A population of small fish lives in a lake with a sandy bottom. Large shore birds prey on these fish. Although most of the fish are the same light brown color as the sandy lake bottom, about 10% are mottled. A construction company begins dumping loads of gravel in the bottom of the lake, giving it a mottled appearance. Which outcome would you predict?
a. As the sand-colored fish are eaten, more will be produced to keep their frequency at 90%.b. There is no way to guess the result, because evolution occurs at random.c. The proportion of mottled fish will increase over time.d. According to the Hardy-Weinberg equilibrium, the ratios will not change.

The initial proportion of 90% light-brown (sand-like) colored fish to 10% mottled fish is the result of the light-brown fish mixing with their background and thus being preyed less by the birds than the mottled fish which are of course more visible above a light brown sandy bottom. The higher predation results in the removal of more mottled fish and thus a smaller number of these fish in the population. When the gravel is added to the bottom of the lake, this situation reverses. Suddenly the light brown fish are more visible to the birds and we can expect that their numbers will decrease, while the mottled fish will become more numerous as they will be harder to spot and capture by the birds, and will therefore survive more than their pale counterparts.