Genetic Variation: Sea turtles have to get to the ocean. Some of the turtles have stronger flippers than the others and have a higher chance of survival.
Overproduction & the Struggle for Existence: The struggle with the sea turtles is that when they try to get to the ocean, there is a chance that they get eaten by the seagulls.
Differential Survival & Reproduction: When the turtles are on there way to the ocean the seagulls can just swoop down to eat them. This is where the stonger flippered turtles have a higher chance of survival because they can get through the sand and to the ocean faster than the slow ones.
Changes in the Gene Pool: In this case since some turtles make it to sea, most of them won't live long enough for them to reproduce but the ones that do reproduce, it means they can pass down their traits so some of the turtles can have strong flippered offspring.