Question

1. 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.
2. human babies usually do not survive if they are born under 4 pounds because their organs are too small to work properly; they cannot be born if they are too big because the mother would not be able to give birth without dying.

Explanation:

These problems relate to evolutionary concepts (natural selection) in Biology. For problem 20, the fish's predation affects snail color survival (directional selection against gray). For problem 21, baby weight is constrained by survival and birth feasibility (stabilizing selection around a middle weight).

Answer:

Problem 20: Directional selection (against gray snails, favoring black/white that blend).
Problem 21: Stabilizing selection (favoring baby weights around a middle range for survival and birth).