Okay, let me preface this with, IQ is an idiotic measure of pretty much anything. Loose gauge of intelligence, fine, but trying to read into it anything is pretty damn stupid. My issue is with this concept that medical students are so different from the rest of the population.
Two standard deviations means that the average medical student is in the top 2% of the population in terms of intelligence. AVERAGE medical students. 135-140 is higher than 2 standard deviations. The vast majority of medical students that I have worked with or have been in school with are hard working. THAT is what is fundamentally different from the rest of the population. They have a drive and a willingness to study harder and more than the average person. In terms of raw intelligence, innate ability, etc, yes, we absolutely select for it, but people drastically overestimate it, example, this thread. I can't even think of that many people that I know with >135 IQs. Maybe it is 110, maybe it is 115, maybe it is 120, but is sure as hell isn't 135+. I spent more time at Wash U than any other school over the past decade with a fair amount of time at Harvard and Northwestern. I don't know why those schools would have a higher proportion of low IQ students than wherever you have guys have spent time.
Quick google search:
120: Hauser, Robert M. 2002. "Meritocracy, cognitive ability, and the sources of occupational success."
http://www.iqcomparisonsite.com/Images/OccsX.jpg
114:
http://motls.blogspot.com/2006/03/iq-in-different-fields.html
For whatever IQ is worth, I have no doubts that medical students are smarter than your average person out there. It certainly makes sense. But, I'm sorry, this notion that as a group medical students and physicians are 2 standard deviations above average is just ludicrous.