Biologists use the term “genetic bottleneck” to refer to a period in which population numbers drop dramatically. The name comes from the shape of the graph used to illustrate the phenomenon. Such an event could occur due to disease, lack of food, or rapid climate change. In the recent history of the world, human decimation of species has also led to population bottlenecks. The reduction in genetic diversity that results from a bottleneck can remain for hundreds of generations and have deleterious effects even if population numbers return to previous levels. Low genetic variety in a population results in higher rates of inbreeding and increased occurrence of negative recessive traits.
The Museum of Life+Science houses red wolves, a species that underwent an extreme genetic bottleneck in the twentieth century. They went extinct in the wild and all wolves today are descendant from only 14 wolves that were used to start a captive breeding population. Even though red wolf numbers have rebounded to around 300, a careful Species Survival Plan is necessary to prevent any further decreases in genetic diversity.
Evolution is driven by a number of factors and results in changes in genetic frequencies over time in populations. Natural selection is the most commonly referenced evolutionary mechanism but other forces can shape the genetic makeup of a species. Genetic drift is another evolutionary mechanism and usually occurs slowly in populations. In this evolutionary scenario, genetic frequencies drift slightly from generation to generation due to the randomness of mate selection. However, genetic drift can accelerate when populations shrink rapidly during a “bottleneck” event.
The picture to the right represents the results of a simulation of genetic drift that ran for 250 generations in a population(N) of 250 individuals. The graphic follows the frequency of a gene (p) that starts out being found in 50% (or 0.5) of the population. Each colored line represents a different simulation; as you can see sometimes the gene becomes more common (e.g. the gene represented by the blue line became much more frequent in subsequent generations) and other times less common. Each trial turns out differently due to the random nature of mating.
Notice what happens when the population size (N) shrinks from 250 to 25. Sometimes the gene frequency goes to zero and that gene variation goes extinct in the population(p=0.0). Other times the gene becomes the only variant in the population (p=1.0). In both cases there is a permanent loss of genetic variety in the population.
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Some other notable examples of genetic bottlenecks in animals include:
You can run your own population simulations at the University of Connecticut’s site.