Final answer:
The question addresses different types of natural selection in humans: stabilizing selection, which narrows phenotypic variation, and directional selection, which favors one extreme phenotype and can increase genetic variance in a population.
Step-by-step explanation:
The question you've raised pertains to the types of natural selection observed in contemporary human populations. There are three primary types mentioned in your question: directional selection, stabilizing selection, and disruptive selection.
Stabilizing selection is a type of natural selection where phenotypes at both extremes of the spectrum are selected against, resulting in a narrower range of phenotypic variation. An example of this in humans is birth weight, as babies that are too small or too large have lower survival rates, keeping the birth weight within a narrower range.
Directional selection occurs when a single phenotype is favored, shifting the phenotypic distribution toward one extreme. This is exemplified by the Grants' study of finch beak sizes in the Galápagos, where a particular beak size became more prominent over generations. This selection can lead to greater genetic variance in a population over time.