Leather back sea turtles are born in big groups. Some of them born with stronger flippers others with weak ones, but they all can sense the polarity of the ocean.
Since the turtles are born in big groups many of them die, either by the birds, sand suffocation, or by predators in the ocean. Although the ones that do survive tend to be the ones with strong flippers since they have faster mobility.
Those turtles that survive will later mature and hopefully live long enough to find a mate to reproduce.
Lastly the mother will bury her eggs in the sand, and the newborn turtles will be born with the advantageous trait of strong flippers changing the gene pool and hopefully allowing more of them to live and make it to the ocean.
That was an explanation of the four major principles of natural selection showing how the leather back sea turtle population can change over time