What is natural selection? Well you probably already know what it is because we've heard it from Hr Huang about 10,000 times but just in case you've forgotten, natural selection is the process whereby organisms better adapted to 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.

But how does natural selection have anything to do with sea turtles? Well ages ago (like a really long time ago) the sea turtle only existed as a normal tortoise living on the land. But tortoises needed to swim to have a better chance of survival, so one day a mummy tortoise and a daddy tortoise made a baby with legs that allowed it to swim better than other turtles. This gene of having a legs like fins was suited to the environment of the turtle and thus became a more common allele in the turtle population. After a long time, the turtle with the fin legs slowly grew into proper fins, a huge difference from the legs of the tortoise.

At this point, the two turtles were so different that they became a different species. You can already notice just by looking at the differences between the picture above and the picture below.