[Why the mutation rate never reaches zero?]

Alfred H. Sturtevant was the first to raise the question: why does the mutation rate not become reduced to zero? Indeed, most new mutations with a phenotypic effect are deleterious. Therefore, individuals who produce less mutants produce more viable and fertile offspring. Consequently, natural selection should increase the frequency of antimutator genotypes and progressively reduce the mutation rate to zero. However, no species has ever been found with a mutation rate equal to zero. Recent analyses suggest that setting the mutation rate above zero depends mainly on the effective size of the genome and the effective population size. The mutation rate is a trade-off between natural selection that operates to improve replication fidelity and the random genetic drift that sets the ultimate lower limit. This trade off illustrates the limitation of the power of natural selection in a world where natural populations have a finite size.