Evolution doesn't work the same way in bacteria as in other organisms due to horizontal gene transfer[1]. Bacteria can't be split into well-defined lines, because they share a common pool of genetic information.

[2] is a good book to get a perspective on antibiotic resistance.

[1] http://en.wikipedia.org/wiki/Horizontal_gene_transfer [2] http://www.perseusbooksgroup.com/dacapo/book_detail.jsp?isbn...