How is discontinuous variation characterized?

Discontinuous variation is characterized by distinct categories or phenotypes, where individuals display one specific trait or another without intermediate forms. This means that traits exhibit clear differences, with no gradual or blended options available. For instance, consider traits like blood type or flower color in certain species, where individuals can be classified distinctly into categories such as type A, B, AB, or O, or specific colors, rather than a blend of the two.

This definition helps clarify why the other options do not accurately describe discontinuous variation. Phenotypic variation on a spectrum implies a gradual transition between traits, which contrasts with the clear-cut nature of discontinuous variation. Variation that includes multiple variants within a trait suggests the presence of intermediate forms, which again falls outside the definition of discontinuous variation. Lastly, gradual changes over generations denote evolutionary processes that lead to a spectrum of traits rather than distinctly separate categories. Thus, the essence of discontinuous variation lies in its binary or categorical nature, making the chosen answer correct.