In a distribution curve for a trait affected by directional selection, the primary change observed is a shift in the mean of the trait's distribution in one direction. This means that individuals with traits that are favored by the selection pressure become more common in the population, leading to an increase or decrease in the average value of that trait.
Specifically, if a particular trait offers a survival or reproductive advantage, individuals with that trait will have higher fitness, and over time, the frequency of that trait will increase. As a result, the distribution curve will show a clear shift toward the favored trait expression, resulting in a more pronounced skew in one direction of the normal distribution—either to the right (higher values) or to the left (lower values)—depending on which trait variant is advantageous.
Overall, this results in a more asymmetric distribution compared to the original bell-shaped curve typically seen in normal distributions, as the population adapts to the selection pressure.