Historically, unquestionably true.
However things appear to be changing going forward. At least for humans, long term. We may need to redefine fitness consequently in that species.
If we take a population with a high reproductive rate, and short lifespan, the old paradigm holds. Most of the genes that survive are in the young.
Change the math by orders of magnitude so that individuals live 300 years, and seldom reproduce. Now most of the surviving genes are in the long lived. No other species is like that.
"Fitness" is now a measure of how long the organism lives, a function of health and wealth, instead of how sexy it was, and profligate its behaviour is.
Considering how much has unpredictably changed in the last 5 years, I hesitate to predict anything. The tools are sitting there. The money has found them, just as AI is poised to go General soon.
The fittest genes may well become the ones we habitually copy out of a databank. They may code for things we know nothing about now. And some of them may very well come from other species, or be engineered de novo.
