John Maynard Smith and John Haigh discuss the hitch-hiking effect of a favourable gene, where a selectively advantageous gene substitution influences the frequencies of alleles at closely linked loci. When a favourable mutation becomes fixed in a population, it can increase the frequency of alleles on the same chromosome, leading to a reduction in heterozygosity at neutral loci. This effect is more significant than random fixation in large populations, potentially explaining why the extent of neutral polymorphism in natural populations does not vary as much as expected. For a selectively maintained polymorphism at a linked locus, the hitch-hiking effect is only important if it leads to complete fixation. The probability of complete fixation at the linked locus is approximately exp(-Nc), where c is the recombinant fraction and N is the population size. In large populations, a selective substitution can occur in a cistron without eliminating a selectively maintained polymorphism in the same cistron. The hitch-hiking effect is more significant in large populations, reducing neutral polymorphism more than random fixation. The effect of hitch-hiking on reducing heterozygosity is greatest when the selective advantage is high, the fittest genotype is much fitter than average, and the population is small. The half-life of a neutral polymorphism against fixation by hitch-hiking is much longer than that against fixation by random drift in large populations. The probability of complete fixation of a linked favourable allele is approximately exp(-θNcR), where θ is a constant, N is the population size, c is the recombination fraction, and R is the proportion of the allele at the linked locus. The hitch-hiking effect is very local, affecting only a small region of the chromosome. In large populations, a selectively maintained polymorphism can survive a substitution in the same cistron, although one or other of the alleles would usually be reduced to a low frequency. The study shows that the hitch-hiking effect can account for the uniformity of neutral polymorphism between species, challenging the neutral mutation theory.John Maynard Smith and John Haigh discuss the hitch-hiking effect of a favourable gene, where a selectively advantageous gene substitution influences the frequencies of alleles at closely linked loci. When a favourable mutation becomes fixed in a population, it can increase the frequency of alleles on the same chromosome, leading to a reduction in heterozygosity at neutral loci. This effect is more significant than random fixation in large populations, potentially explaining why the extent of neutral polymorphism in natural populations does not vary as much as expected. For a selectively maintained polymorphism at a linked locus, the hitch-hiking effect is only important if it leads to complete fixation. The probability of complete fixation at the linked locus is approximately exp(-Nc), where c is the recombinant fraction and N is the population size. In large populations, a selective substitution can occur in a cistron without eliminating a selectively maintained polymorphism in the same cistron. The hitch-hiking effect is more significant in large populations, reducing neutral polymorphism more than random fixation. The effect of hitch-hiking on reducing heterozygosity is greatest when the selective advantage is high, the fittest genotype is much fitter than average, and the population is small. The half-life of a neutral polymorphism against fixation by hitch-hiking is much longer than that against fixation by random drift in large populations. The probability of complete fixation of a linked favourable allele is approximately exp(-θNcR), where θ is a constant, N is the population size, c is the recombination fraction, and R is the proportion of the allele at the linked locus. The hitch-hiking effect is very local, affecting only a small region of the chromosome. In large populations, a selectively maintained polymorphism can survive a substitution in the same cistron, although one or other of the alleles would usually be reduced to a low frequency. The study shows that the hitch-hiking effect can account for the uniformity of neutral polymorphism between species, challenging the neutral mutation theory.