Population Genetics Cheat Sheet

The core ideas of Population Genetics distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

Hardy-Weinberg Equilibrium

A principle stating that allele and genotype frequencies in a population remain constant from generation to generation in the absence of evolutionary forces (no selection, no mutation, no migration, random mating, and infinite population size). The genotype frequencies are predicted by the equation p^2 + 2pq + q^2 = 1.

Genetic Drift

Random changes in allele frequencies from one generation to the next due to the chance sampling of gametes, particularly significant in small populations. Drift can lead to the fixation or loss of alleles regardless of their fitness effects.

Natural Selection

The process by which alleles that confer higher fitness (survival and reproductive success) increase in frequency within a population over generations. Selection can be directional, stabilizing, or disruptive depending on which phenotypes are favored.

Gene Flow (Migration)

The transfer of alleles between populations through the movement of individuals or their gametes. Gene flow tends to homogenize allele frequencies between populations and can introduce new genetic variation into a population.

Mutation

The ultimate source of all new genetic variation, mutation is any heritable change in the DNA sequence. While most mutations are neutral or deleterious, rare beneficial mutations provide the raw material upon which natural selection acts.

Effective Population Size (Ne)

The number of individuals in an idealized population that would show the same rate of genetic drift as the actual population. It is often much smaller than the census population size due to unequal sex ratios, variance in reproductive success, and population size fluctuations.

Fixation Index (FST)

A measure of population differentiation due to genetic structure. FST ranges from 0 (no differentiation, identical allele frequencies) to 1 (complete differentiation, populations fixed for different alleles). It quantifies how much genetic variation is partitioned between versus within populations.

Coalescent Theory

A retrospective model of population genetics that traces the genealogy of alleles backward in time to their most recent common ancestor (MRCA). It provides a framework for inferring demographic history, migration patterns, and selection from present-day genetic data.

Linkage Disequilibrium

The non-random association of alleles at different loci on a chromosome. When alleles at two loci occur together more or less often than expected by chance, they are in linkage disequilibrium. LD decays over time through recombination.

Bottleneck Effect

A dramatic reduction in population size that severely reduces genetic diversity and increases the impact of genetic drift. The surviving population carries only a fraction of the original population's genetic variation, and rare alleles may be lost entirely.

Key Terms at a Glance

Allele Frequency:The proportion of a specific allele among all allele copies at a given locus in a population.

Assortative Mating:Non-random mating in which individuals preferentially mate with others that are similar (positive) or dissimilar (negative) to themselves for a given trait.

Balancing Selection:Natural selection that maintains genetic polymorphism through mechanisms such as heterozygote advantage, frequency-dependent selection, or varying selection across environments.

Bottleneck:A sharp reduction in population size that reduces genetic diversity and increases the influence of genetic drift.

Coalescence:The merging of two lineages into a single ancestral lineage when traced backward in time, forming the basis of coalescent theory.

Directional Selection:Selection that consistently favors one extreme phenotype, shifting the population mean in one direction over time.

Effective Population Size:The size of an ideal Wright-Fisher population that would experience the same amount of genetic drift as the observed population.

Epistasis:The interaction between genes at different loci where the effect of one gene depends on the presence of other genes, complicating the prediction of phenotype from genotype.

Fitness:The relative reproductive success of a genotype, measured as its contribution of offspring to the next generation relative to other genotypes.

Fixation:The state in which one allele has reached a frequency of 100% in a population, eliminating all other alleles at that locus.

Founder Effect:A form of genetic drift that occurs when a small group of individuals establishes a new population with reduced genetic diversity relative to the source population.

Frequency-Dependent Selection:A form of selection in which the fitness of a phenotype depends on its frequency in the population, often favoring rare phenotypes (negative frequency dependence).

Gene Flow:The movement of alleles between populations through migration, which tends to homogenize allele frequencies and counteract differentiation.

Genetic Drift:Random fluctuations in allele frequencies across generations due to finite population size, independent of the alleles' effects on fitness.

Genotype Frequency:The proportion of individuals in a population that carry a particular genotype at a given locus.

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