Molecular Population Genetics

Molecular population genetics analyzes DNA sequence variation within and between populations to infer the demographic history and selective forces that have shaped genomes.

Definition

Molecular population genetics is the study of genetic variation at the level of DNA sequences within populations. It combines population-genetic theory with sequence data to estimate parameters such as nucleotide diversity, recombination, and selection, and to reconstruct demographic history.

Scope

This topic covers the description of molecular variation through diversity statistics and the site frequency spectrum, the use of coalescent theory to model genealogies, tests that distinguish selection from neutral demography, and the inference of population size changes, migration, and admixture from genomic data.

Core questions

• How is sequence variation within a population summarized and quantified?
• How does coalescent theory model the genealogy of sampled sequences?
• Which statistical tests distinguish natural selection from neutral demographic processes?
• What can the site frequency spectrum reveal about population history and selection?

Key theories

Neutral and nearly neutral theory

Much molecular variation and divergence is governed by mutation and drift acting on neutral or slightly deleterious variants, providing the null expectation against which selection is detected.

Coalescent theory

The genealogy of sampled alleles, traced backward in time to common ancestors, provides a powerful framework for predicting patterns of variation and for statistical inference from sequence data.

Mechanisms

Variation is summarized by statistics such as the number of segregating sites, nucleotide diversity, and the site frequency spectrum, which records how common derived variants are. The coalescent models these patterns by following lineages backward to their most recent common ancestors, with expectations depending on effective population size, mutation, recombination, and demography. Neutrality tests compare observed diversity and divergence to neutral expectations to detect selection. Demographic events such as expansions, bottlenecks, and admixture leave distinctive signatures in the frequency spectrum and in linkage disequilibrium that can be inferred from genome-wide data.

Clinical relevance

These methods reconstruct human population history and migration, detect signatures of recent adaptation in the human genome, and track the evolution and spread of pathogens, informing public-health surveillance and the interpretation of disease-associated variants.

History

The field began with allozyme electrophoresis in the 1960s, which revealed unexpectedly high variation and sparked the neutralist-selectionist debate. DNA sequencing, coalescent theory in the 1980s, and genome-scale sequencing transformed it into population genomics, enabling fine-grained inference of selection and demography.

Debates

Detecting selection against a demographic background

Because demographic events and selection can produce similar patterns of variation, reliably distinguishing the two from genomic data is a persistent methodological challenge.

Key figures

• Motoo Kimura
• Tomoko Ohta
• John Maynard Smith
• Richard Lewontin

Seminal works

• saetreRavinet2019
• hartlClark2007
• ohta1973

Frequently asked questions

What is the coalescent?

The coalescent is a mathematical model that traces the ancestry of a sample of alleles backward in time until they merge into common ancestors, providing the basis for much modern population-genetic inference.

Can we tell selection apart from population history in DNA data?

Often, but not always easily; selection and demographic changes can leave overlapping signatures, so researchers use multiple tests and genome-wide comparisons to separate them.