AP BIOLOGY

Detailed explanation-1: -Polygenic traits in a population often form a bell curve distribution. Natural selection on polygenic traits can take the form of: Stabilizing selection: Intermediate phenotypes have the highest fitness, and the bell curve tends to narrow. Directional selection: One of the extreme phenotypes has the highest fitness.

Detailed explanation-2: -A graph of a population undergoing stabilizing selection. In directional selection, one of the extreme traits is selected. A graph of a population undergoing directional selection. In disruptive selection both extreme traits are selected.

Detailed explanation-3: -Graph 1 shows directional selection, in which a single extreme phenotype is favored. Graph 2 depicts stabilizing selection, where the intermediate phenotype is favored over the extreme traits. Graph 3 shows disruptive selection, in which the extreme phenotypes are favored over the intermediate.

Detailed explanation-4: -Blue curve represents the phenotype frequency distribution of an initial population before encountering a change in selection due to niche discordance; red curve represents the population’s initial evolutionary response to the new selection regime.