Directional selection
Selection causing a consistent directional change in the form of a population through time (e.g., selection for larger body size).
2. Selection resulting in a shift in the population mean in the direction desired by the breeder or in the direction of greater adaptation by nature.

Disassortative mating
The mating of two individuals with dissimilar phenotypes.

Discontinuous variation
Phenotypic variation in which the phenotypes fall into distinct classes, such as tall vs. short, yellow vs. green, etc.

Disruption
The morphologic defect of an organ or larger region of the body resulting from the disturbance or destruction of an originally normal developmental process.

Disruptive selection
Selection favoring forms that deviate in either direction from the population average. Selection favors forms that are larger or smaller than average, but works against the average forms between the extremes.
2. The selection of divergent phenotypic extremes in a population until, after several generations of selection, two discontinuous strains are obtained.