How does the natural selection work in the case of viroids? And why they cannot have multiple hosts?

Because they are a simple strands of circular RNA, I can't understand how any mutation of the genetic material would affect their reproduction...and if any mutation could be propagated in the future generations without any consequences, there wouldn't be a lot of species? This thing doesn't seem very obvious from the Wikipedia article(https://en.wikipedia.org/wiki/Viroid) that presents only a few. Using the same reasoning, why they can't have multiple hosts if they only need RNA polymerase II to multiplicate?