Final answer:
Directional selection leads to the increase in frequency of beneficial alleles as a phenotypic trait becomes more adaptive, demonstrating adaptive evolution and contributing to evolutionary fitness.
Step-by-step explanation:
With directional selection, the frequency of a favored allele will increase as a phenotypic variation becomes more adaptive. During this process, alleles that contribute to traits at one end of the spectrum are selected for, while those at the other end are selected against. This results in an increase in frequency of beneficial alleles and a decrease in those considered deleterious. An example of directional selection is the evolution of peppered moths, where those with darker wing coloration became more frequent due to the industrial revolution causing soot-covered trees.
Adaptive evolution is the outcome of these selective pressures, often seen in natural populations adapting to changing environments. It can lead to a change in allele frequency in populations over time, which is a fundamental aspect of study in population genetics. Evolutionary fitness is enhanced for those individuals whose traits better suit them to their environment, playing a pivotal role in this type of evolutionary change.