Final answer:
Selection can favor certain traits resulting in evolutionary change. Stabilizing selection decreases genetic variance by favoring the average phenotype while diversifying selection increases genetic variance by favoring multiple distinct advantageous phenotypes.
Step-by-step explanation:
Selection, which is also known as "survival of the fittest", refers to the process where natural selection favors individuals with certain traits, leading to evolutionary change. When selection favors an average phenotype and selects against extreme variations, we witness stabilizing selection.
This leads to a decrease in a population's genetic variance. One example is mice living in a uniformly colored environment where those that blend in are more likely to survive and reproduce. Directional selection is another type of natural selection where one of two extreme phenotypes is favored, causing a shift towards that extreme in the phenotypic distribution.
Lastly, diversifying selection increases genetic variance by favoring two or more distinct phenotypes that are each advantageous, while intermediate phenotypes are selected against.