Stabilizing selection tends to favor intermediate phenotypes in a stable environment.

• A. Favors extreme phenotypes at both ends
• B. Occurs when evolutionary lineages diverge greatly
• C. Favors a single extreme due to a changing environment
• D. Favors intermediate phenotypes in a stable environment

Answer: (D)

Stabilizing selection preserves average traits by favoring intermediate phenotypes when the environment remains constant. In a stable environment, individuals with the most common, middle-range traits tend to have the highest survival and reproductive success because these traits best fit the regular conditions and resources. Extremes—too small or too large, too fast or too slow, etc.—often have lower fitness, so they are weeded out over generations. Over time, variation around the average decreases and the population stays centered around that intermediate form. If the environment were changing, you’d see different patterns: shifting toward one extreme (directional selection) or promoting extremes in different contexts (disruptive selection). The idea here is that a steady environment keeps the successful trait near the middle.