Why do Jews often so closely resemble the members of the nationalities amongst whom they have been living for only a few hundred years? How do features spread through such an exclusive population so quickly to the extent that it is easier to identify on sight national origin (with some degree of accuracy) than religious affiliation?
(I think this question might be founded on faulty assumptions about any subset of {genetics; statistics; physiology; Judaism}. If so, please correct.)
I suspect that its a combination of some superset of the following factors:
As a demonstration of the importance of my four proposed non-hereditary factors, I submit that Ashkenazic Jews who grew up in Israel look subtly but noticeably different from those who grew up in the US, even if they came from the same family origins, going back just a generation or two.
Another example: There's a guy I work with who was born in India. On his employee ID photo, which is from a few years ago, he looks incredibly more Indian than he does now, almost like a different person. I think part of it is that he's put on a little weight , but there's something harder to put my finger on that I'll bet is an American attitude's effect on how he holds his face. I saw a photo in his cubicle that was probably taken between his ID and now, and his "Indianness" seems in between to me.
There is a Gemara where Hillel was asked about facial features of different nationalities and responded that the location effected them. Similarly we see by animals that those that are in very cold environments have thicker furs.
The genetic potential is there. There are environmental factors and genetic factors, some that are independent of eachother and others that have some mysterious interplay. We just do not know enough yet to determine the conclusive answer to this question.
To add to the other answers:
Environmental factors can cause genes to be expressed or repressed, and changes in gene expression can be heritable. It's possible that a single human's DNA could contain repressed genes with the potential for many ethnic appearances, and environmentally induced changes in expression of these genes could be heritable. (Incidentally, the large proportion of noncoding DNA in humans might include latent, repressed genes that affect appearance).
Where other mechanisms are nonviable or insufficient to explain changes in appearance, epigenetic mechanisms may be partially responsible for adaptation of local phenotypic traits by migratory populations within as little as one or two generations.
This may explain the fact that Jews throughout world are generally physically indistinguishable from non-Jews of their host countries despite the genetic evidence that suggests "that most Jewish communities have remained relatively isolated from neighboring non-Jewish communities during and after the Diaspora." (Hammer, et al.)
The Mishna (Kilaim 8:6) spoke almost 2000 years ago about micro-evolution: "Rabbis: the wild ox is a species of "behema", Rabbi Yossi says: it is a species of "chaya", Bartenura: "The Rabbis hold it used to be domesticated but it fled civilization and eventually transformed into the wild ox specie".
There's a fascinating book by Dr. Lee Spetner called "Not By Chance".
There he brings evidence that the DNA code has been found to change itself to adapt to the environment. ie NOT by "random" mutations, the dna reprograms itself
According to this, you can have the genetic code change even without any intermarriage. The facial features such as hair color or eye color, skin etc. change because they are advantageous in the climate of the host nation.
the other possibility is that this mechanism is not through genes modifying themselves but rather through the activation of dormant genes through some built-in mechanism which senses when to active them.
