What type of natural selection causes the frequency of a particular allele to move in one direction?

Directional selection is the type of natural selection that leads to a consistent change in the frequency of a particular allele in one direction. This process occurs when individuals with a trait at one extreme of the phenotypic range have a higher fitness compared to individuals with other traits. As a result, the allele associated with that advantageous trait increases in frequency over time.

For example, if a specific allele leads to a trait that enhances survival or reproductive success in a changing environment, individuals expressing that trait are more likely to survive and reproduce. This selective pressure pushes the overall population's trait distribution toward that advantageous phenotype, effectively moving the allele frequency in one direction.

In summary, directional selection is characterized by its focus on favoring one extreme trait, which drives the overall genetic makeup of the population in a specific direction over time.