Bottleneck Effect Calculators

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The bottleneck effect is a type of genetic drift that occurs when a population undergoes a severe reduction in size, causing random loss of alleles from the gene pool. Unlike natural selection, the bottleneck removes alleles regardless of their adaptive value — even beneficial alleles can be lost by chance. After the bottleneck, the surviving population has reduced genetic diversity, potentially altered allele frequencies, and may show increased homozygosity and inbreeding depression. Notable examples include cheetahs, northern elephant seals, and the human population during the Toba supervolcano eruption ~74,000 years ago.

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Genetic Consequences of Bottlenecks

  • Reduced allelic richness: Rare alleles are disproportionately lost — with only 10 individuals surviving, any allele present at < 5% frequency has a high probability of being absent
  • Reduced heterozygosity: Expected H decreases proportionally with 1 − 1/(2Ne); single severe bottleneck (Ne = 10) reduces H by ~5% per generation
  • Allele frequency shifts: Random survivors determine post-bottleneck allele frequencies regardless of pre-bottleneck adaptive value
  • Inbreeding depression: Increased homozygosity exposes deleterious recessive alleles

Famous Examples

Cheetahs (Acinonyx jubatus): extreme genetic uniformity — skin grafts accepted between unrelated individuals; reproductive problems (low sperm quality, high infant mortality). Northern elephant seals: reduced to ~20 individuals in the 1890s; recovered to 170,000+ but with very low genetic diversity at all loci studied. Florida panther: severe inbreeding depression reversed by genetic rescue (Texas pumas introduced 1995).

Founder Effect

Related concept: a small number of individuals establishes a new population, carrying only a subset of the source population's alleles. Examples: Amish population (founder effect for Ellis-van Creveld syndrome); Ashkenazi Jewish disease alleles; island colonization events.

Glossary

Bottleneck Effect
A form of genetic drift following severe population size reduction; randomly removes alleles from the gene pool regardless of adaptive value; reduces heterozygosity and allelic richness.
Founder Effect
Genetic drift in a new population established by a small number of founders; founders carry only a subset of source alleles; causes allele frequency differences from source population.
Allelic Richness
The number of distinct alleles per locus; more sensitive to bottlenecks than heterozygosity; rare alleles are disproportionately lost during severe population reductions.

Frequently Asked Questions

The bottleneck effect occurs when a population drastically shrinks, randomly eliminating alleles from the gene pool. Key effects: (1) Loss of rare alleles — alleles at < 5–10% frequency in the original population are unlikely to be represented in a small survivor group. (2) Reduced heterozygosity — fewer distinct alleles means less variation. (3) Altered allele frequencies — surviving allele frequencies may not reflect the original population. (4) Increased inbreeding — small post-bottleneck populations have elevated inbreeding coefficients. Effects are irreversible in the short term — lost alleles cannot be recovered without immigration from other populations.

Bottleneck effect: an existing population is drastically reduced in size by a catastrophic event (disease, hunting, habitat loss, natural disaster). The surviving individuals represent a random subset of the original gene pool. Founder effect: a small group leaves an existing population to establish a new one in a different location (island colonization, migration, religious community isolation). The founders carry only a subset of source population alleles. Both are forms of genetic drift causing random allele loss and frequency changes, but differ in mechanism — catastrophic reduction vs. colonization. Both reduce genetic diversity in the resulting population.

Cheetahs (Acinonyx jubatus) experienced one or more severe population bottlenecks, estimated to have occurred ~10,000–12,000 years ago during the megafaunal extinctions and possibly earlier. The genetic consequences are extreme: virtually no variation at MHC (immune) loci — skin grafts between unrelated cheetahs are accepted without rejection (unusual in outbred species); extremely low microsatellite diversity; reduced sperm motility and high proportion of morphologically abnormal sperm. Despite population recovery to ~7,000 individuals, genetic diversity remains very low, making cheetahs highly vulnerable to novel disease outbreaks and environmental change.

Recovery of genetic diversity after a bottleneck depends on: population size, mutation rate, and immigration from other populations. Mutation alone is very slow — new mutations arise at ~10⁻⁸ per locus per generation; restoring allelic richness through mutation alone takes thousands of generations. Gene flow (immigration) from genetically distinct populations is much faster for restoration. Heterozygosity recovers faster than allelic richness after a bottleneck — a single migrant from a diverse source population can dramatically increase allelic richness. Cheetahs have recovered numerically but not genetically after ~10,000 years — demonstrating that numerical recovery does not equal genetic recovery.