27. Which of the following best describes directional selection?
(A) Two or more phenotypes are common and drive diversity.
(B) It occurs when one extreme phenotype has an advantage over all other phenotypes.
(C) Larger individuals in a population have higher rates of survival than average and small individuals.
(D) All of the above.
(E) B and C are correct.

Directional selection is when one particular value for a trait is more successful than others, so over time the average for the entire population shifts in that direction.

Stabilizing selection is selection which maintains the status quo because individuals with extreme values (high or low) for a trait do less well.

Disruptive selection is the opposite of of stabilizing selection. Instead of the individuals with the current average value for a trait doing better, individuals with extreme values do better which results in selection away from the status quo. The difference from directional selection is that the change isn’t in a particular direction, but is simply away from the current average value. Whereas directional selection will cause the distribution for a trait to shift either higher or lower, disruptive selection results in a distribution with more individuals at both higher and lower values, since they do better than individuals with intermediate values.