In population genetics, a genetic bottleneck is a severe genetic impoverishment of a species and the associated change in allele frequencies , which is caused by reducing it to a very small population , often consisting of only a few individuals ( founder effect ).
This can be a key problem in the conservation of endangered species if an allele that affects reproduction cannot be balanced by a second allele that is more advantageous for reproduction (see recessive hereditary disease ). A genetic bottleneck can therefore result in inbreeding depression .
Occurrence of genetic bottlenecks
Several species have passed through genetic bottlenecks in the last two hundred years or are just passing through them, including the David deer , the Californian condor ( Gymnogyps californianus ), the Arabian oryx ( Oryx leucoryx ), the Alpine ibex ( Capra ibex ), the Kakapo ( Strigops habroptilus ), the wisent ( Bison bonasus ) and the Przewalski horse ( Equus przewalski ). In the above cases, all animals living today go back to numbers from about a dozen to less than a hundred individuals. In prehistoric times, the cheetah ( Acinonyx jubatus ) passed through such an extremely narrow genetic bottleneck that today tissue can be transferred from one cheetah to any other without any rejection reaction, which is otherwise only possible with identical twins .
In the case of zoo animals, of which only a few individuals can be kept due to lack of space - for example elephants or rhinos - genetically different breeding animals are laboriously imported from other zoos in order to preserve genetic diversity and to avoid inbreeding. This consideration becomes even more important when breeding animals are to be released into the wild.
Genetic bottlenecks are also not uncommon in pet breeding and occur in particular in pedigree dogs , pedigree cats and small pets (e.g. golden hamsters ). This has the consequence, especially in dogs and cats, that certain hereditary diseases, which are rare in the entire population, occur very frequently in certain breeds.
Genetic bottleneck in humans
Statistical analyzes of the mitochondrial DNA (mtDNA) of anatomically modern humans ( Homo sapiens ) have shown an unexpectedly low genetic diversity and led to the assumption that around 70,000 to 80,000 years ago there could also have been a genetic bottleneck in humans. At that time, only around 1,000 to 10,000 individuals of Homo sapiens lived , mostly in Africa.
According to a controversial hypothesis by the anthropologist Stanley Ambrose ( Toba catastrophe theory ), this genetic bottleneck can be traced back to a super-eruption of the Toba volcano on Sumatra about 74,000 years ago. This eruption was followed by an extreme cold period ( volcanic winter ), which brought Homo sapiens to the brink of extinction. This hypothesis combines two conflicting findings on the genetic development of Homo sapiens : On the one hand, starting in Africa, a relatively rapid spread of humans can be proven through fossil finds, which can also be demonstrated on the basis of mitochondrial differences. Both can be used to explain the very low genetic variability of people living today (compare mitochondrial Eve and Adam of the Y chromosome ). On the other hand, there is the view that after the first expansion, there was a regionally different, isolated development of the populations , as a result of which the external appearance of people began to differentiate, which prompted earlier anthropologists to define various so-called large races and races .
Another study calculated that 1.2 million years ago only around 18,500 individuals from the direct ancestral line of Homo sapiens lived.
Another genetic bottleneck effect is believed to have occurred in Africa 120,000 years ago for Homo sapiens . According to this, only a few hundred individuals of the human species are said to have survived in a few places before they left Africa - after a cold period of around 60,000 years - including in the cave Pinnacle Point 13B (PP13B) near Mossel Bay on the coast of South Africa .
- James Q. Jacobs: Population Bottlenecks and Volcanic Winter. In: jqjacobs.net. July 4, 2000 (English).
- David Whitehouse: Humans came 'close to extinction'. In: BBC News . September 8, 1998 (English).
- Axel Bojanowski: Toba super volcano. The greatest crisis of mankind. In: sueddeutsche.de . May 17, 2010, archived from the original on June 22, 2010 .
- Franz Embacher: Endangered Species, the Fate of Genes and Chance in Evolution. In: Franz Embacher's website at the University of Vienna. Retrieved January 15, 2019 (genetic drift model).
- An overview of the mtDNA can be found in: Doron M. Behar et al .: The Dawn of Human Matrilineal Diversity. In: The American Journal of Human Genetics , Volume 82, No. 5, 2008, pp. 1130-1140, doi : 10.1016 / j.ajhg.2008.04.002
- Chad D. Huff et al .: Mobile elements reveal small population size in the ancient ancestors of Homo sapiens. In PNAS , Volume 107, No. 5, 2010, pp. 2147-2152, doi: 10.1073 / pnas.0909000107
- Curtis W. Marean: When humans nearly died out. In: Spectrum of Science Special. Archeology, history, culture. The creative person between biology and culture , 2/2013