Oooh, ooh, let me!
This paper and others it cites shows both in vitro and in vivo that antibiotic resistance to cefotaxime, cefuroxime, ceftazadime, and aztreonam evolved by single amino-acid substitutions from an ancestral penicillinase gene. The authors conclusion (and I quote) "The authors take this result as evidence that their in vitro evolution technique accurately mimics natural evolution and can therefore be used to predict the results of natural evolutionary processes. "
Predicting evolutionary potential: In vitro evolution accurately reproduces natural evolution of the TEM b-lactamase. Barlow, Miriam; Hall, Barry G. Biology Department, University of Rochester, Rochester, NY, USA. Genetics (2002), 160(3), 823-832.
Don't look now, but your inner primate is showing. Ooh, ooh, ook!