Originally, More of the bunny offspring were being produced then surviving. Most of the bunnies were eaten/killed before they were able to reproduce.
competetion
There are different variations in the bunny population. In this environment their are 2 variations. long legs and short legs. One variation is selected for whereas the other variation is selected against.
selection
The predator of the bunnies (the fox), have the same habitat. The foxes are threats to the bunny species.
change in over all species
The bunnies are always competing with one another to get food, shelter, and lots more. Each bunny is fighting to survive. There is also always a possibility of get eaten by a fox.
The 2 variations play a huge role in survival. The organisms with the helpful adaptions (long legs) are more likely to survive then the organisms with the short legs because they are unable to run or get away from their predators.
After many many years, evolution and natural selection, and lots of reproducing, they all work together to make a population with almost all bunnies with the more successful survival rate. Their are only a couple bunnies with shorter legs.