There are 2 different types of prey for these bats. There are Light colored mice (loud noises) and dark colored mice (quiet noises).
Changes in the gene pool
It is hard for the light colored mice to survive because in the caves, the bats hear them moving and find them easier to eat. The dark colored mice are able to survive because they're a lot quieter than the light colored mice.
In the current environment of these mice, the dark colored mice will survive and be able to pass their genes to the upcoming generations.
The dark mice are the only type that has survived and are able to reproduce, so in the gene pool of mic in these caves, there will only be dark mice.