One pattern of natural selection polygenic traits is directional selection, in which one end of the spectrum leads to increased fitness and the other end decreased fitness. Disruptive selection is when both ends of the spectrum lead to increased fitness and the middle leads to decreased fitness and it leads to two distinct phenotypes being selected for. It's opposite is stabilizing selection, in which the middle has the best fitness and the two extremes have decreased fitness.

Stabilizing selection

Fitness.

(got this answer from my Biology Textbook)

Disruptive selection

This is designated as the organisms fitness.