Population Genetics, Selection, Drift, and Demography
Allele frequency dynamics
Foundational equations
Population genetics models how mutation, selection, drift, migration, and recombination alter allele frequencies over time. Under Hardy-Weinberg assumptions, genotype frequencies follow , , and for alleles with frequencies and , but real populations often deviate because of nonrandom mating, structure, and selection.
Under Hardy-Weinberg equilibrium, what is the expected frequency of heterozygotes for alleles with frequencies and ?
The heterozygote frequency is .
Correct answer:
What does genetic drift describe?
Drift is strongest in small populations.
Correct answer: Random changes in allele frequency due to finite population sampling.
Selection and demography
Evolutionary pressures
Selection can be directional, stabilizing, disruptive, balancing, or purifying. Demographic events such as bottlenecks, expansions, and admixture leave signatures that can mimic or obscure selection. Accurate inference therefore requires integrating allele frequency spectra, haplotype structure, and historical models.
Evolutionary forces
| Force | Primary effect | Typical signature |
|---|---|---|
| Selection | Nonrandom fitness differences | Reduced variation or sweep signals |
| Drift | Random fluctuation | Loss of alleles in small populations |
| Migration | Gene flow | Allele sharing across populations |
| Mutation | Novel variation | Low-frequency new alleles |
| Recombination | Breakdown of LD | Haplotype reshuffling |
Which event can create a genome-wide reduction in diversity without requiring selection?
A bottleneck reduces effective population size and diversity via drift.
Correct answer: Bottleneck
Why can demographic history confound tests for selection?
Inference methods must separate shared demographic signals from adaptive ones.
Correct answer: Because demographic events change allele frequency distributions and linkage patterns in ways that can resemble selection.
Research application
Modern scans for selection often combine statistics from single variants, haplotypes, and site frequency spectra to improve robustness.