Question: Ashkenazi Jews and the largest single groups of foreigners in 2010 may be genetically similar due to what two factors?

Knowledge 1: Switzerland
In 2012, resident foreigners made up 23.3% of the population. Most of these (64%) were from European Union or EFTA countries. Italians were the largest single group of foreigners with 15.6% of total foreign population. They were closely followed by Germans (15.2%), immigrants from Portugal (12.7%), France (5.6%), Serbia (5.3%), Turkey (3.8%), Spain (3.7%), and Austria (2%). Immigrants from Sri Lanka, most of them former Tamil refugees, were the largest group among people of Asian origin (6.3%). Additionally, the figures from 2012 show that 34.7% of the permanent resident population aged 15 or over in Switzerland, i.e. 2,335,000 persons, had an immigrant background. A third of this population (853,000) held Swiss citizenship. Four fifths of persons with an immigration background were themselves immigrants (first generation foreigners and native-born and naturalised Swiss citizens), whereas one fifth were born in Switzerland (second generation foreigners and native-born and naturalised Swiss citizens). In the 2000s, domestic and international institutions expressed concern about what they perceived as an increase in xenophobia, particularly in some political campaigns. In reply to one critical report the Federal Council noted that "racism unfortunately is present in Switzerland", but stated that the high proportion of foreign citizens in the country, as well as the generally unproblematic integration of foreigners", underlined Switzerland's openness. [SEP] Knowledge 2: Ashkenazi Jews
A 2010 study on Jewish ancestry by Atzmon-Ostrer et al. stated "Two major groups were identified by principal component, phylogenetic, and identity by descent (IBD) analysis: Middle Eastern Jews and European/Syrian Jews. The IBD segment sharing and the proximity of European Jews to each other and to southern European populations suggested similar origins for European Jewry and refuted large-scale genetic contributions of Central and Eastern European and Slavic populations to the formation of Ashkenazi Jewry", as both groups – the Middle Eastern Jews and European/Syrian Jews – shared common ancestors in the Middle East about 2500 years ago. The study examines genetic markers spread across the entire genome and shows that the Jewish groups (Ashkenazi and non Ashkenazi) share large swaths of DNA, indicating close relationships and that each of the Jewish groups in the study (Iranian, Iraqi, Syrian, Italian, Turkish, Greek and Ashkenazi) has its own genetic signature but is more closely related to the other Jewish groups than to their fellow non-Jewish countrymen. Atzmon's team found that the SNP markers in genetic segments of 3 million DNA letters or longer were 10 times more likely to be identical among Jews than non-Jews. Results of the analysis also tally with biblical accounts of the fate of the Jews. The study also found that with respect to non-Jewish European groups, the population most closely related to Ashkenazi Jews are modern-day Italians. The study speculated that the genetic-similarity between Ashkenazi Jews and Italians may be due to inter-marriage and conversions in the time of the Roman Empire. It was also found that any two Ashkenazi Jewish participants in the study shared about as much DNA as fourth or fifth cousins.
Knowledge 1: Switzerland
In 2012, resident foreigners made up 23.3% of the population. Most of these (64%) were from European Union or EFTA countries. Italians were the largest single group of foreigners with 15.6% of total foreign population. They were closely followed by Germans (15.2%), immigrants from Portugal (12.7%), France (5.6%), Serbia (5.3%), Turkey (3.8%), Spain (3.7%), and Austria (2%). Immigrants from Sri Lanka, most of them former Tamil refugees, were the largest group among people of Asian origin (6.3%). Additionally, the figures from 2012 show that 34.7% of the permanent resident population aged 15 or over in Switzerland, i.e. 2,335,000 persons, had an immigrant background. A third of this population (853,000) held Swiss citizenship. Four fifths of persons with an immigration background were themselves immigrants (first generation foreigners and native-born and naturalised Swiss citizens), whereas one fifth were born in Switzerland (second generation foreigners and native-born and naturalised Swiss citizens). In the 2000s, domestic and international institutions expressed concern about what they perceived as an increase in xenophobia, particularly in some political campaigns. In reply to one critical report the Federal Council noted that "racism unfortunately is present in Switzerland", but stated that the high proportion of foreign citizens in the country, as well as the generally unproblematic integration of foreigners", underlined Switzerland's openness. [SEP] Knowledge 2: Ashkenazi Jews
The genome-wide genetic study carried out in 2010 by Behar et al. examined the genetic relationships among all major Jewish groups, including Ashkenazim, as well as the genetic relationship between these Jewish groups and non-Jewish ethnic populations. The study found that contemporary Jews (excluding Indian and Ethiopian Jews) have a close genetic relationship with people from the Levant. The authors explained that "the most parsimonious explanation for these observations is a common genetic origin, which is consistent with an historical formulation of the Jewish people as descending from ancient Hebrew and Israelite residents of the Levant".
Knowledge 1: Ashkenazi Jews
The genome-wide genetic study carried out in 2010 by Behar et al. examined the genetic relationships among all major Jewish groups, including Ashkenazim, as well as the genetic relationship between these Jewish groups and non-Jewish ethnic populations. The study found that contemporary Jews (excluding Indian and Ethiopian Jews) have a close genetic relationship with people from the Levant. The authors explained that "the most parsimonious explanation for these observations is a common genetic origin, which is consistent with an historical formulation of the Jewish people as descending from ancient Hebrew and Israelite residents of the Levant". [SEP] Knowledge 2: Infection
A number of studies have reported associations between pathogen load in an area and human behavior. Higher pathogen load is associated with decreased size of ethnic and religious groups in an area. This may be due high pathogen load favoring avoidance of other groups, which may reduce pathogen transmission, or a high pathogen load preventing the creation of large settlements and armies that enforce a common culture. Higher pathogen load is also associated with more restricted sexual behavior, which may reduce pathogen transmission. It also associated with higher preferences for health and attractiveness in mates. Higher fertility rates and shorter or less parental care per child is another association that may be a compensation for the higher mortality rate. There is also an association with polygyny which may be due to higher pathogen load, making selecting males with a high genetic resistance increasingly important. Higher pathogen load is also associated with more collectivism and less individualism, which may limit contacts with outside groups and infections. There are alternative explanations for at least some of the associations although some of these explanations may in turn ultimately be due to pathogen load. Thus, polygny may also be due to a lower male:female ratio in these areas but this may ultimately be due to male infants having increased mortality from infectious diseases. Another example is that poor socioeconomic factors may ultimately in part be due to high pathogen load preventing economic development.

Answer: inter-marriage and conversions in the time of the Roman Empire