Genetic Drift Calculators

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Genetic drift is the random fluctuation in allele frequencies in finite populations due to sampling error in reproduction. In each generation, alleles are randomly sampled from the gene pool — by chance, some alleles are over-represented and others under-represented in offspring. The effect is inversely proportional to population size: the smaller the population (lower Ne), the greater the drift. Genetic drift causes alleles to eventually reach fixation (frequency = 1) or loss (frequency = 0), reducing genetic diversity over time. In very small populations, drift can fix even slightly deleterious alleles (overriding purifying selection), leading to inbreeding depression.

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Mathematical Foundation

In a Wright-Fisher population of size Ne: the variance in allele frequency change per generation = p(1−p)/(2Ne). Rate of heterozygosity loss per generation = 1/(2Ne). Expected time to fixation (given eventual fixation): t_fix ≈ 4Ne generations. Expected time to loss (of rare allele): t_loss ≈ (2Ne/p₀) × ln(1/p₀) / 2Ne generations ≈ very short for rare alleles.

Bottleneck Effect

A sudden severe reduction in population size → dramatic loss of alleles → reduced genetic diversity. Even if population recovers numerically, genetic diversity remains low ('genetic bottleneck'). Examples: cheetahs (post-Ice Age bottleneck); northern elephant seals; Florida panther. Bottleneck removes rare alleles; the surviving population's allele frequencies may be very different from the source population.

Founder Effect

A new population established by a small number of individuals ('founders') carries only a fraction of the source population's genetic diversity. Allele frequencies in the new population depend on chance sampling of founders → may be very different from source. Examples: Amish community; Afrikaner population of South Africa; island colonization events. Can lead to high frequency of rare autosomal recessive diseases if founders happen to be carriers.

Drift vs. Selection

Effective population size Ne determines whether selection or drift dominates: Ne × s >> 1: selection determines fate of allele. Ne × s << 1: drift dominates (allele behaves as if neutral).

Glossary

Genetic Drift
Random fluctuation in allele frequencies in finite populations due to sampling variation; larger effect in small populations; causes allele fixation or loss; reduces genetic diversity over time.
Bottleneck Effect
Loss of genetic diversity from a severe population size reduction; rare alleles lost; diversity remains low even after population recovery; examples: cheetahs, northern elephant seals.
Founder Effect
Reduced genetic diversity in a new population established by few individuals; allele frequencies reflect random sampling of founders; can lead to high frequency of specific disease alleles.

Frequently Asked Questions

Genetic drift is the change in allele frequencies in a population caused by random sampling variation — chance events in which individuals survive, reproduce, or pass on particular alleles. In any finite population, allele frequencies change randomly from generation to generation, even in the absence of mutation, selection, migration, or non-random mating. The magnitude of drift: larger in small populations (high random sampling error); smaller in large populations (law of large numbers smooths random variation). Mathematical analogy: tossing a fair coin 10 times might give 7 heads (70% H, not 50%) by chance — small samples show high random variation. Alleles can be fixed (reach 100% frequency) or lost (reach 0%) by drift, reducing genetic diversity over time.

The founder effect occurs when a new population is established by a small number of individuals from a larger source population. The founders carry only a random sample of the source population's allele frequencies — not a representative sample. Consequences: Reduced genetic diversity: only alleles present in the founders are in the new population; many source alleles are absent. Random allele frequencies: the founder population's allele frequencies reflect chance sampling of the founders, not the source population's allele structure. Genetic diseases: if one or more founders happened to be carriers of a recessive disease allele, that allele may be at high frequency in the new population. Examples: Ellis-van Creveld syndrome is highly prevalent in the Old Order Amish (founder effect from a couple who were carriers); Huntington's disease is unusually common in the Lake Maracaibo region of Venezuela due to a single 19th-century founder.

Bottleneck: a severe, sudden reduction in the size of an existing population (due to disaster, disease, hunting, habitat loss). The population that survives has reduced genetic diversity — rare alleles are likely lost. Even if the population later recovers numerically, it retains the reduced genetic diversity of the bottleneck population. Examples: cheetahs have extremely low genetic diversity from a post-Pleistocene bottleneck (~10,000 years ago) — they can receive skin grafts from unrelated individuals without rejection. Northern elephant seals were hunted to < 20 individuals in the 1890s; despite recovering to > 200,000, genetic diversity is extremely low. Founder effect: specifically involves establishment of a new isolated population from few individuals; both result in reduced diversity, but 'bottleneck' refers to a squeeze in an existing population while 'founder effect' refers to colonization of a new area.

Genetic drift affects all loci equally (genome-wide); natural selection acts on specific loci (or loci in linkage with selected loci). Distinguishing methods: FST outlier analysis: genome-wide FST (population differentiation) is elevated above the neutral background at loci under positive or divergent selection (scan for FST outliers that deviate from the neutral distribution expected from drift alone). Tajima's D test: D < 0 indicates excess rare variants (purifying selection or population expansion); D > 0 indicates excess intermediate-frequency variants (balancing selection or bottleneck). Selective sweep detection: reduced diversity around a recently fixed beneficial allele (reduced heterozygosity, long haplotype blocks). Molecular clock + neutral model: compare observed divergence rates to neutral prediction — accelerated divergence at non-synonymous sites suggests positive selection.