Their are two types of snakes in the desert a brown snakes and green snakes.
Genetic Variation- the brown snakes passes on their genes to the next generation because they survive more the green due to predators not spotting them that easily.
Overpopulation/Struggle for Survival- pressures-the brown snakes tend to survive more due to the birds not being able to see them therefor the green are more likely to die because they can easily be spotted in this case the selective pressure is predators and color.
Differential Survival and Reproduction- snakes who inherit the brown color tend to survive more and pass that trait on to their offspring but some way their will always be a green snake.
Change in the gene pool- every generation will have more brown snakes than green snakes because it is an advantage used to survive in which the green snake doesn't have so they will decrease in numbers so their would be more brown than green snakes.
In a few years later their will be at least one green snake with a bunch of brown snakes.