AP BIOLOGY

Detailed explanation-1: -Stabilizing Selection–The extremes are selected against. Example: height; mostly beings tend to the average height-not too many really short ones or really tall ones. Directional selection–One extreme value is selected for. Example: speed; faster is always better so a population will tend to get faster over time.

Detailed explanation-2: -Stabilizing selection results in a decrease of a population ‘s genetic variance when natural selection favors an average phenotype and selects against extreme variations. In directional selection, a population’s genetic variance shifts toward a new phenotype when exposed to environmental changes.

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: -Examples. An example of directional selection is fossil records that show that the size of the black bears in Europe decreased during interglacial periods of the ice ages, but increased during each glacial period. Another example is the beak size in a population of finches.