On the Evolution of Spirits

Spirits in old China could not pass over large steps or around right angle corners.  This was known.  It was also known to some Europeans that ghosts could not cross running water.

A doorway in Guangzhou, raised to foil spirits.  

Such defenses against spirits beg some questions.  Notably, is it possible to select for spirits that can defeat these defenses?  This seems rather iffy.  For a population of spirits to evolve, several things must be true.

1. Spirits must be able to reproduce somehow.
2. There must be variation within the population of spirits.
3. There must be some advantage gained by being able to circumvent antispirit defenses, such that those who can are more likely to reproduce.

The very first condition, well maybe.  At first blush this would seem unlikely.  Spirits that are not of human origin are generally thought of as not being in the family way, and spirits that are a result of dead humans definitely don't.  However, humans themselves evolve.  It stands to reason that resultant ghosts have changed over time as well.  This also leads directly to point 2.  In that there is variation within the human population, there will also be variation within the resultant ghosts.  And here I am confident in saying that there is also variation within spirits not of human origin.  Chinese imagined spirits are not Irish imagined spirits are not Tanzanian imagined spirits.

 Point 3 actually seems the most unlikely to be true.  What exactly do spirits gain from getting entrance to human domiciles?  It's not like they are going to cease to exist if they don't. The anime answer would be that there are spirits that eat human souls, but I'm not currently enough up on my folklore to venture a guess on whether this is true outside of anime.  And even if it is true, humans can't spend all their time cowering behind spirit defenses unless they are really rich.  Since the really rich are always vastly outnumbered by the really poor,  there will always be easier and less-protected prey.

I actually doubt, considering point 3, that humans are likely to be selecting for defense-resistant spirits. If we are, it is going to be on a significantly smaller scale than the rate at which we are selecting for antibiotic-resistant bacteria and antimalarial-resistant malaria.