Genetic Drift

Definition and Overview

Concept

Genetic drift: stochastic change in allele frequencies due to random sampling of gametes across generations. Occurs independently of natural selection. Affects all sexually reproducing populations.

Historical Context

Term coined by Sewall Wright (1930s). Early population genetics emphasized drift’s role alongside selection and mutation. Central to modern evolutionary synthesis.

Scope

Relevant in small populations. Causes divergence among isolated populations. Contributes to genetic differentiation and speciation.

"Drift is the mechanism of evolution arising from random sampling error in finite populations." -- Motoo Kimura

Mechanism of Genetic Drift

Random Sampling

Alleles passed to next generation sampled randomly from gene pool. Sampling variance causes allele frequency fluctuations.

Generational Impact

Each generation: new allele frequencies differ slightly due to chance. Cumulative effect leads to divergence or fixation.

Neutral Alleles

Drift primarily affects neutral or nearly neutral alleles. Selection does not bias their frequency changes.

Effects on Populations

Genetic Variation Loss

Drift reduces heterozygosity over time, decreasing genetic diversity within populations.

Population Divergence

Random differences accumulate among isolated populations, increasing genetic differentiation (FST).

Random Fixation

Alleles may become fixed or lost purely by chance, independent of fitness.

Bottleneck Effect

Definition

Severe reduction in population size causing drastic genetic drift. Survivors’ allele frequencies may not reflect original population.

Consequences

Loss of rare alleles, reduced heterozygosity, altered allele frequencies. Increased inbreeding and genetic load risk.

Examples

Northern elephant seal bottleneck; cheetah population crash; human populations after natural disasters.

Founder Effect

Definition

New population established by few individuals carrying limited genetic variation. Drift effects amplified in founders.

Genetic Consequences

Reduced diversity, potential fixation of rare alleles, altered allele frequencies relative to source population.

Examples

Amish communities, island colonizations, invasive species founder events.

Influence of Population Size

Effective Population Size (Ne)

Ne determines magnitude of drift. Smaller Ne = stronger drift effects. Ne often less than census size.

Inverse Relationship

Drift magnitude ∝ 1/Ne. Large populations: drift slower, selection dominates. Small populations: drift dominates.

Demographic Fluctuations

Population bottlenecks, founder events reduce Ne temporarily, increasing drift impact.

Allele Fixation and Loss

Definition

Fixation: allele frequency reaches 100%. Loss: allele frequency reaches 0%. Both can occur by drift.

Probability of Fixation

Neutral allele fixation probability equals its initial frequency. Rare alleles less likely to fix.

Time to Fixation

Dependent on Ne and initial frequency. Smaller populations fix alleles faster due to stronger drift.

Expected time to fixation (neutral allele):t ≈ 4Ne generations (diploid population)

Neutral Theory of Molecular Evolution

Concept

Most molecular variation caused by genetic drift of neutral mutations rather than selection.

Motoo Kimura’s Contribution

Proposed neutral mutations fix via drift, explaining molecular clock constancy.

Implications

Drift fundamental to molecular evolution. Selection important but not sole driver of variation.

Interaction with Natural Selection

Opposing Forces

Selection favors advantageous alleles; drift causes random changes. Balance depends on Ne and selection coefficient (s).

Selection Coefficient vs Drift

If |Ns| << 1 (N=Ne, s=selection coefficient), drift dominates. If |Ns| >> 1, selection prevails.

Examples

Weakly selected alleles behave neutrally in small populations. Strong selection overrides drift in large populations.

Condition for drift dominance:|Ne * s| < 1 → drift > selection

Mathematical Models

Wright-Fisher Model

Discrete generations, fixed Ne, binomial sampling of alleles. Baseline drift model.

Moran Model

Overlapping generations, continuous time. One birth and one death per time step.

Diffusion Approximations

Continuous approximation of allele frequency changes. Used to calculate fixation probabilities and times.

Empirical Examples

Island Populations

Isolated islands show allele frequency divergence due to founder effects and drift.

Endangered Species

Small populations of cheetahs, Florida panthers show reduced genetic diversity from drift.

Human Populations

Founder effects in Ashkenazi Jews, Amish communities demonstrate drift’s role in allele frequency changes.

Implications for Conservation Genetics

Maintaining Genetic Diversity

Minimize drift by preserving large effective population sizes. Avoid bottlenecks and fragmentation.

Inbreeding Depression

Drift increases homozygosity, exposing deleterious recessive alleles, reducing fitness.

Management Strategies

Gene flow introduction, captive breeding programs, habitat restoration reduce drift effects.

References

• Kimura M., "The Neutral Theory of Molecular Evolution," Cambridge University Press, 1983, pp. 1-367.
• Wright S., "Evolution in Mendelian Populations," Genetics, vol. 16, 1931, pp. 97-159.
• Hartl D.L., Clark A.G., "Principles of Population Genetics," 4th ed., Sinauer Associates, 2007, pp. 1-682.
• Nei M., "Molecular Evolutionary Genetics," Columbia University Press, 1987, pp. 1-512.
• Frankham R., Ballou J.D., Briscoe D.A., "Introduction to Conservation Genetics," 2nd ed., Cambridge University Press, 2010, pp. 1-617.