Population bottleneck followed by recovery or extinction
A population bottleneck or genetic bottleneck is a sharp reduction in the size of a population due to environmental events (such as famines, earthquakes, floods, fires, disease, or droughts) or human activities (such as genocide). Such events can reduce the variation in the gene pool of a population; thereafter, a smaller population, with a smaller genetic diversity, remains to pass on genes to future generations of offspring through sexual reproduction.
Genetic diversity remains lower, increasing only when gene flow from
another population occurs or very slowly increasing with time as random mutations occur. This results in a reduction in the robustness of the population and in its ability to adapt to and survive selecting environmental changes, such as climate change or a shift in available resources. Alternatively, if survivors of the bottleneck are the individuals with the greatest genetic fitness, the frequency of the fitter genes within the gene pool is increased, while the pool itself is reduced.
The genetic drift caused by a population bottleneck can change the proportional random distribution of alleles and even lead to loss of alleles. The chances of inbreeding and genetic homogeneity can increase, possibly leading to inbreeding depression. Smaller population size can also cause deleterious mutations to accumulate.
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.
Population bottlenecks play an important role in conservation biology and in the context of agriculture (biological and pest control).
Examples
Humans
According to a 1999 model, a severe population bottleneck, or more specifically a full-fledged speciation, occurred among a group of Australopithecina as they transitioned into the species known as Homo erectus two million years ago. It is believed that additional bottlenecks must have occurred since Homo erectus
started walking the Earth, but current archaeological, paleontological,
and genetic data is inadequate to give much reliable information about
such conjectured bottlenecks. That said, the possibility of a severe
recent species-wide bottleneck cannot be ruled out.
A 2005 study from Rutgers University theorized that the pre-1492 native populations of the Americas are the descendants of only 70 individuals who crossed the land bridge between Asia and North America.
Toba catastrophe theory
The controversial Toba catastrophe theory, presented in the late 1990s to early 2000s, suggested that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to perhaps 10,000–30,000 individuals when the Toba supervolcano in Indonesia erupted and triggered a major environmental change. Parallel bottlenecks were proposed to exist among chimpanzees, gorillas, rhesus macaques, orangutans and tigers. The hypothesis was based on geological evidence of sudden climate change and on coalescence evidence of some genes (including mitochondrial DNA, Y-chromosome DNA and some nuclear genes) and the relatively low level of genetic variation in humans.
However, subsequent research, especially in the 2010s, appeared
to refute both the climate argument and the genetic argument. Recent
research shows the extent of climate change was much smaller than
believed by proponents of the theory. In
addition, coalescence times for Y-chromosomal and mitochondrial DNA
have been revised to well above 100,000 years since 2011. Finally, such
coalescence would not, in itself, indicate a population bottleneck,
because mitochondrial DNA and Y-chromosome DNA are only a small part of
the entire genome, and are atypical in that they are inherited
exclusively through the mother or through the father, respectively.
Genetic material inherited exclusively from either father or mother can
be traced back in time via either matrilineal or patrilineal ancestry.
In 2000, a Molecular 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. This would be consistent with suggestions that in sub-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 the Late Stone Age.
Other animals
European bison, also called wisent (Bison bonasus),
faced extinction in the early 20th century. The animals living today
are all descended (except those in South Dakota at the time), from 12
individuals and they have extremely low genetic variation, which may be
beginning to affect the reproductive ability of bulls. The population of American bison (Bison bison) fell due to overhunting, nearly leading to extinction around the year 1890, though it has since begun to recover.
Overhunting
pushed the northern elephant seal to the brink of extinction by the
late 19th century. Though they have made a comeback, the genetic
variation within the population remains very low.
A classic example of a population bottleneck is that of the northern 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. The golden hamster
is a similarly bottlenecked species, with the vast majority of
domesticated hamsters descended from a single litter found in the Syrian desert around 1930, and very few wild golden hamsters remaining.
An extreme example of a population bottleneck is the New Zealand Black 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.
The genome of the giant panda shows evidence of a severe bottleneck about 43,000 years ago. There is also evidence of at least one primate species, the golden 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 low genetic diversity observed in both species.
Further deductions can sometimes be inferred from an observed population bottleneck. Among the Galápagos Islands giant tortoises — themselves a prime example of a bottleneck — the comparatively large population on the slopes of the Alcedo 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). About 100,000 YBP the volcano erupted violently, deeply burying much of the tortoise habitat in pumice and ash.
Before Europeans arrived in North America, prairies served as habitats to greater prairie chickens. In Illinois
alone, the number of greater prairie chickens plummeted from over 100
million in 1900 to about 50 in 1990. These declines in population were
the result of hunting and habitat 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. The greater prairie chicken is currently experiencing low reproductive success.
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
the IUCN. 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.
Genetic bottlenecks exist in cheetahs.
Selective breeding
Bottlenecks also exist among pure-bred animals (e.g., dogs and cats: pugs, Persian) because breeders limit their gene pools
by 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 a popular sire effect.
Selective breeding for dog breeds caused constricting breed-specific bottlenecks. These bottlenecks have led to dogs having an average of 2-3% more genetic loading than gray wolves.
The strict breeding programs and population bottlenecks have led to the
prevalence of diseases such as heart disease, blindness, cancers, hip
dysplasia, cataracts, and more.
Selective breeding to produce high-yielding crops has caused genetic bottlenecks in these crops and has led to genetic homogeneity.
This reduced genetic diversity in many crops could lead to broader
susceptibility to new diseases or pests, which threatens global food
security.
Plants
Research showed that there is incredibly low, nearly undetectable amounts of genetic diversity in the genome of the Wollemi pine (Wollemia nobilis).
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. 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 endangered Mauna Kea silversword (Argyroxiphium sandwicense ssp. sandwicense).
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.
Minimum viable population size
In conservation biology, minimum viable population (MVP) size helps to determine the effective population size when a population is at risk for extinction.
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. There is considerable debate about the
usefulness of the MVP.
