Which form of natural selection promotes one extreme phenotype over others?

Directional selection promotes one extreme phenotype over others by favoring individuals with traits that are at one end of the spectrum. This type of selection occurs when environmental conditions change or when a population migrates to a different habitat, making certain traits more advantageous for survival and reproduction in that specific context.

For example, if a population of birds has varying beak sizes and the available food becomes primarily larger seeds, birds with larger beaks may be more successful in feeding compared to those with smaller beaks. Over time, the frequency of the larger beak phenotype will increase within the population. This selective pressure drives a shift in the population's average phenotype toward the favored extreme.

In contrast, disruptive selection would favor individuals at both extremes of a phenotype distribution while selecting against intermediate traits, which does not apply in this scenario. Stabilizing selection favors the intermediate phenotypes and acts against extremes, thus reducing variation. Sexual selection involves mate choice and does not directly relate to survival of certain phenotypes in the context of environmental pressures.