Are today's Jews truly the ethnical successors of the ancient people of Judea, or are they rather the inheritors of the Jewish culture and religion, with only loose ethnical ties?

TheFairyGuineaPig

First, we have to look at who today's Jews are.

Some will be converts. Conversion has happened historically, even in Nazi occupied countries^1 (!) and some descendants with Jewish heritage may therefore have one or multiple convert ancestors. The majority of these historic converts probably ended up marrying a non convert Jew, so they would still have some Jewish ancestors generations before the conversion. Converts and the descendants of converts are of course Jewish, but are not ethnic successors in that they are unlikely to share the same or similar genetic links a Jew descended from a non convert would with ancient Judeans, although I would say they were cultural successors.

Within that group of converts and descendants of converts are some very old and large ethnic groups. For example, Beta Israel, and also the Falash Muras (converted Christian Beta Israel^3 ) are of Ethiopian origin. There is controversy over their origins, with some having claimed they are descended from the tribe of Dan, from King Solomon^4 and so on. Various studies have been done to decipher their genetic origins, and mostly they are shown to be of Cushitic descent^5 and distinct from other Jewish ethnic groups^6 .There may be similarities with Yemeni Jews^7 , but the sample size for the study showing a possible link was too small to say anything conclusively, and even this link was weak. Most likely, some Jewish travellers from outside Ethiopia came to the area, converted some of the local population and married into local families, and that was what led to the creation of Beta Israel. So they are probably not the ethnic descendants of ancient Judeans, but again, with religious and cultural ties, they are instead inheritors of Jewish culture, as you put it.

Ashkenazi Jewish genetics has been dealt with elsewhere, so I'll do Sephardim. Sephardic Jews are shown to have a genetic link with the Fertile Crescent, and also a genetic link to Ashkenazi Jews. North African Jews are shown to be a distinct group from local populations which aren't Jewish, but often have traces of Berber blood as well, due to intermixing, intermarriage etc^8 .

The historic Jews of northern Portugal also show a close link with the Middle East. About 70% of their paternal lineages are shown to be more common and typical in the Middle East than Portugal, although there is still a significant European ancestry in the majority, however it must also be understood that frankly to have that 70% is amazing in itself, as it shows that even though intermarriage (between Jews of different ethnic backgrounds) must've occurred, it probably occurred very little, over a millennia.

In fact, for the majority of Jewish ethnic groups, there are genetic ties, and close ones at that, between Mountain Jews, Georgian Jews (descendants of Persian and Iraqi Jews), Yemenite Jews, Ashkenazi Jews, Romaniote Jews, Italian Jews and the many ethnic groups making up the Mizrahi Jews^9. But that isn't to say there isn't any mixing, for although the majority of the Jewish population not descended from recent converts are likely to have Judean ancestors, they are also likely to have had a few more local ancestors, which is why Ashkenazi Jews tend to be paler than the average Arab person, for example.

So, to sum it up, it depends. There are many ethnic groups that Jews belong to, some will have few genetic links to ancient Judea due to being descendants of converts (however many generations ago), and the majority will have a minority of non-Jewish ancestors due to mixing, intermarriage and so on. But yes, there is an ethnic link between most Jews and ancient Judeans.

^1 Conversion to Christianity and Conversion to Judaism During the Era of Nazi Rule, Professor Dan Mechman
^2 The Theory and Practice of Welcoming Converts to Judaism, Lawrence Epstein
^3 The Beta Israel in Ethiopia and Israel: Studies on the Ethiopian Jews, Emanuela Trevisan Semi
^4 The Beta Israel: From Earliest Times to the Twentieth Century, Steven Kaplan
^5 Reconstruction of patrilineages and matrilineages of Samaritans and other Israeli populations from Y-Chromosome and mitochondrial DNA sequence Variation, P. Shen et al
^6 Beta Israel are on a distinct distal branch outside of the shared Jewish population cluster identified by PCA, pg9.
^7 Mitochondrial DNA reveals distinct evolutionary histories for Jewish populations in Yemen and Ethiopia
^8 The Genetic Legacy of Religious Diversity and Intolerance: Paternal Lineages of Christians, Jews, and Muslims in the Iberian Peninsula
^9 http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3543766/