What is Natural Selection
Natural selection is those who are most '"fit" will survive, reproduce and take over an enviroment.
Deer mice had dark brown hair when they migrated to the hills of Nebraska but they changed to light brown hair to hid from there predators. The ones that didn't change there hair color didn't hid from there predators well.
What determines who is "selected"?
The Enviroment does. The Deer mice relay off there Enviroment to live so if they can't blended in then they can't live.