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.