A population of voles (small mammals) live in a shady forest. Originally, the voles are a light tan color, but then a dark brown coloration arises by a new mutation. The darker voles are harder for predators to see in the leaf litter, thus they are less susceptible to predation. After 10 generations the majority of voles in the population are the dark brown color. What type of selection are the population of voles going through