The effects of stabilizing directional and disruptive selection on variations in a trait over time?

Answer 1

Let us use tallness in men and women as our example.

In stabilizing selection a normal distribution of heights would be expected as the selective environment would not select against having all variants present in the population.

In directional selection you would see men and women getting increasingly taller ( or, perhaps shorter ) as natural selection selected the variants that were more reproductively successful in the immediate environment. ( say women's mate choice went into overdrive for tall men )

Disruptive selection is the distribution of traits that are at either tail of the distribution. Both short and tall people are represented, but no intermediates. These two dichotomous traits are seen in nature when one species feeds on two different food sources in the same area. Rather like small and large beaked birds. How humans could go through disruptive selection I leave to your analysis.

Answer 2

Stabilizing selection reduces variation in a trait by favoring the intermediate phenotype, leading to a narrower range of trait values over time. In contrast, disruptive selection increases variation in a trait by favoring extreme phenotypes, potentially leading to divergence into distinct subgroups with different trait values. Both types of selection can influence the evolution of a trait by shaping the distribution of phenotypic variation within a population.