The turtles live on a beach. The turtles are eaten by big birds.
Differential Survival
More baby turtles are born than can survive on the beach. There are not enough resources for all of them. Some of the turtles always get spotted by the birds and get eaten.
Differential Reproduction
Most turtles are light green but some are a dark green color due to a mutation. Competition based on adaptations will allow turtles to be eaten or not to be eaten by the birds.
Natural Selection
The dark green turtles are better at avoiding the birds than the bright green turtles. Dark green turtles get eaten less often and survive longer than the bright green turtles.
The turtles that survive reproduce. Most of the turtles are dark green and they pass on that trait to their offspring. The next generation has a greater percentage of dark green turtles than bright green.
The bright green turtles stand out more than the dark green turtles. This makes it easier for the birds to spot them.