Population Genetics II: Drift, Effective Population Size, and Coalescent Thinking
Random Genetic Drift
Stochastic change
Drift is random allele-frequency change due to finite sampling of gametes or individuals. It is strongest in small populations, can fix neutral alleles by chance, and can overpower weak selection when , where is effective population size.
Effective population size
Effective population size is the size of an idealized population that would experience the same degree of drift as the real population. It is often smaller than census size because of unequal sex ratios, fluctuating sizes, variance in reproductive success, and selection at linked sites.
Interpretive caution
is not simply the number of individuals alive. It is a parameter summarizing the genetic consequences of demographic and reproductive structure.
Coalescent theory
Coalescent models trace sampled alleles backward in time to common ancestors. They provide a powerful way to infer demographic history, recombination, selection, and population structure from sequence data. Under neutrality, the expected time to coalescence scales with .
Selection versus drift
Selection
- Directional and environment-dependent.
- Acts on fitness differences and can be predictable.
Drift
- Stochastic and stronger in small populations.
- Can fix or lose alleles regardless of effect.
Which population is most affected by drift, all else equal?
Drift scales inversely with effective population size.
Correct answer: Very small population
Give one reason effective population size can be much smaller than census size.
Any factor increasing variance in gamete contribution reduces .
Correct answer: Unequal reproductive success among individuals is one valid reason, as are fluctuating population size and skewed sex ratios.
Using coalescent logic in inference
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1
Sample aligned sequences from the population of interest.
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2
Estimate genealogical relationships and summary statistics such as diversity and linkage patterns.
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3
Fit neutral or demographic models to the data.
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4
Test whether the observed patterns require selection, population structure, or growth/bottlenecks.
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5
Compare competing models using likelihood or Bayesian approaches.
Why is coalescent theory especially useful in genomics?
Coalescent reasoning links observed genetic variation to ancestral histories and demographic processes.
Correct answer: It translates sequence data into genealogical and demographic inference
What does a selective sweep do to local genetic variation?
This is a classic signature of positive selection and linkage effects.
Correct answer: It reduces nearby variation by driving a beneficial allele to high frequency and carrying linked haplotypes with it.