Looking at it pragmatically, we know that the Surgery and Pharmacology skills, while reliant on Knowledge (as all skills are), will also utilize attributes in determining both performance quality and growth rate of the skills themselves (though, of course, player skill remains chief in import, but I would argue that these other factors should not really be ignored). As such, and given that the tribes have varying minimum/maximum possible scores as well as different growth rates for the various attributes, it seems fairly apparent that tribe choice would probably play a role in prospective Physicians, as it would for someone of any profession.
Obviously this enters the realm of speculation in at least two primary ways: What attributes are used for the skill(s)? and what tribes are "good at" what attributes? To provide my own opinion on these two questions, and to give my own answer to the question posed by the OP, I think the safest assumption is that Surgery uses Agility and Reason, at least, and Pharmacology uses Reason. The latter probably has some other attribute associated with it (perhaps Will or Intuition, or perhaps both), but it's hard to say and as such I personally wouldn't rely on that. Given what we "know" of the tribes, then, and using only these criteria, I would lean towards the Kypiq/To'resk, depending on region. Kypiq seem most inclined towards it, given that they have the highest predisposition to Agility, and have one of the highest Reason. To'resk might beat them in the Reason department but they probably don't come even close when it comes to Agility.
However, I would also posit another prominent choice here: Dras. Per their writeup, they are very much into natural philosophy, alchemy, and "bardic skills" (an old part of the skill tree which has since been divided between Art and Lore, and the two medicine skills are found in Lore). Furthermore, they seem to have quite a few associations with the idea of healing, ranging from the altruism espoused by their true faith and their response to the Searing Plague (going out and acting as plague doctors among other tribes, even though their disease immunity didn't protect them from this particular illness). It seems likely to me that being a physician would be a fairly common profession among the Dras, and there may be plenty of Knowledge of the medical arts among them. While Dras don't really get sick much, nor do they get hurt by poison much, they seem to have studied both of these things highly extensively, which would be helpful to healers seeking knowledge of Pharmacology.
EDIT: In retrospect, an interesting pattern emerges with these three tribes being looked to as physicians. These three tribes are the least inclined towards warfare and hostilities, by design at least. In some ways this makes sense. Peacemakers being adept at the practice of healing and aiding others, etc. The Kypiq and Dras faith are both fairly oriented around nurturing others (whether they be beasts or mann), and the To'resk are there too I guess. Something else to consider, though, is that these tribes may not need the services of a physician quite so much as other, more martially inclined tribes would. Perhaps Kypiq physicians accompany Neran soldiers as they march off to war, To'resk doctors consult the Janoa shamans and aid in dealing with the wounds inevitably endured by the brutal hunters, etc. Seems like hanging around those who cause more conflict and damage would be ideal for growing your medical skills, anyway, instead of hanging around a place more about diplomacy and whatever people do instead of murdering each other and everything around them.