Genetic Drift Examples

Why This Matters

Genetic drift is one of the four fundamental mechanisms of evolution—alongside natural selection, mutation, and gene flow—and AP exams love testing whether you understand the difference between random and selective evolutionary change. While natural selection acts on fitness differences, genetic drift operates purely through chance, making it especially powerful in small populations where random sampling effects can override adaptive pressures. Understanding drift helps you explain everything from why endangered species struggle to recover to why human populations carry different disease alleles.

You're being tested on your ability to recognize when drift dominates over selection, how population size affects evolutionary outcomes, and why reduced genetic diversity matters for long-term survival. The examples below demonstrate these principles in action—from theoretical mechanisms like allele fixation to real-world cases like the cheetah bottleneck. Don't just memorize the examples; know what concept each one illustrates and be ready to compare how different scenarios lead to similar evolutionary outcomes.

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Mechanisms That Drive Drift

Genetic drift ultimately stems from the random sampling of alleles during reproduction. Because not every individual reproduces, and gametes carry only half of each parent's alleles, chance determines which genetic variants make it to the next generation.

Random Sampling of Gametes

• Stochastic allele transmission—each parent passes only 50% of their alleles to offspring, meaning some variants are lost purely by chance
• Binomial sampling creates generation-to-generation fluctuations in allele frequencies, even without any fitness differences
• Small populations amplify this randomness because fewer "draws" from the gene pool mean greater deviation from expected frequencies

Neutral Mutations

• No fitness effect means natural selection cannot "see" these mutations—their fate is determined entirely by drift
• Molecular clock applications rely on neutral mutation accumulation to estimate divergence times between species
• Most genetic variation at the molecular level is thought to be selectively neutral, making drift a major force in genome evolution

Genetic Hitchhiking

• Linkage disequilibrium causes neutral or even slightly harmful alleles to increase in frequency when physically linked to beneficial alleles under selection
• Selective sweeps can drag nearby neutral variants to high frequency, reducing local genetic diversity
• Complicates adaptation studies because not every common allele was favored by selection—some just "rode along"

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Population Size Effects

The strength of genetic drift is inversely related to population size. In small populations, random fluctuations have outsized effects because each individual represents a larger fraction of the total gene pool.

Small Population Size

• Effective population size ($N_e$) determines drift intensity—populations with fewer breeding individuals experience stronger random effects
• Allele frequency changes of 10-20% per generation are common in populations under 50 individuals, compared to <1% in large populations
• Overwhelms natural selection when $N_e$ is small enough that $s < \frac{1}{2N_e}$, where $s$ is the selection coefficient

Allele Fixation

• 100% frequency means an allele has completely replaced all other variants at that locus—no more variation exists
• Probability of fixation for a neutral allele equals its starting frequency (a rare allele at 5% has a 5% chance of eventual fixation)
• Time to fixation averages $4N_e$ generations for neutral alleles, meaning small populations fix alleles faster

Loss of Genetic Diversity

• Heterozygosity declines by approximately $\frac{1}{2N_e}$ each generation in the absence of new mutations
• Quantitative trait variation decreases, limiting the raw material available for future adaptive evolution
• Extinction vortex risk increases as populations lose the diversity needed to respond to environmental challenges

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Events That Reduce Population Size

Sudden demographic crashes create opportunities for drift to reshape populations dramatically. When few individuals survive to reproduce, their allele frequencies—not the original population's—become the new baseline.

Population Bottleneck

• Dramatic reduction in population size due to catastrophic events (disease, habitat destruction, climate shifts) eliminates most genetic variants
• Surviving individuals carry only a subset of original alleles, and rare variants are often lost entirely
• Recovery doesn't restore diversity—even if numbers rebound, the genetic variation lost during the bottleneck is gone permanently without new mutation or gene flow

Founder Effect

• Colonization events occur when a small number of individuals establish a new, isolated population
• Sampling bias means founders carry allele frequencies that may differ substantially from the source population
• Rare alleles can become common—if a founder happens to carry an unusual variant, it may reach high frequency in descendants (e.g., Ellis-van Creveld syndrome in Amish populations)

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Real-World Case Studies

These examples demonstrate how drift operates in natural populations and why conservation biologists worry about genetic diversity loss. Theory becomes testable when we can measure the genetic signatures drift leaves behind.

Genetic Drift in Island Populations

• Geographic isolation prevents gene flow, allowing drift to push allele frequencies in unique directions
• Endemic species often show reduced heterozygosity compared to mainland relatives, reflecting long-term small population effects
• Founder events compound because islands are typically colonized by few individuals, then remain isolated—double drift pressure

Cheetah Population Bottleneck

• Late Pleistocene crash approximately 10,000-12,000 years ago reduced cheetahs to a tiny breeding population
• Genetic uniformity is so extreme that skin grafts between unrelated cheetahs are not rejected—they're essentially immunological clones
• Conservation implications include high susceptibility to disease outbreaks and low reproductive success, demonstrating why genetic diversity matters for species survival

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Quick Reference Table

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Self-Check Questions

1. Which two examples both involve the establishment of new populations with reduced genetic diversity, and how do their underlying causes differ?

2. If an FRQ describes a population where a harmful allele has reached high frequency despite reducing survival, which mechanisms could explain this outcome? Identify at least two examples from this guide.

3. Compare and contrast how population bottlenecks and small population size affect genetic diversity—what distinguishes a one-time event from an ongoing condition?

4. A researcher finds that two isolated island populations of the same species have fixed different alleles at the same locus. Which concept best explains this observation, and why would this be unlikely in a large mainland population?

5. Why might genetic hitchhiking complicate a study attempting to identify alleles that were directly favored by natural selection? What evidence would help distinguish hitchhiking from direct selection?