Principle #1 Genetic Variation There are 2 different types of pigs, thin and thick. The thinner pigs don't eat as much and tend to be the more quiet ones. The thicker pigs stuff themselves with food causes them to grow which is better for a farmer it gives him more money.
Principle #2 Overproduction of offspring/ Struggle Not all pigs are going to be killed for food for the year because the thinner pigs don't give enough food for markets, homes, etc.
Principle #3 Differential Survival and Reproduction The thicker pigs will be killed for meat. The thinner ones will survive to produce more and be ready for next year.
Principle #4 Change in the Gene Pool There will be an increase in thinner pigs than thicker ones because us humans get them for food.