Evolution can be observed in the wild, such as the formation of anti-biotic resistant bacterial strains.
Additionally Bacterial evolution has been observed in a large experiment, specifically the evolution of E.coli to metabolise citrate, a process that E.coli normally cannot undertake. Read more about it here: http://en.wikipedia.org/wiki/E._coli_long-term_evolution_experiment
For more examples of experimental evolution read here: http://en.wikipedia.org/wiki/Experimental_evolution