Every cell, virus, plant, or animal mutation or characteristic is explained by Natural Selection.
It’s almost like there is some motivation for it to make sure life exists and survives. But why do anything at all? If there is a God then it makes sense that it was put in place for life to exist but if there is no God then why don’t we just have a universe of non-living matter?
I often compare Natural Selection to Survivorship Bias, because as far as I can tell that is what it is.
There is no “drive” or mythical force to be better. A mutation occurs and the result works or doesn’t.
Those that work have survived until today, and those that don’t failed to reproduce sufficiently to reach today.
That said, today we actually have what I call “Un-natural Selection”, and that is when we humans take something that would have failed naturally and ensure its success through our intervention. Think seedless plants or humans/animals with chronic disabilities. Natural selection would likely have eliminated them for failure to function or reproduce, but through our will they endure. For now.
You can include what you deem unfit in the Natural Selection process too. Humans are part of reality, part of nature, part of Natural Selection process/environment.
A crop that can only survive with human culturing may be unfit without human intervention, but humans are there culturing it. The crop survives. The crop even replaces other crops that humans may have cultivated otherwise. The crop is still being naturally selected. So you have to include humans and that process in Natural Selection.