One way it is evolving is with the informal 'super-GM' title. At one time, there weren't many GMs. Being a GM was being in an elite club. Now there are a lot of GMs and the differences in their strengths are beginning to show. (I'm not talking about a GM's performance degrading due to age...)
From the wiki linked below:
In 1972 there were only 88 GMs with 33 representing the USSR. The
current FIDE ratings list includes over 1000 grandmasters.
Perhaps 20 years ago, a GM rated 2600 was a 'super GM'. Now 2700 could well be that mark, with some GMs peaking at 2850 or so.
Also from the article:
December 2008 saw a record number of GMs (1,192) and IMs (2,916)
causing some FIDE officials to suggest that FIDE should consider an
"elite grandmaster" title. The unofficial title, "Super
Grandmaster", is often used by players to refer to those with a 2700+
rating to distinguish the most serious world champion contenders.
Read more here. This article also includes a synopsis of how GM and IM norms are earned, including rules changes by year. It's very interesting.