Genetic Bottleneck
A genetic bottleneck is a sharp reduction in the size of a population due to an environmental event (such as a famine, disease, or habitat destruction), resulting in a dramatic loss of genetic diversity in the surviving population. The surviving individuals carry only a small, random subset of the original genetic variation, and subsequent generations inherit this reduced genetic repertoire regardless of population size recovery. Bottlenecks increase homozygosity, reduce adaptive potential, and can cause rare alleles to be lost or increase in frequency by chance.
Consequences of Genetic Bottlenecks
| Effect | Mechanism | Outcome | Real-World Example |
|---|---|---|---|
| Reduced genetic diversity | Random subset of alleles survives | Fewer alleles per locus | Cheetah: extremely low MHC diversity |
| Increased homozygosity | Common alleles become even more common | More individuals homozygous | Florida panther inbreeding depression |
| Loss of rare alleles | Low-frequency alleles likely absent in survivors | Permanent allele loss | Northern elephant seal — 1 MHC allele remains |
| Founder-like effects | Small gene pool limits future variation | Reduced adaptive capacity | Mauritius kestrel after bottleneck of 4 |
| Genetic drift amplification | Small N means drift dominates over selection | Non-adaptive allele changes | Isle Royale wolves |
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Related Terms
Founder Effect
The founder effect is the loss of genetic variation that occurs when a new population is established by a small number of individuals (founders) from a larger source population. Because the founders carry only a random, limited sample of the original alleles, the new population will differ genetically from the source population and will have lower genetic diversity. The founder effect is a special case of genetic drift and is often responsible for the high prevalence of certain genetic conditions in isolated human populations.
Gene Flow
Gene flow, also called gene migration, is the transfer of alleles or genes from one population to another through the movement and interbreeding of individuals. It tends to homogenise allele frequencies between populations, reducing genetic differentiation, and can introduce new alleles into a population or change the frequencies of existing ones. Gene flow counteracts the genetic divergence produced by natural selection, drift, and mutation, and is a critical factor in whether populations will diverge enough to speciate.
Evolutionary Fitness
Evolutionary fitness is a measure of an organism's reproductive success relative to other individuals in the population, quantified as the average number of offspring that survive to reproductive age. It is not a measure of physical strength but of how well an organism's genotype is represented in the next generation. Absolute fitness is the actual number of reproducing offspring, while relative fitness normalises this value against the most successful genotype in the population.
The term "bottleneck" is a metaphor: just as a bottle's narrow neck restricts the flow of liquid, a population bottleneck restricts the passage of genetic diversity to future generations. "Genetic" derives from Greek "genetikos" (of birth/origin). The concept was developed by Sewall Wright in the 1930s.