I'm pretty sure the fact of microevolution goes a long way in justifying our belief in evolution generally. As far as I'm aware, the theory of evolution was developed by observing macro phenomena. Once the theory developed, the idea that evolutionary principles should apply to all levels of living organisms suggested -- in fact predicted -- that if we observed micro-organisms we would see the same principles at work. This became a prediction which could be tested, and which, once confirmed, greatly increased our justification for holding the entire theory (i.e., that evolutionary principles apply to all living organisms) true. That's the way (or one of the ways) that science works.