".... But we can explore the built in adaptations designed to help the organism survive changing conditions, like maybe a larger jaw if the only prey is large,..."
This is the simple thing that I was keying in on.
Could a smallie's physical geometry of jaw change slightly to allow it to eat more bluegill.
My buddy has a newly filled pond, and we've stocked it with (27) adult bluegill from my neighborhood pond. When it comes time for predators, I'd selfishly like him to put SMB in so I can 'reappropriate' some to my neighborhood pond, from time to time.