They don't really "evolve" per se, but previously I always felt they were more complete as each game went on (polished is arguable. III was really glitchy compared to II, but I still preferred III).
It's when IV came around that the series started going downhill to me, what with the massive lack of content, the removal of both a worthwhile story mode (Tale of Souls) and the awesome mini-games that were Weapon Master, or moreso with Chronicles of the Sword (easily my favorite part of SC3).
SC5, on the other hand, completely changed up the mechanics, and not for the better. Guard Impacts were changed into something completely different, and weren't worth the effort to pull off. Not to mention the removal of several key characters and the half-assed story mode, and...
Yeah, going into it anymore kinda derails the topic into HG101 territory, but I think I got the gist of it across.