Variation: in the picture above the variation of the population is the color of the shell. Three of the snails have brown shells and one of them have a lighter brown shell.
Over Production and selection pressure: In the image shown overproduction has happened and so some may die off because of the competition of food with the offspring, parents, and other animals.
Differential Survival & Reproduction: All the lighter shell snails will die quicker because they are easily spotted by predators. Due to that all the brown shell snails will survive longer and will be able to create more offspring.
Changes in the Gene Pool: Because of all the light brown shells dying the population in brown shells has grown way more faster then the light brown shells.