Stabilizing selection is a condition where extreme traits have a negative outcome in the indivuals of a population, therefore the average value of the trait has the advantage. In this example, pine trees that are in the extremes of height (either tall or short) might have physiological disadvantages. On the other hand, pine trees with average height might escape this issues. For example, tall trees might have troubles with water transport, while short trees might have troubles with getting enough sunlight exposure. Average pine trees would not have disadvantages in either of the physiological examples described. With time, this trees (average height) will dominate in the population structure, whereas the other type of trees while be diminished.
