What happens to an allele at one locus that is linked to another allele undergoing positive selection?

When an allele at one locus is linked to another allele undergoing positive selection, the process known as "hitchhiking" can occur. Hitchhiking refers to the phenomenon where the increase in frequency of the positively selected allele can indirectly increase the frequency of the linked allele. This happens because both alleles are located close together on the same chromosome, making it less likely for them to be separated by recombination during meiosis.

As the advantageous allele increases in frequency due to natural selection, alleles that are physically linked to it also rise in frequency. If the linked allele has no detrimental effects and is not subject to opposing selective forces, it may eventually become fixed in the population—that is, reach a frequency of 100%. This process illustrates how selection at one locus can influence the genetic composition of nearby loci in the genome, leading to changes in allele frequencies that may not be directly related to the selected trait itself.