It's just a THEORY, stupid 2
Reader zhanzhao made the comment in the previous post that evolution "cannot be observed under lab conditions, vaildated [sic] nor replicated".
He is partly correct.
Evolution has been observed and validated in a lab condition, but curiously, it could not be replicated*.
Leng Hiong covered this interesting experiment briefly earlier. If you want a more detailed account but do not want to read the original paper, Bad Science has a post on an exchange between the investigator and a critic which covers some of the details of the experiment.
Both are well-worth a read.
* - Edit: On closer reading of the posts I realise that the citrate+ trait does "re-evolve" in the 'ancestors' of the populations of E. coli that evolved the trait, but not in the other 11 populations in the study, so in a sense evolution has been observed to be replicated. I apologise for the error.