Final answer:
Directional selection is a form of natural selection that shifts a population's phenotype distribution toward one extreme, favoring one extreme phenotype over others.
Step-by-step explanation:
The primary characteristic of directional selection in the context of continuous traits in a population is that natural selection favors one extreme phenotype over the average or other extreme. This causes a shift in the population's phenotypic distribution towards that favored extreme. For example, if environmental changes make it advantageous for a plant species to grow taller to avoid being eaten by ground-dwelling herbivores, over time, the average height of the plants in the population will increase as those traits are passed on through successive generations.