Principle 1: Genetic Variation The genetic variation between the snakes is that there is dark green snakes and light greens snakes.
Principle 2: Overproduction & the Struggle for Existence The dark green snakes are getting picked off by the owls because they aren't able to camouflage with the environment. All the snakes that are being born are getting eaten quickly, giving them no chance to survive.
Principle 3: Differential Survival & Reproduction The light green snakes will survive because they will be difficult to be spotted by predators. The light green snakes will take over the population in a couple of generations because it has a higher fitness for that environment.
Principle 4: Changes in Gene Pool The better trait which is light green will take over the snake population for better survival, the gene pool changes from differential to light green.