19. the rocks at the bottom of a stream are either black or white. snails that live on these rocks are a range of color from solid black to gray to solid white. a local freshwater fish loves to munch on these snails, but it cannot see the solid black or solid white snails when the snails are on matching rocks. grey does not blend into either rock color and is much easier to see which trait is natural selection selecting for? how will the population change over time?

Natural selection is selecting for extreme phenotypes. Over time their frequency will increase in population and frequency of intermediate phenotype will decrease in population.

Explanation:

This is a classic case of disruptive selection where both the extreme phenotypes are favoured over the intermediate phenotype. Since black snails can hide on black rocks and white snails can hide on white rocks, snails having these colours have better chances of survival.

Grey snails wont be able to hide on either type of rock so they will be targeted more by the predator and their population will decrease. Eventually, the extreme black and white coloured snails will increase in number and intermediate grey coloured snails will decrease.