Final answer:
In directional selection, one extreme phenotype is selected for, leading to a shift in the population's traits towards that extreme, unlike stabilizing selection which narrows variation and diversifying selection which increases genetic variance by favoring multiple extremes.
Step-by-step explanation:
In directional selection, both phenotypic extremes are not equally selected against. Instead, this type of natural selection favors one end of the phenotypic range, leading to a shift in the population's phenotypic distribution towards one extreme. The classic example provided is the case of the Galápagos finches observed by the Grants, where finch beak size changed over time due to environmental pressures that favored one extreme of beak size over another.
Directional selection differs from stabilizing selection, where the average phenotype is favored and both phenotypic extremes are selected against, and diversifying selection, where two or more extreme phenotypes are favored over the intermediate types. An example of stabilizing selection is the human birth weight, where very large or very small babies have lower survival rates, resulting in a narrower range of birth weights in the population. Meanwhile, diversifying selection can be seen in certain animal populations where multiple male forms exist, and both extremes have advantages, while intermediate forms are at a disadvantage, thus increasing the population's genetic variance.