Le Scraf's Ghost Theory, continued...
Here I explain Mega Gengar, and how it is based off of a human soul, like all ghosts. They live on a plane between Dimension 1 (our dimension) and Dimension 2 (alternate dimension) and is accessable by going through a door between worlds. What fun.
What does this have to do with Ghost Evolution? Well, let's take that Human Soul theory and put it to the test against a newer line of Ghosts, Phantump and Trevenant.
According to old tales, these Pokémon are stumps possessed by the spirits of children who died while lost in the forest. (PokeDex entry from Y)
So Phantump are dead children. I'm just going to get to the point here and say that it's tragic that children die. But this is what Phantump is based off of. But it's not just based off of any child...
It is 0.4m tall, or about 1' 4". What size is that to you? A newborn baby?
Yes, Phantump is based off of a newborn baby that was lost/seperated from family in the forest and died somehow. Then, the soul of the baby possessed a tree stump, and voila.
Now, once it matures, it starts to become more social and it needs friends, like most children.
...well, it can't really make friends, can it? Since it lives in abandoned forests, it just needs to wait for people to go in the forest. Then, it beckons poor person with it's childish voice, since it is "capable of imitating the sound of children's voices. (Also Bulbapedia)"
Okay, Phantump is based off of a dead baby. Fun stuff. But what does this have to do with evolution? Well, let's get onto Trevenant.
It can control trees at will. It will trap people who harm the forest, so they can never leave. (X)
It's kind to the Pokémon that reside in its body. (Y)
It's PokeDex entries suggest it is evil to people who harm things, and is kind of a parent to Pokemon residing in the forest, like Caterpie and Wurmple. This means the soul that was once a Phantump has learned good from bad, learns its true power, and is parental to Pokemon that it cares about, and is a sort of leader of the forest it has called its home for most of its "lifetime".
Now, let's look at its height. It is 1.5m tall, or about 4' 11". In other words, it's the size of your average teenager. It displays all of the qualities of a teenager such as leadership and beginning to act parental, so it is very likely Trevenant is a teenage tree. And this is where evolution comes in...
Ghost souls don't develop physically like other Pokemon evolutions such as Wooper to Quagsire. They develop mentally. Ghost souls grow like human brains so to speak, they learn lessons and get smarter as they evolve. (It should be noted that they can develop physically, such as a stump growing into a tree in ~13 years)
tl;dr, ghosts don't physically evolve, but they mentally evolve.
Either that or Game Freak felt like all Pokemon should equally evolve.