Heil Hitler!
If natural selection still worked like that, I wouldn't've survived very long. I'm sorry but 'natural selection' isn't a very moral viewpoint, and this thread is kind of all about morals. If your argument is "I have none" then fair enough :P
In some sort of 'respect' to natural selection... well it's kind of agreed that we're evolving mentally, socially and technologically a lot faster than physically.
Physically weak people (in what way, by the way? I'd like to hear just how discriminative you're going :P) aren't necessarily mentally weak.
Giving organs is more about having the choice to save multiple lives after you die. That sort of reach from the grave isn't really what you get if you lived amongst the rabbits and the tigers.