You assume incorrectly.
In most previous (and some subsequent) wars, your thesis would hold: those who fought bravely and cannily would live to fight another day and thus to reproduce.
Unfortunately for all concerned, the leadership was (as usual) fighting the last war, with horrific consequences in the age of machine guns, HE, and poison gas. The individuals who would have been "selected" for their sterling qualities in a more traditional war were overridden by the stupidity of the brass hats, most particularly French, Haig, Foch, and Joffre.
Darwinist theories were submerged in total war and total slaughter. The statistics, while incomplete, do indicate a noticeable falling-off in physical size and strength. I don't know about the IQ issue, Binet invented the concept just before the war, so I don't know what kind of a baseline they had. Probably just anecdotal, but the physical statistics are easy to find. I'm most familiar with Britain, and I know that the recruiters in WWII were disturbed enough by the falling-off in health and strength to take note of it.
BTW, rather than deride the professor, you might give this some serious consideration. I majored in history with a concentration in military history, and he has a point.
P.S., the Swiss stayed out of it, thus proving once again their essential good sense.
Nope. You forgot that only those, who were "dispensable" were send into those sections were no survival was possible. The men used for "suicide squads" or "Himmelfahrtskommandos" (in German) were usually not the elite. A simple example: The most intellectual men (the officers) in a war are always in safe places behind the lines because they are needed for better purposes than direct hostilities.