The tendency for one extreme phenotype, such as large size or extreme coloration, to be favored over average phenotypes is called

• Diatonic254:

  The answer is disruptive selection. The allelic frequency of the population shifts to the extreme traits that are being favoured by natural selection as opposed to the intermediate traits.  The variance in the population increases and two species begin to emerge.  Directional selection, on the other, favours one end of the extreme traits. Disruptive selection is a form of sympatric speciation.  

• Bistai:

  Directional selection

  A directional selection is the tendency for an extreme phenotype such as large size or extreme coloration to be favored over other phenotypes. Direct selection allows the favored or selected allele to increase as a result of differences in survival and reproduction among different phenotypes.