Hundreds of infant sea turtles are hatched at once. Some of the sea turtles have strong flippers and others do not. The strength of the flippers are inherited from the parents by the offspring.
Many sea turtles are born, but not all of them survive because they are unable to move to the ocean fast enough. Baby sea turtles that do not have strong enough flippers to get to the ocean most often get eaten by predators such as raccoons. In this scenario, the selective pressure is predators.
The turtle that inherit the gene for strong flippers make it to the ocean. These turtle survive long enough to reproduce and pass on the gene of strong flippers. The surviving turtle come back to lay their eggs.
The further generations of turtles will have an increase in the amount of turtle with strong flippers. The trait of weak flippers will slowly become less prevalent in the population. Due to the increase in stronger turtles, more are able to make it to the ocean.