In a distribution curve for a trait affected by directional selection, there is a noticeable shift in the mean or average value of the trait towards one extreme of the distribution. This is because directional selection favors individuals with traits that are advantageous in their environment, thereby increasing the frequency of those traits in the population over time. As a result, the distribution curve becomes skewed towards the direction of the favored trait, causing the mean to move in that direction. Additionally, there may also be a decrease in the genetic diversity of the population as individuals with less advantageous traits are selected against, further contributing to the shift in the distribution curve.
