If you don't explicitly label a method as virtual in the base class, then the child class overrides will ignore it. The Pop Framework has deliberately made move a non-virtual method, because it would be a bad idea for you to override it, like tinkering with the delicate innards of a watch. Even if you were to write a move method inside one of the child classes, your code would be ignored (unless you were to put virtual in front of the move prototype in critter.h ).
Let's see what happens if we make update non-virtual. This will mean that all of the critters' specialized update methods will be ignored and they'll just use the base class update method, which in fact does very little.
Open up the Pop project file and remove the world 'virtual' from the line virtual void update(CPopView *pactiveview, Real dt) at the bottom of the critter.h file. Build and run. You'll find that you're no longer able to shoot bullets in the Spacewar game. This is because the shooting behavior is part of the cArmedCritter::update code, which is now not being used.
Try changing the default _mutationstrength value, and make some changes to the cPolygon and cPolyPolygon mutate methods as well. See what kinds of interesting polypolygons you can come up with.