote History, 16th Century History, 17th Century History, 18th Century History, 19th Century History, 20th Century Humans Jews/genetics,history Life Expectancy Male Mutation Poverty Selection, Genetic Socioeconomic Factors