100 years ago there lived a small community of snakes. 50% of them were green, and 50% of them were yellow.
One day, all the green snakes happened to be standing in the same place. A dead tree fell right on top of the green snakes, killing four of the five.
Now, the green snakes represented only 1/6 of the population, whereas before, they represented 1/2 of the population. When the snakes repopulated, only yellow snakes were born, and the trait giving the snakes a green color was not passed on.
After the last green snake died of old age, there were no more blue snakes in the population.