In the rat species they where brown and some light brown rats. This was a genetic variation. A variation is a diffrence of a organism physical looks to the population. For example, eye color.
.Brown rats got eaten the most often because of the falcons seeing them more easily. Being the light skin rat however, had better chances of surviving because their skin color mtached the enviorment floor.
Since the light skin rats selection helped them to live longer they survied and reproduced. A selction is a trait of organism that helpes them survive.
Since light skin rats lived longer they reproduced more often then they did in the past which is called overproduction.Overproduction means that more organism are procduced then in the past.
Now a light skin adaptation has been added into the species