I think you've got the fact/theory idea reversed. That natural selection occurs and results in changes in population characteristics is not a theory. It is known to be a fact as it has been directly observed. Also evolution is a fact in the sense that changes in population characteristics have been directly observed. In fact a population that is observed to change due to changes introduced through natural selection has evolved. It has changed. The classic melanistic moth thing is an example (see http://www.christs.cam.ac.uk/darwin200/ ... page_id=g5" onclick="window.open(this.href);return false;).Evolution is a fact, but natural selection is the theory behind it. We can and do observe evolution in real-time with microorganisms.
What's not known to be fact is the overall theory of evolution. That is the theory that the kind of change...or evolution...that has been seen in real time with microorganisms as well as with multicellular organisms such as the moth and darwin's finches (changes in predominant occurence of beak morphology) led to changes such as populations of single celled organisms giving rise to multicellular organisms and eventually...over billions of years...to populations of such things as human beings and blue whales.
Anyway, population change through natural selection is known to be a fact. The overall theory of evolution is not.













