Sand explores Koestler's work, but doesn't explicitly endorse it as the thing that absolutely happened. Secondly, no one knows whether it was just the elites who converted to Judaism, as history focuses on elites and disregards the poor, and finally, his main assertion is that irregardless of whether the 13th tribe idea happened, there was undoubtedly significant proselytism amongst the Jewish communities already spread throughout North Africa, Southern Europe and the Middle East from the 3rd Century, which is attested to in many historical works. Furthermore, he tackles the Jewish Gene problem in his book, using the same research this author relies on. The conclusions? Genetically, Ashkenazim have more in common with Welsh people than with the Mizrahim. Mizrahim have more in common with Arabs than with the Ashkenazim. Sorry, Jewishness doesn't represent the homogeneous race you describe.
Guatemala judge orders detention of president in corruption scandal (Reuters)
from the article: DNA links prove Jews are a 'race,' says genetics expert