There is a ton of small rabbits their genes are smaller legs and they are a bit cubbier.
new species of rabbits appear
Foxes start to appearing and eating the rabbits.
new rabbits start evolving
The rabbits have a hard time getting away from the foxes because of the way their generation is. If they want to survive there needs to be a mutation or natural selection will take them out.
new species of rabbits better for surviving
One of the offspring of a short rabbit is a bit skinner and has longer legs. The traits of this offspring makes it easier to escape from the foxes.
The genes of this rabbit over time evolve and there is a variation of rabbits now.
The bigger rabbits do much better than the smaller rabbits with escaping from the foxes.