The paper by John Maynard Smith and John Haigh explores the "hitch-hiking effect" of a selectively favourable gene substitution on the genetic polymorphism of a population. The authors analyze how changes in gene frequencies occur at closely linked loci when a selectively favourable gene substitution happens. They find that the reduction in average heterozygosity due to such substitutions can be significant, especially in large populations (of size $10^6$ or more). This effect can explain why the extent of polymorphism in natural populations does not vary as much as expected from the equilibrium between mutation and random fixation in different-sized populations. For a selectively maintained polymorphism at a linked locus, the authors show that 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. This means that in a large population, a selective substitution can occur without eliminating a selectively maintained polymorphism in the same cistron. The paper also discusses the impact of the hitch-hiking effect on average heterozygosity and the probability of complete fixation of a linked locus. It concludes that in large populations, the hitch-hiking effect can be more important than random drift in determining the level of heterozygosity for neutral alleles, and may even be predominant in smaller populations (of size $10^4$ or more). The authors use mathematical models and simulations to support their findings, providing insights into the dynamics of genetic polymorphism and the role of hitch-hiking in evolutionary processes.The paper by John Maynard Smith and John Haigh explores the "hitch-hiking effect" of a selectively favourable gene substitution on the genetic polymorphism of a population. The authors analyze how changes in gene frequencies occur at closely linked loci when a selectively favourable gene substitution happens. They find that the reduction in average heterozygosity due to such substitutions can be significant, especially in large populations (of size $10^6$ or more). This effect can explain why the extent of polymorphism in natural populations does not vary as much as expected from the equilibrium between mutation and random fixation in different-sized populations. For a selectively maintained polymorphism at a linked locus, the authors show that 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. This means that in a large population, a selective substitution can occur without eliminating a selectively maintained polymorphism in the same cistron. The paper also discusses the impact of the hitch-hiking effect on average heterozygosity and the probability of complete fixation of a linked locus. It concludes that in large populations, the hitch-hiking effect can be more important than random drift in determining the level of heterozygosity for neutral alleles, and may even be predominant in smaller populations (of size $10^4$ or more). The authors use mathematical models and simulations to support their findings, providing insights into the dynamics of genetic polymorphism and the role of hitch-hiking in evolutionary processes.