Which factors influence the expected time to fixation of a beneficial allele?

• A. Mutation rate at other loci
• B. Selection strength s, effective population size Ne, and initial frequency
• C. Recombination rate
• D. Migration rate only

Answer: (B)

The expected time for a beneficial allele to fix in a population is determined by how quickly selection can push it upward and how much random drift can interfere with that rise. The key factors are the strength of selection (s), the effective population size (Ne), and the starting frequency of the allele.

A stronger selective advantage makes each generation’s increase larger, so the allele climbs to high frequency more rapidly and fixation occurs sooner. If selection is weak, random fluctuations due to drift can dominate early on, slowing or even preventing fixation, so the process takes longer.

Effective population size matters because drift is stronger in smaller populations. In a small population, an advantageous allele can be lost by chance before selection can act strongly enough to fix it, lengthening the waiting time. In larger populations, drift is weaker, so selection acts more deterministically and the sweep proceeds faster, shortening the time to fixation.

The starting frequency is crucial: an allele present at a higher frequency needs fewer generations to reach fixation than one that starts rare. If the allele is standing variation at a moderate frequency, fixation occurs quickly; if it begins as a new mutation at a very low frequency, there’s a longer path to fixation.

Mutation rate at other loci, recombination, or migration influence the broader genetic landscape and patterns around the sweep, but the primary determinants of how long it takes for a given beneficial allele to fix are s, Ne, and the initial frequency.