Genetic variation is caused by mutations in the gametes and meiosis.
The large ears of the rabbits may help them hear when predators are coming. If they hear them coming, then they may be able to escape.
A polar bear with dark fur would be more obvious to its prey, who would more easily avoid him. The other white ones can blend in better and have a less chance of being detected by prey.
Organisms have genetic variation
The conditions change
The ones better at coping... survive
Survivors breed and pass on their variation
natural selection occurs
Some bacteria are more resistant
Antibiotic is added
Some bacteria survive
The survivors reproduce
Low levels of antibiotic remain
Charles Darwin
Hope I helped