What effect does a selective sweep have on DNA sequence variation near an allele undergoing strong positive selection?

A selective sweep occurs when a beneficial allele increases in frequency rapidly within a population due to strong positive selection. As this advantageous allele spreads through the population, it tends to "sweep" along nearby genetic variants, reducing genetic diversity in those regions.

This phenomenon is largely due to the fact that as the advantageous allele becomes more common, the surrounding genetic material (haplotypes) also gets pulled along with it, which can lead to a significant reduction in variation in those adjacent regions. The close linkage between the selected allele and surrounding markers leads to what is known as increased linkage disequilibrium. Linkage disequilibrium refers to the non-random association of alleles at different loci, which becomes pronounced when strong selection affects not just the allele itself but also the adjacent genetic material.

In summary, a selective sweep decreases genetic variation near the allele that is undergoing positive selection and simultaneously increases linkage disequilibrium, as the beneficial allele and its linked variants tend to rise and fall together in frequency throughout the population.