Using concepts of Mendelian genetics and natural selection explain the increase in the phenotype frequency for black fur from the initial population through generation 40

Black fur is more fit to survive in the environment than other colors of fur.

Explanation:

"Fitness", in terms of natural selection, is an organism's likelihood to survive in a given environment. Assuming that the increase in black fur is significant (Not attributable to pure chance), we can safely assume this is because Black Fur provides a competitive advantage in said environment over other colors of fur. There may be black terrain that allows them to blend into the environment and ovoid predators, or maybe it keeps them warmer if it is located in a cold region, thereby reducing the amount of resources they need to spend to stay warm.