Apopulation bottleneck orgenetic bottleneck is a sharp reduction in the size of apopulation due to environmental events such asfamines,earthquakes,floods, fires,disease, anddroughts; or human activities such asgenocide,speciocide, widespread violence or intentionalculling. Such events can reduce the variation in thegene pool of a population; thereafter, a smaller population, with a smallergenetic diversity, remains to pass on genes tofuture generations ofoffspring. Genetic diversity remains lower, increasing only when gene flow from another population occurs or very slowly increasing with time as randommutations occur.[1][self-published source] This results in a reduction in the robustness of the population and in its ability to adapt to and surviveselecting environmental changes, such asclimate change or a shift in available resources.[2] Alternatively, if survivors of the bottleneck are the individuals with the greatest geneticfitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.
Thegenetic drift caused by a population bottleneck can change the proportional random distribution ofalleles and even lead toloss of alleles. The chances ofinbreeding and genetic homogeneity can increase, possibly leading toinbreeding depression. Smaller population size can also cause deleterious mutations to accumulate.[3]
Population bottlenecks play an important role inconservation biology (seeminimum viable population size) and in the context of agriculture (biological andpest control).[4]
Inconservation biology, minimum viable population (MVP) size helps to determine theeffective population size when a population is at risk forextinction.[5][6] The effects of a population bottleneck often depend on the number of individuals remaining after the bottleneck and how that compares to the minimum viable population size.
A slightly different form of bottleneck can occur if a small group becomes reproductively (e.g., geographically) separated from the main population, such as through a founder event, e.g., if a few members of a species successfully colonize a new isolated island, or from small captive breeding programs such as animals at a zoo. Alternatively,invasive species can undergo population bottlenecks through founder events when introduced into their invaded range.[7]
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According to a 1999 model, a severe population bottleneck, or more specifically a full-fledgedspeciation, occurred among a group ofAustralopithecina as they transitioned into the species known asHomo erectus two million years ago. It is believed that additional bottlenecks must have occurred sinceHomo erectus started walking the Earth, but current archaeological, paleontological, and genetic data are inadequate to give much reliable information about such conjectured bottlenecks.[8] Nonetheless, a 2023 genetic analysis discerned such ahuman ancestor population bottleneck of a possible 100,000 to 1,000 individuals "around 930,000 and 813,000 years ago [which] lasted for about 117,000 years and brought human ancestors close to extinction."[9][10]
The controversialToba catastrophe theory, presented in the late 1990s to early 2000s, suggested that asupervolcano eruption in Indonesia approximately 75,000 years ago caused ahuman population bottleneck to 10,000–30,000 individuals.[11] The hypothesis was based on the apparent coincidence of geological evidence ofsudden climate change,coalescence evidence of some genes,[12] and the relatively low level of genetic variation in humans.[11] However, subsequent research, especially in the 2010s, appeared to refute both the genetic argument and the extent of climate change at the time.[13]
In 2000, aMolecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change.[8] This would be consistent with suggestions that insub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in theLate Stone Age.[14]
TheNeolithic Y-chromosome bottleneck refers to a period around 5000 BC where the diversity in the male y-chromosome dropped precipitously acrossAfrica,Europe andAsia, to a level equivalent to reproduction occurring with a ratio between men and women of 1:17.[15] Discovered in 2015, the research suggests that the reason for the bottleneck may not be a reduction in the number of males, but a drastic decrease in the percentage of males with reproductive success[16] inNeolithic agropastoralist cultures, compared to the previoushunter gatherers.[17]
| Year | American bison (est) |
|---|---|
| Before 1492 | 60,000,000 |
| 1890 | 750 |
| 2000 | 360,000 |
European bison, also called wisent(Bison bonasus), faced extinction in the early 20th century. The animals living today are all descended from 12 individuals and they have extremely low genetic variation, which may be beginning to affect the reproductive ability of bulls.[18]
The population ofAmerican bison(Bison bison) fell due to overhunting, nearly leading to extinction around the year 1890, though it has since begun to recover (see table).
A classic example of a population bottleneck is that of thenorthern elephant seal, whose population fell to about 30 in the 1890s. Although it now numbers in the hundreds of thousands, the potential for bottlenecks within colonies remains. Dominant bulls are able to mate with the largest number of females—sometimes as many as 100. With so much of a colony's offspring descended from just one dominant male, genetic diversity is limited, making the species more vulnerable to diseases and genetic mutations.
Thegolden hamster is a similarly bottlenecked species, with the vast majority of domesticated hamsters descended from a single litter found in theSyrian desert around 1930, and very few wild golden hamsters remain.
An extreme example of a population bottleneck is the New Zealandblack robin, of which every specimen today is a descendant of a single female, called Old Blue. The Black Robin population is still recovering from its low point of only five individuals in 1980.
Thegenome of thegiant panda shows evidence of a severe bottleneck about 43,000 years ago.[19] There is also evidence of at least one primate species, thegolden snub-nosed monkey, that also suffered from a bottleneck around this time. An unknown environmental event is suspected to have caused the bottlenecks observed in both of these species. The bottlenecks likely caused the lowgenetic diversity observed in both species.
Other facts can sometimes be inferred from an observed population bottleneck. Among theGalápagos Islandsgiant tortoises—themselves a prime example of a bottleneck—the comparatively large population on the slopes of theAlcedo volcano is significantly less diverse than four other tortoise populations on the same island. DNA analyses date the bottleneck to around 88,000 years before present (YBP).[20] About 100,000 YBP thevolcano erupted violently, deeply burying much of the tortoise habitat in pumice and ash.
Another example can be seen in thegreater prairie chickens, which were prevalent in North America until the 20th century. InIllinois alone, the number of greater prairie chickens plummeted from over 100 million in 1900 to about 46 in 1998.[21] These declines in population were the result of hunting andhabitat destruction, but the random consequences have also caused a great loss in species diversity. DNA analysis comparing the birds from 1990 and mid-century shows a steep genetic decline in recent decades. Management of the greater prairie chickens now includesgenetic rescue efforts including translocation of prairie chickens betweenleks to increase each population's genetic diversity.[21]
Population bottlenecking poses a major threat to the stability of species populations as well.Papilio homerus is the largest butterfly in the Americas and is endangered according to theIUCN. The disappearance of a central population poses a major threat of population bottleneck. The remaining two populations are now geographically isolated and the populations face an unstable future with limited remaining opportunity for gene flow.[22]
Genetic bottlenecks exist incheetahs.[23][24]
Bottlenecks also exist among pure-bred animals (e.g.,dogs andcats:pugs,Persian) because breeders limit theirgene pools to a few (show-winning) individuals for their looks and behaviors. The extensive use of desirable individual animals at the exclusion of others can result in apopular sire effect.
Selective breeding fordog breeds caused constricting breed-specific bottlenecks.[25] These bottlenecks have led to dogs having an average of 2–3% moregenetic loading thangray wolves.[26] The strict breeding programs and population bottlenecks have led to the prevalence of diseases such as heart disease, blindness,cancers, hip dysplasia, and cataracts in domestic dogs.[25]
Selective breeding to produce high-yielding crops has caused genetic bottlenecks in these crops and has led to genetic homogeneity.[27] This reduced genetic diversity in many crops could lead to broader susceptibility to new diseases or pests, which threatens global food security.[28]
Research showed that there is incredibly low, nearly undetectable amounts of genetic diversity in the genome of theWollemi pine (Wollemia nobilis).[29] The IUCN found a population count of 80 mature individuals and about 300 seedlings and juveniles in 2011, and previously, the Wollemi pine had fewer than 50 individuals in the wild.[30] The low population size and low genetic diversity indicates that the Wollemi pine went through a severe population bottleneck.
A population bottleneck was created in the 1970s through the conservation efforts of the endangeredMauna Kea silversword (Argyroxiphium sandwicense ssp.sandwicense).[31] The small natural population of silversword was augmented through the 1970s with outplanted individuals. All of the outplanted silversword plants were found to be first or subsequent generation offspring of just two maternal founders. The low amount of polymorphic loci in the outplanted individuals led to the population bottleneck, causing the loss of the marker allele at eight of the loci.
