[quote=@Techspert]
<Snipped quote by Darlit Glitch>

I understand the concept. However, in an adaptive context, OCD only makes sense as something good to improve the current situation. For example, Dark's is cleanliness, which causes him to go out of his way to be sanitary. But in this case, it actually hinders his ability to be clean.
[/quote]

OCD isn't an adaptive thing. It's a negative adaption that would result in being less likely to reproduce, hence not propagating. Problem is, the things that cause it aren't strictly hereditary. 
Basically, OCD means my neurons have created connections in such a way that the "signal of completeness" doesn't get sent. You know how, like, if you were to wash something, you would eventually reach a point where your brain says "that's enough."? My brain just doesn't do that, so in order to get that signal of "that's enough" I have to do a lot more, or just go without it which can lead to severe anxiety. It's actually pretty interesting if you want to look into the disorder more.