Mice living on a granite hillside have coloration ranging from light to dark. The surfaces of the granite rocks are either very dark or very light. Dark mice hiding on dark patches are difficult to see and light mice hiding on light patches are equally camouflaged. Mice with gray fur can be seen everywhere and are preyed upon more heavily. What kind of selection is this?

Disruptive selection.

Explanation:

Natural selection is the result of the phenotype-environment interaction which determines gene destiny in space and time, selecting beneficial alleles and increasing their frequency in the population. There are different types of natural selection: sexual selection, stabilizing selection, directional selection, frequency-dependent selection, and disruptive selection.  

The disruptive selection causes an increase in the two types of extreme phenotypes over the intermediate forms. Limits between one extreme and the other are frequently very sharped. Individuals belonging to one phenotype can not live in the same area as individuals belonging to the other phenotype, due to the traits differences between them, competition, predation, etcetera.

In the proposed case, each population of mice is adapted to its environment: one of them is dark-furred as the color of the dark patches it lives on, and the other population is light-furred as it lives in light patches. These adaptations are probably related to survival strategies and help them to avoid predators as they camouflage with the substrate.  

Both extreme phenotypes have been favored over intermediate forms, resulting in the development of two groups with very marked differences.  Grey-furred mice population belongs to the intermediate phenotype which can be seen everywhere and be preyed upon more heavily. Disruptive selection does not favor this phenotype.

Disruptive selection can lead to speciation, driving to evolution. This is why it is also called "diversifying selection".  

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