"Fitness" is all about the environment in which the organism resides. Change the environment and the currently "fit" becomes unfit, and the currently "unfit" becomes "fit"
There was an interesting experiment that I read about long ago. Two types of bacteria were kept together in the same petri dish. As long as they were kept separated by centrifuge, they both flourished.
But when they were allowed to mix, in every instance one of the species disappeared. And in every instance, it was the same one.
However, it the temperature was altered a few degrees, or the pH altered, I forget which, the other species was wiped out instead.
Makes me think.
In the Carboniferous Epoch we were promised abundance for all,
By robbing selected Peter to pay for collective Paul;
But, though we had plenty of money, there was nothing our money could buy,
And the Gods of the Copybook Headings said: "If you don't work you die."
“pH altered”...
Andromeda Strain?