"What or who defines "what is possible?"
The possibilities are not defined. They're the possible phenotypes that can arise from the dynamics and reactions that can occur in the given physical system.
And yet unguided physical systems have not been shown to get anywhere. Indeed, Newton worried that such "dynamics and reactions that can occur in the given physical system" would, if left on their own, eventually culminate in the disorder of the system, such that it could no longer preserve its physical integrity.
If you're interested in Newton's take on this problem, maybe this article might help.