The genetic variation of bat is some are brown and some are black.
The thing with over production means something is over produces, sometimes bats are in this scenario
The raccoon predator come and eats the bats. The black one are most likely to survive ad reproduce because they camouflage in their dark environment. The brown ones would just get spotted by the predator and die off.
The changes in this gene pool are that the black bats genes are getting passed down and therefore are better adapted to the environment to stay alive