Which of the following correctly defines directional selection?

Directional selection is a key concept in evolutionary biology that occurs when individuals with a particular extreme phenotype confer a fitness advantage in a specific environment. This leads to an increase in the frequency of that particular trait over time, pushing the overall trait distribution in one direction.

In the context of evolutionary processes, this means that if one extreme variation of a trait is advantageous — for example, larger size providing better survival against predators — those individuals are more likely to survive and reproduce, thus passing on that trait to future generations. This shifts the population's traits toward that advantageous extreme.

The other options do not accurately characterize directional selection. For instance, stabilization of a trait describes stabilizing selection, which maintains traits that are average and reduces variation. Favoring individuals in the middle range pertains to stabilizing selection as well, which tends to select against extremes. Finally, while directional selection may inadvertently lead to the reduction of less fit individuals over time, that is not its defining characteristic. The essence of directional selection lies in favoring one extreme variation over the other possibilities.