Their environment tend to survive and produce more offspring while the lesser adapted organisms die before being allowed to pass on their genes at all.
Turtles needed to swim to have a better chance of survival, this gene of having legs like fins was suited to the environment of the turtle and thus become a more common allele in the turtle population.
The turtles were so different that they became a different species. The 75% or turtles of all species suddenly go extinct.
They are able to stay underwater for a very long time,do to the adaptations in their body, their adaptation is to convert salt water to fresh water and let our the salt, this helps them travel long distance underwater, making them hard to track and count.
During the over production is where and organisms creates many off springs more than the amount that could survive, many of the off springs can survive, in a leather back sea turtle species.
During genetic variation in a species each individuals has their own set of traits, but only some are better adapted other aren't that lucky and don't get to reproduce or adapt which leads to the species evolving over time.