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.[17] 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.