Two bacterial species are compared: Species A has an effective population size of 10⁹ and Species B has an effective population size of 10⁴. Which prediction does the drift barrier hypothesis make about their per-nucleotide mutation rates?
ASpecies A will have a higher per-nucleotide mutation rate because large populations generate more total mutations
BBoth species will have the same mutation rate because natural selection optimizes fidelity to the same level in all bacteria
CSpecies B will have a higher per-nucleotide mutation rate because drift overpowers weak selection for improved fidelity in small populations
DSpecies B will have a lower mutation rate because small populations cannot afford to waste resources on deleterious mutations
The drift barrier hypothesis predicts that selection can refine replication fidelity only down to the point where the fitness benefit of further improvement exceeds the noise introduced by genetic drift. In large populations, even very small fitness differences are 'visible' to selection, allowing evolution of very high fidelity. In small populations, drift overpowers weak selection — a slightly higher-fidelity polymerase that provides only a tiny fitness advantage can drift out by chance. Species B cannot evolve as high a fidelity, predicting higher per-nucleotide mutation rates. This is confirmed empirically: multicellular eukaryotes with smaller effective population sizes have higher per-base error rates than bacteria.
Question 2 Multiple Choice
Under which conditions would a 'mutator allele' — a variant that disables mismatch repair and increases mutation rate — be expected to rise in frequency in a bacterial population?
AWhen the environment is stable and the current bacterial genotype is well-adapted
BWhen the population faces strong directional selection (such as antibiotic pressure), creating opportunity for beneficial mutations to hitchhike with the mutator
CWhen the mutator allele directly increases fitness, independent of any other mutations it generates
DIn large populations where drift cannot purge the mutator allele despite its fitness cost
Mutator alleles are favored by indirect selection — they don't directly increase fitness, but they increase the rate at which beneficial mutations arise in linked genes. Under strong directional selection (like sub-therapeutic antibiotic pressure), the rare beneficial mutations generated by the high mutation rate can sweep to fixation, carrying the mutator allele with them as a hitchhiker. In stable environments, the mutator allele only generates deleterious mutations with no beneficial ones to hitchhike on, so it accumulates mutational load and is disfavored.
Question 3 True / False
Organisms with smaller effective population sizes tend to have higher per-nucleotide mutation rates, as predicted by the drift barrier hypothesis.
TTrue
FFalse
Answer: True
This is a well-supported empirical pattern. Bacteria with effective population sizes of 10⁸–10⁹ have per-base error rates around 10⁻¹⁰, while multicellular eukaryotes with much smaller effective population sizes have error rates around 10⁻⁸–10⁻⁹. The drift barrier explanation: selection can push fidelity higher only when the fitness advantage of improvement exceeds drift noise, which requires a large population. In small populations, even modest improvements in fidelity cannot be selected for reliably, leaving mutation rates higher.
Question 4 True / False
The optimal mutation rate for any organism is zero — any mutation rate above zero increases mutational load and reduces mean fitness.
TTrue
FFalse
Answer: False
Higher fidelity has real costs: more accurate polymerases replicate more slowly, elaborate repair machinery requires energy and gene products, and there are physical limits to discrimination between correct and incorrect nucleotides. At some point, the cost of improving fidelity exceeds the benefit of reducing mutational load. Additionally, the drift barrier means selection cannot fix alleles whose fitness advantages are smaller than the noise introduced by drift. The optimum is a balance point set by these opposing pressures, not zero — and that optimum varies predictably with population size.
Question 5 Short Answer
What is the core tradeoff that determines the evolutionarily optimal mutation rate, and why can't natural selection simply drive mutation rates to zero?
Think about your answer, then reveal below.
Model answer: The tradeoff is between mutational load (the fitness cost of harmful mutations produced by higher rates) and the cost of fidelity (more accurate replication machinery is slower, more energy-intensive, and subject to physical limits on discrimination accuracy). Selection cannot drive mutation rates to zero because: (1) more accurate polymerases have real biophysical costs; (2) the marginal benefit of improving fidelity eventually falls below the marginal cost; and (3) in small populations, genetic drift prevents selection from fixing alleles with only tiny fitness advantages — the 'drift barrier' — below which fidelity cannot be further refined.
Applying cost-benefit tradeoff thinking to the mutation machinery itself reveals that mutation rate is not a passive property of chemistry but an evolved optimum. This optimum varies predictably with population size: large populations can afford the evolutionary 'investment' in higher fidelity because selection is strong enough to fix the responsible alleles; small populations cannot.