Final answer:
Directional selection is the mode of selection where a consistent preference for a higher or lower value of a trait over time changes the average value of that trait in a population. It results in a shift towards one of the extremes in the phenotypic distribution.
Step-by-step explanation:
When selection consistently favors a higher (or lower) value of a trait or phenotype, leading to a change in the average value of that trait in the population over time, this mode of selection is known as directional selection. An example of directional selection is noted in the beak size of Galápagos finches, where one of two extreme phenotypes is favored, causing a shift in the phenotypic distribution toward that favored extreme.
In contrast, stabilizing selection occurs when natural selection favors an average phenotype and selects against extreme variation, which results in a decrease in the population's genetic variance. Meanwhile, disruptive selection (also known as diversifying selection) occurs when extreme phenotypes are favored and the average phenotype is selected against, leading to greater genetic variance in the population.