MtDNA evidence for a genetic bottleneck in the early history of the Ashkenazi Jewish population:

These diversity patterns provide evidence for a prolonged period of low effective size in the history of the Ashkenazi population. The data best fit a model of an early bottleneck (~100 generations ago), perhaps corresponding to initial migrations of ancestral Ashkenazim in the Near East or to Europe. A genetic bottleneck followed by the recent phenomenon of rapid population growth are likely to have produced the conditions that led to the high frequency of many genetic disease alleles in the Ashkenazi population.

I believe that Hammer (one of the authors of the above paper) has argued for a lower level of admixture with European populations for Ashkenazi Jews than others.