Thomas Shey
Legend
As an example, early on, most games had something like classes and levels. Over time, more and more games have been made with weaker constructions of class and level (like, say, World of Darkness games) or to have no such concepts at all (games that are skill-based, and point buy, like Fate or Gumshoe games). RPGs have evolved to contain such concepts.
In fact, even games with classes have tended to erode the edges of them over time, as the number of people who found the rigidity of early classes unsatisfactory have impacted the hobby.