Consider the data on cliff swallow mortality. Why is this an example of directional selection? Overall genetic variation decreased. Selection did not occur, because no reproduction occurred (just survival). Individuals with intermediate phenotypes survived best. The average trait value changed in one direction (in this case, larger size).

The average trait value changed in one direction. (In this case, larger size)

Explanation:

In evolution a natural selection can be disruptive, directional or stabilizing

In stabilizing no extreme trait is favored hence provides intermediate values.

Disruptive selection both extreme traits are favored over the intermediate trait.

Directional, the enviroment will favor the survival of one trait hence a change in direction either towards the left or the right.

In the case of swallow cliff mortality, selection favored the larger size.