THEORY: Evolution of infixes/ablaut?

Hi. I'm wondering if anyone has any information or even ideas about how
languages develop alternation inside of morphemes? That is to say, where
morphemes can be inflected or otherwise modified by changing, adding, or
deleting elements _inside_ the morphemes themselves, such as with infixes
and ablaut (vowel alternation). I'm really fascinated by the idea but I
can't figure out how the mind would allow a morpheme to be modified from
the inside -- just seems like morphemes "should" be concrete, unbreakable
elements to me. It's a bias in my conlanging instinct I guess :)

The point of this is that I'd like to use infixes and/or vowel alternation
in a conlang, but I'd like to be able to demonstrate that they evolved
(intra-conlang) somehow from an earlier form without internal alternation.

Thanks :)