Watch a beneficial allele sweep to fixation—see linked neutral variants hitchhike along
A selective sweep occurs when a beneficial mutation rapidly increases in frequency and goes to fixation in a population. As the beneficial allele "sweeps" through the population, it drags along ("hitchhikes") neutral variation at physically linked sites on the same chromosome.
In a hard sweep, a single beneficial mutation arises on one chromosomal background and sweeps to fixation. This creates a characteristic signature:
The fitness of each genotype depends on selection coefficient s and dominance h:
When h = 0.5 (codominant), heterozygotes have intermediate fitness. When h = 0 (fully recessive), the mutation is only beneficial when homozygous.
Neutral variants at linked sites are affected by their recombination distance from the selected site. The probability that a neutral allele hitchhikes to high frequency depends on the recombination rate r. Very tightly linked sites (low r) are strongly affected, while distant sites recombine away and retain diversity.
Explore Hardy-Weinberg Equilibrium to understand population genetics fundamentals, and Linkage Disequilibrium to see how non-random associations between alleles form and decay.
The theory of selective sweeps was developed by John Maynard Smith and John Haigh (1974), who showed that strong selection reduces variation at linked neutral sites. Modern genomics uses sweep signatures to identify recent positive selection in populations, including human adaptation to agriculture, high altitude, and pathogens.