Answer:
Natural selection is a factor of biological evolution, by means of which individuals who possess a certain characteristic tend to prevail over those who lack such characteristic. Thus, by reproducing, individuals with favorable characteristics end up imposing their own set of characteristics, taking control of the environment in which they develop and relegating other individuals or condemning them to their extinction or displacement.
Thus, in the case, if some mice entered the environment of others, most likely they would not be able to adapt to the conditions to which the original group of that environment had previously adapted. In this way, natural selection itself would make the new mice end up disappearing from it, in favor of the mice that already lived there and that, therefore, had adapted their characteristics to those of the environment.