Hill-Robertson effects are interference between two locus under selection. When recombination is weak, selection on the two locus interfere. Selection is less efficient than when it acts on two independent locus.
Consider A and B, two moderately advantageous allele, in different population. They are under positive or directional selection. If recombination is weak, an optimal AB combination can never appear. A and B interfere with each other.
Those are the typical Hill-Robertson effects, as described by Hill and Robertson in 1966. They have been described under other declination since.
Selective Sweep
Consider A, a strongly advantageous allele, and b a weakly deleterious allele. b is linked to A1. The advantageous effects of A leads to its invasion in the population. It can even lead to a point of fixation. It is thus a selective sweep of A.
But b is linked to A. b will also invade the population, even if it is deleterious. This is a case of “genetic hitch-hiking”2.
Background selection
It is the opposite case of selective sweep interference. Consider now B, a strongly deleterious allele, and a a weakly advantageous allele. a is linked to B. The deleterious effects of B lead to the extinction of B carrying individuals.
But a is linked to B. a will disappear from the population, even if it is advantageous. This is a case of background selection. All polymorphism linked to a strongly deleterious allele is purged.
Muller’s Ratchet
Muller’s ratchet was first described in 1932.
It happens in clonal species, with small population sizes. A population carrying no deleterious mutation is a population subgroup. When the first deleterious mutation appears, the population ratchets up on the mutational burden. Since recombination is weak, given the small population size, this mutation has no chance to be cured. When further deleterious mutation occurs, population degenerate. Its fitness irresistibly drops down, to the point of extinction.
Conclusion
Hill-Robertson effects only occurs when recombination is weaker than mutation. Polymorphism decreases and selection efficacy decreases too. One must consider selection intensity, mutation and recombination respective rates. The size of the genomic window affected depends on those three parameters : if recombination is weak and selection strong, genome can be affected on a large scale.
Hill-Robertson effects can explain counter intuitive observations. The spread of deleterious allele can be explained by genomic interference, as is the case with cystic fibrosis.