Differences Between Stabilizing & Directional Selection

Directional Selection

• Imagine that you have a measurable trait such as bill length that varies throughout a population. Some birds in the population have long bills, others short bills and still others bills somewhere in between. Selection pressures that select for variants at one end of the range are called directional selection. In the bird bill example, an environment that selects for birds with longer bills is an example of directional selection. Over time, it tends to push the genetic makeup of the population in one direction or the other.

Stabilizing Selection

• Imagine the environment these birds inhabit changes in some way. Perhaps a different kind of seed becomes more abundant, for example. Now birds with medium-length bills have the advantage. This kind of situation exemplifies stabilizing selection, in which the environment selects for intermediate variants. Variants at either extreme -- very long or very short bills -- are gradually eliminated from the population by stabilizing selection, and the genetic makeup of the population tend toward the middle of the range.

Examples

• According to "Biology," most human babies are between 6.6 lbs. and 8.8 lbs. Babies with much larger or much smaller birth weights are far less likely to survive. Because the intermediate variant is more likely to survive than the extremes, birth weight in humans is an example of stabilizing selection. Finches in the Galapagos Islands offer a famous example of directional selection. During a drought in 1976 and 1977, scientists found that birds with larger bills were more likely to survive. When the rains returned the following year, the population of birds now had a larger bill size on average. The average bill size had shifted due to directional selection.

Trends

• Stabilizing selection tends to decrease variation in a population. Because the intermediate variants are selected, over time the percentage of the population with genes that give rise to extreme variants decreases. It also preserves the status quo. Directional selection, by contrast, causes a shift in the genetic makeup of the population over time.