What is fitness?

My kids bedtime interrupted me looking back through the thread - and so I had forgotten about your reply so thanks for the reminder. I had watched the moth video. And I just read through the mouse thread. Those are both interesting examples of evolution and natural selection but that data doesn’t show genetic entropy to be false.

Sorry if I miss things. I try to get to as much if what people share as I can.

That’s what I’m asking for - which thread should I look at?