For those who don't know (just in case this succumbs to controversy), Darwinism is incredibly simple when you think about it, and is not at all evolution. (Not neccessarily, just related)
Simple fact is - the smarter or stronger you are, the more likely you are to survive, whereas the guy who's weaker and dumber is probably going to die eventually. Simple probability.
Evolution is a related concept that introduces genomes into the equation, but it is not natural selection. Natural selection is short term, evolution is long long long term.
Natural selection:
Person A is stronger, tougher, and smarter than Person B. Person A has a higher probability of surviving. Person B has a lower probability of surviving.
Evolution:
Person A is stronger, tougher, and smarter (compared to it's environment) than Person B. Person A has a higher probability of surviving. Person B has a lower probablity of surviving. If Person B dies, he will not breed and give birth to weak, sickly, and dumb offspring. If Person A lives, he _will_ be able to breed and give birth to strong, tough, and smart offspring. Person A's offspring then gain an advantage over everyone else, so they start reproducing even better than everyone else. Soon the whole species is stronger, tougher, and smarter than it ever was.
That's just the simplistic version, of course.
And yes, I am a firm beleiver that natural selection takes place _everywhere_.