The turtle species, Speedolous Stiltskin, lives on a beige, sandy beach. The turtles are eaten by hungry birds that spot them from above.
More juvenile turtles are born than can survive on the beach. There is not enough food or space for all of them to live. some of the snails are always spotted by the birds and eaten.
Not all of the turtles are brown. Some turtles are born with other colors. All of the turtles on the beach are competing to stay hidden from the birds and not get eaten.
The brightly colored turtles really stand out against the brown, sandy beach. That makes them easy to spot from above. The brown turtles match the color of their environment, making it easier for them to hide.
Because their color is an adaptation on the beach, brown turtles are better at avoiding the birds. Brown turtles are eaten less often and survive longer.
Only surviving turtles get a chance to reproduce. Because most of those turtles are brown, they pass on their brown color trait to their offspring. Therefore, a greater percentage of turtles are brown in th next generation.