In a region of low recombination, a beneficial mutation arises on a chromosome that also carries several deleterious alleles nearby. What outcome does Hill-Robertson interference predict?
ASelection efficiently purges the deleterious alleles while fixing the beneficial mutation
BThe beneficial mutation and deleterious alleles are separated quickly by drift
CThe beneficial mutation may be dragged to extinction with the deleterious alleles, or the deleterious alleles may hitchhike to fixation with the beneficial mutation
DThe beneficial mutation spreads rapidly because it is more visible to selection in low-recombination regions
Hill-Robertson interference describes how selection at one locus impedes selection at linked loci when recombination is rare. A beneficial mutation on a chromosome carrying deleterious alleles cannot easily be 'liberated' — the linked deleterious load drags the beneficial variant down, potentially eliminating it. Conversely, a deleterious allele can hitchhike to high frequency if it is linked to a sweeping beneficial mutation. Option A describes what happens in high-recombination regions where selection can act independently on each locus.
Question 2 Multiple Choice
Why does a modifier allele that increases recombination rate in a region spread through a population, even though the modifier itself has no direct effect on the organism's fitness?
AIt is favored by kin selection, because relatives benefit from the better genotypes it generates
BIt becomes statistically associated with higher-fitness chromosomes because it breaks apart unfavorable allele combinations created by Hill-Robertson interference
CIt increases the effective population size, reducing genetic drift
DIt directly improves the efficiency of DNA repair, reducing the deleterious mutation rate
This is indirect selection. The recombination modifier does not improve individual fitness directly; instead, it tends to generate chromosomes that carry beneficial alleles separated from deleterious ones. Over time, the modifier allele ends up on those higher-fitness chromosomes more often than chance would predict. Selection thus acts on the genetic backgrounds the modifier creates. This is the same logic as selection on any modifier of genetic architecture — the modifier is selected for its consequences, not its own phenotypic effect.
Question 3 True / False
Hill-Robertson interference is most severe in large populations where genetic drift is negligible and selection acts cleanly on most locus.
TTrue
FFalse
Answer: False
Hill-Robertson interference actually requires finite population size to be significant. In an infinite population with no drift, linkage disequilibrium between selected loci can still interfere with selection, but the effect is weaker and more tractable theoretically. In finite populations, drift creates and maintains linkage disequilibrium even in the absence of epistasis, amplifying the interference between linked selected sites. The effect is strongest when populations are finite, selection is operating across many loci simultaneously, and recombination is low — conditions common in natural populations.
Question 4 True / False
Non-recombining regions of Y chromosomes show progressive degeneration (gene loss, repeat accumulation) over evolutionary time, consistent with Hill-Robertson interference operating without recombination's counterbalancing effects.
TTrue
FFalse
Answer: True
Y chromosomes (and W chromosomes in female-heterogametic species) mostly do not recombine. Without recombination, every deleterious mutation on the Y is permanently linked to every other locus on the Y, making it impossible for selection to remove bad alleles without eliminating linked good ones. The result is mutational decay: deleterious alleles accumulate, genes are lost, and repetitive elements spread — a process called Muller's ratchet. This is direct empirical evidence for the fitness cost of low recombination predicted by Hill-Robertson theory.
Question 5 Short Answer
Explain why recombination rates can evolve — why would natural selection favor alleles that modify crossover rates, given that a recombination modifier has no direct effect on the organism's fitness?
Think about your answer, then reveal below.
Model answer: Recombination modifiers evolve through indirect selection. In a finite population, Hill-Robertson interference means that low recombination allows deleterious mutations to accumulate and beneficial mutations to be lost, because selection cannot act independently on linked loci. A modifier that increases recombination will tend to generate chromosomes where beneficial alleles are freed from deleterious neighbors and vice versa. These better chromosomes have higher fitness, and the modifier allele — by being physically linked to the chromosomes it improves — becomes statistically associated with them. Over generations, the modifier increases in frequency not because it is intrinsically beneficial, but because it is found on better genetic backgrounds.
This is a form of second-order selection — selection acting on the genetic architecture itself rather than on individual alleles. The intensity of selection on recombination modifiers depends on how much Hill-Robertson interference is operating, which in turn depends on population size, mutation rate, and the density of selected sites in the genome. This theory explains why recombination rates are not evolutionarily fixed but vary across taxa and genomic regions in predictable ways.