What is the name of natural selection that favors a single extreme phenotype over others?

Natural selection that favors a single extreme phenotype over other forms is called directional selection. As generations pass, the population’s average trait value tends to shift toward the favored extreme.

For example, if unusually large beaks help a bird population crack available seeds, birds with larger beaks may survive and reproduce more successfully than birds with smaller or intermediate beaks. The relevant genetic variants can then become more common. Directional selection can also favor smaller size, lighter coloration, greater speed, or increased resistance, depending on environmental conditions.

Directional selection is different from stabilizing selection, which favors intermediate phenotypes and selects against extremes. Disruptive selection favors both extremes while disadvantaging intermediate forms, potentially splitting a population’s trait distribution. Artificial selection is also different because humans deliberately choose which organisms reproduce. A classic modern example of directional selection is the spread of antibiotic resistance in bacteria exposed to antibiotics.