Principle #1: Genetic Variation In the horse population some horses have longer and stronger legs than others. Some horses have shorter and puny legs.
Principle #2: Overproduction & Struggle for Survival An average horse has usually 1 offspring at a time. Their selective pressure are predators such as lions who eat any horse they can catch. If the horse has long and strong legs it can run away from the lion. However, if it has short, puny legs then its chances of survival are minimum.
Principle #3: Differential Survival and Reproduction The horses with long and strong legs are able to run away from predators and survive. Therefore, they reproduce and pass on their traits to their offspring that will also help them survive and reproduce. However, the horses that have short and puny legs don't manage to escape from the lions, so they get eaten and don't get to pass on their genes to future generations.
Principle #4: Changes in the Gene Pool As time passes by there will be a decrease in the amount of horses with short, puny legs in the population. Meanwhile, the number of horses that have long and strong legs in the population will increase in new generations.