When resources are more scarce selection pressure weeds out the less fit ones. Really cool relationship btw community diversity & speed of evolution to a new environment.
These are very controlled & optimized conditions. Without competition from other species (only between slightly different strains) the bacteria can run free.
Slowly evolving multiple antibiotic resistances & becoming a real problem for hospitals. It sounds like you want an example of a new animal that has like some new arm or something.
Well its most easy to see evolution in bacteria because there are so many of them & they have short generation times. Eg Methicillin-resistant Staphylococcus aureus.
(Eg when oxygen could be used as an electron acceptor). All of a sudden the organisms have all the energy they could want. Under these conditions, the number of mutants increases.
In nature everything is under constant pressure to survive, leaving little room for this “experimentation” between strains. Room is given when there are large shifts.