A mutation happened in the bunny family. There are the light brown original bunnies and dark brown bunnies.
Then comes a hungry wolf. However, he only spots the light brown bunnies. He was able to sneak up on the bunnies before they have a chance to run away. Of course the darker brown colored got a chance to run away.
Only a little population of bunnies is left but not gone because the darker brown colored bunnies survived and were able to reproduce.
More bunnies of a darker fur color are being produce because they can survive and bunnies with lighter fur are becoming less because the inability to hide.