I find this twist on our understanding of evolution as quite interesting. It may still be true that only the fittest survive but fit means whatever actually works, not strongest or otherwise simplistically, linearly more.
If the weakest works best for whatever reason, then the weakest survive.
In this case we are before a case of proteins that work worse... and by working worse, they begin to interact and therefore create complexity.
That is what, for researchers Ariel Fernández and Michael Lynch, caused the rise of biological complexity: the evolution from prokaryote to eukaryote and so on: errors in proteins, weaker proteins... which eventually caused protein binding and compelxity.
Ariel Fernández and Michael Lynch, Non-adaptive origins of interactome complexity. Nature 2011. Freely accessible.
Story at Science Daily.