Sea Turtles produce more offspring than that will survive. Some may not make it to adulthood because of being eaten, not having enough food, etc.
The variation of the Sea Turtles is hard and soft shells. One variation will be selected by nature and one will be selected against in the process called natural selection.
The Sea Turtles preditor are sharks, which are in the same environment as the Sea Turtles. Sharks will hunt these turtles so, the turtles will see them as a threat.
These Sea Turtles are always in a never-ending competition to get food, shelter, mates, to not get eaten, etc.
The variations are selected for and against through the natural selection process. Organisms with help with adaptation like, hard-shell turtles which are more likely to survive. The hard shell allows them to protect themselves, unlike the soft turtle shells.
Over many years, evolution and natural selection occur and they work together to form a population of successful Sea Turtles. Since nature selected against the soft shell turtles, there will be few to no turtles with the soft shells, and they will eventually die out.