Off topic, but possibly the people here might be able to help: Is there a name for the type of music where the theme evolves over time. Not (obviously) repeating, no echoing on another instrument, just progression. The bass line in Free's Mr Big (https://youtu.be/Azkef2lXW88?t=282) is the classic example for me. Is it a one-part fugue? or something with no name that just happens and has no name?

I asked a musician theorist friend and he said Minimalism, but that doesn't seem quite right.