What has occurred when natural selection shifts allelic frequencies toward the extremes of a range of genetic expressions for a particular trait? 

A)  directional natural selection 
B)  discontinuous natural selection 
C)  diversifying natural selection 
D)  stabilizing natural selection

Allelic Frequencies

The relative frequencies of different alleles of a particular gene within a population, which can change over time due to evolutionary pressures.

Genetic Expressions

The process by which information from a gene is used in the synthesis of a functional gene product, often leading to the manifestation of a particular trait.