Organisms of either extreme characteristic dying out while organisms with the medium characteristic have a higher fitness is identified as?

Stabilization selection

Explanation:

Natural selection involves the differential survival and growth of organisms which have suitable traits to survive in unfavorable or adverse environment. Such traits are passed on to the next generation. Stabilization selection is a type of natural selection in which the nature selects the non-extreme phenotypic traits. Middle traits are selected and such organisms grow and reproduce. Example can be given that of human babies in which babies with low weight lose more heat and babies with high weight are difficult to be delivered from the pelvis. Therefore, babies with middle weight are expected to survive more than that of low or middle weight.