There are many types of natural selection like direct selection, disruptive selection and stabilizing selection.
The situation given is that a population of mice lives in a city. The largest mice tend to be killed by predators and the smallest mice cannot compete for food.
The type of selection here is stabilizing selection. It is a type of natural selection in which there is decrease in genetic diversity as the population stabilizes.
In a human? Need more context here but should be outside the cell. At equilibrium you will have a more negative environment in the cell.
