Population structure and migratory exchanges between populations are extremely important for understanding human evolution. They are also beginning to play a major role in medical genetics, for a simple reason. It is becoming clear that human population “isolates” are very common. They are populations who have been through a demographic bottleneck at some stage in their lifetime and have limited migratory exchange with neighbors. They are strongly subject to drift, and the result is that their genetic epidemiology is quite different from that of the general human population. Many hereditary diseases common elsewhere are rare or absent in some of these isolates, while other genetic diseases rare or absent in the general population are common. Examples of such isolates are Ashkenazi Jews, French Canadians, Afrikaner, and many other smaller populations like that of the island of Tristan da Cunha, etc. Most hereditary diseases like schizophrenia or allergies are due to many different genetic causes and are therefore difficult to study in the general population, but responsible genes are easier to identify in these isolates. In fact it is likely that a single gene is responsible for all of a given disease found in an isolate, or at last it is much easier to dissect there a complex causal genetic system.

In summary, the combined results suggest that a major portion of NRY biallelic diversity present in most of the contemporary Jewish communities surveyed here traces to a common Middle Eastern source population several thousand years ago. The implication is that this source population included a large number of distinct paternal and maternal lineages, reflecting genetic variation established in the Middle East at that time. In turn, this source diversity has been maintained within Jewish communities, despite numerous migrations during the Diaspora and long-term residence as isolated subpopulations in numerous geographic locations outside of the Middle East.