Olivia Judson's column is a very useful essay on selection incidental to laboratory life for model organisms ("Laboratory Life"). She discusses fruit flies and wasps, but I'll give you a passage about mice:
Mice show a host of changes, too. Compared to their wild relations, laboratory mice are typically bigger, more docile, reach sexual maturity earlier and die younger. Some of these changes can appear quickly: one study found that the ability to reproduce later in life declined within 10 generations of the mice being bred in the laboratory.
Intriguingly, laboratory mice also have longer telomeres than wild mice. (Telomeres are the segments of DNA at the ends of chromosomes; they are thought to play a role in aging and cancer.) Since no one is deliberately breeding mice for extra-long telomeres, this must arise as some consequence of laboratory life. But what?
Take away predators and foraging requirements, and select for fecundity and docility. Lots of things can happen, not all good.