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A population bottleneck or genetic bottleneck is a sharp reduction in the size of a
population Population typically refers the number of people in a single area whether it be a city or town, region, country, or the world. Governments typically quantify the size of the resident population within their jurisdiction by a process called a ...

population
due to environmental events such as famines, earthquakes, floods, fires,
disease A disease is a particular abnormal condition that negatively affects the structure A structure is an arrangement and organization of interrelated elements in a material object or system A system is a group of Interaction, interactin ...
, and
drought A drought is an event of prolonged shortages in the water supply, whether atmospheric (below-average precipitation In meteorology Meteorology is a branch of the (which include and ), with a major focus on . The study of meteorolog ...

drought
s; or human activities such as specicide, widespread violence or intentional culling, and
human population planning Human reproduction planning is the practice of intentionally controlling the rate of growth of a human population. Historically, human population planning has been implemented with the goal of increasing the rate of human population growth. Howe ...
. Such events can reduce the variation in the
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
of a population; thereafter, a smaller population, with a smaller
genetic diversity Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species In biology, a species is the basic unit of biological classificati ...
, remains to pass on genes to
future generations Future generations are the generations of people to come in the future, after the currently living generations of humans. The future generation is contrasted with current and past generations, and evoked in order to create thinking about interge ...
of
offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspring may be known as a brood or progeny in a more ...

offspring
through
sexual reproduction Sexual reproduction is a type of reproduction Reproduction (or procreation or breeding) is the biological process Biological processes are those processes that are vital for an organism In biology, an organism (from Ancient Greek, ...
. Genetic diversity remains lower, increasing only when gene flow from another population occurs or very slowly increasing with time as random
mutation In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechan ...
s 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 Contemporary climate change includes both the global warming caused by humans, and its impacts on Earth's weather patterns. There have been previous periods of climate change, but the current changes are more rapid than any known even ...
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 The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
is increased, while the pool itself is reduced. The
genetic drift Genetic drift (allelic drift or the Sewall Wright effect) is the change in the frequency of an existing gene In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical stru ...

genetic drift
caused by a population bottleneck can change the proportional random distribution of
allele An allele (, ; ; modern formation from Greek ἄλλος ''állos'', "other") is one of two, or more, forms of a given gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance ...
s and even lead to loss of alleles. The chances of
inbreeding Inbreeding is the production of offspring In biology, offspring are the young creation of living organisms, produced either by a Asexual reproduction, single organism or, in the case of sexual reproduction, two organisms. Collective offspri ...
and genetic homogeneity can increase, possibly leading to
inbreeding depression Inbreeding depression is the reduced biological fitness Fitness (often denoted w or ω in population genetics models) is the quantitative representation of natural and sexual selection File:Sexual Selection with Peafowl.gif, 250px, Sexual se ...
. Smaller population size can also cause deleterious mutations to accumulate. Population bottlenecks play an important role in
conservation biology Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an int ...
(see
minimum viable population Minimum viable population (MVP) is a lower bound on the population of a species, such that it can survive in the wild. This term is commonly used in the fields of biological sciences, biology, ecology, and conservation biology. MVP refers to the s ...
size) and in the context of agriculture (biological and
pest control Pest control is the regulation or management of a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often def ...
). Scientists have witnessed population bottlenecks in American bison, greater prairie chickens, northern elephant seals, golden hamsters, and cheetahs. The New Zealand black robins experienced a bottleneck of five individuals, all descendants of a single female. Geneticists have found evidence for past bottlenecks in pandas, golden snub-nosed monkeys, and humans.


Minimum viable population size

In
conservation biology Conservation biology is the study of the conservation of nature and of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction and the erosion of biotic interactions. It is an int ...
, minimum viable population (MVP) size helps to determine the
effective population size The effective population size is the number of individuals that an idealised population would need to have in order for some specified quantity of interest to be the same in the idealised population as in the real population. Idealised populations ...
when a population is at risk for
extinction Extinction is the termination of a kind of organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biol ...

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.


Founder effects

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 An invasive species is an introduced organism that becomes overpopulated and negatively alters its new environment. Although their spread can have beneficial aspects, invasive species adversely affect the invaded habitat Ibex in an ...
can undergo population bottlenecks through founder events when introduced into their invaded range.


Examples


Humans

According to a 1999 model, a severe population bottleneck, or more specifically a full-fledged
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

speciation
, occurred among a group of
Australopithecina Australopithecina or Hominina Australopithecina or Hominina is a subtribe in the tribe Hominini The Hominini form a Tribe (biology), taxonomic tribe of the subfamily Homininae ("hominines"). Hominini includes the extant genera ''Homo'' (hum ...
as they transitioned into the species known as ''
Homo erectus ''Homo erectus'' (meaning "upright Body relative directions (also known as egocentric coordinates) are geometrical orientations relative to a body such as a human Humans (''Homo sapiens'') are the most abundant and widespread s ...

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 are 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 Year 1492 ( MCDXCII) was a leap year starting on Sunday A leap year starting on Sunday is any year with 366 days (i.e. it includes 29 February) that begins on Sunday, 1 January, and ends on Monday, 31 December. Its dominical letters hence are AG. ...
native populations of the
Americas The Americas (also collectively called America) is a landmass comprising the totality of North America, North and South America. The Americas make up most of the land in Earth's Western Hemisphere and comprise the New World. Along with th ...

Americas
are the descendants of only 70 individuals who crossed the
land bridge In biogeography Biogeography is the study of the distribution of species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A sp ...
between Asia and North America.


Toba catastrophe theory

The controversial
Toba catastrophe theory The Youngest Toba eruption was a Supervolcano, supervolcanic types of volcanic eruptions, eruption that occurred around 75,000 years ago at the site of present-day Lake Toba in Sumatra, Indonesia. It is one of the Earth's List of largest volcanic ...
, presented in the late 1990s to early 2000s, suggested that a bottleneck of the
human Humans (''Homo sapiens'') are the most abundant and widespread species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A speci ...

human
population occurred approximately 75,000 years ago, proposing that the human population was reduced to perhaps 10,000–30,000 individuals when the Toba
supervolcano A supervolcano is a large volcano that has had an eruption with a Volcanic Explosivity Index The Volcanic Explosivity Index (VEI) is a relative measure of the explosiveness of volcanic eruptions. It was devised by Chris Newhall of the Unite ...
in Indonesia erupted and triggered a major environmental change. Parallel bottlenecks were proposed to exist among
chimpanzee The chimpanzee (''Pan troglodytes''), also known simply as chimp, is a species of Hominidae, great ape native to the forest and savannah of tropical Africa. It has four confirmed subspecies and a fifth proposed subspecies. The chimpanzee and t ...

chimpanzee
s,
gorilla Gorillas are herbivorous, predominantly ground-dwelling great apes that inhabit the tropical forests of equatorial Africa. The genus ''Gorilla'' is divided into two species: the eastern gorilla and the western gorilla, and either four or f ...

gorilla
s,
rhesus macaque The rhesus macaque (''Macaca mulatta''), colloquially rhesus monkey, is a species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A ...

rhesus macaque
s,
orangutan Orangutans are great apes The Hominidae (), whose members are known as great apes or hominids (), are a taxonomic family In human society A society is a Social group, group of individuals involved in persistent Social relati ...

orangutan
s and
tiger The tiger (''Panthera tigris'') is the largest extant taxon, living Felidae, cat species and a member of the genus ''Panthera''. It is most recognisable for its dark vertical stripes on orange fur with a white underside. An apex predator, i ...

tiger
s. The hypothesis was based on geological evidence of sudden climate change and on coalescence evidence of some genes (including
mitochondrial DNA Mitochondrial DNA (mtDNA or mDNA) is the DNA Deoxyribonucleic acid (; DNA) is a molecule File:Pentacene on Ni(111) STM.jpg, A scanning tunneling microscopy image of pentacene molecules, which consist of linear chains of five car ...

mitochondrial DNA
, Y-chromosome DNA and some
nuclear geneA nuclear gene is a gene In biology, a gene (from ''genos'' "...Wilhelm Johannsen coined the word gene to describe the Mendelian_inheritance#History, Mendelian units of heredity..." (Greek language, Greek) meaning ''generation'' or ''birth'' ...
s) 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 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 Sub-Saharan Africa (commonly called Black Africa) is, geographically, the area of the continent of Africa that lies south of the Sahara. According to the United Nations, it consists of all list of sovereign states and dependent territories i ...

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 The Later Stone Age (LSA) is a period in African prehistory that follows the Middle Stone Age. The Later Stone Age is associated with the advent of modern human behavior in Africa, although definitions of this concept and means of studying it are ...
.


Other animals

European bison The European bison (''Bison bonasus'') or the European wood bison, also known as the wisent ( or ), or the zubr (), is a European species In biology, a species is the basic unit of biological classification, classification and a taxonomic ...

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. The population of
American bison The American bison or simply bison (''Bison bison''), also commonly known as the American buffalo or simply buffalo, is an American species In biology, a species is the basic unit of biological classification, classification and a taxonomic ...

American 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 the
northern elephant seal The northern elephant seal (''Mirounga angustirostris'') is one of two species In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical proces ...
, 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
Syria Syria ( ar, سُورِيَا or ar, سُورِيَة, ''Sūriyā''), officially the Syrian Arab Republic ( ar, ٱلْجُمْهُورِيَّةُ ٱلْعَرَبِيَّةُ ٱلسُّورِيَّةُ, al-Jumhūrīyah al-ʻArabīyah as-S ...

Syria
n desert around 1930, and very few wild golden hamsters remaining. An extreme example of a population bottleneck is the New Zealand
black robin The black robin or Chatham Island robin (''Petroica traversi'') is an endangered bird from the Chatham Islands off the east coast of New Zealand. It is closely related to the South Island robin (''P. australis''). It was first described by Walter ...
, 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 In the fields of molecular biology Molecular biology is the branch of biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, M ...

genome
of the
giant panda The giant panda (''Ailuropoda melanoleuca''; ), also known as the panda bear (or simply the panda), is a bear Bears are carnivoran mammals of the family (biology), family Ursidae. They are classified as caniforms, or doglike carnivoran ...

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 The golden snub-nosed monkey (''Rhinopithecus roxellana'') is an Old World monkey in the subfamily Colobinae The Colobinae are a subfamily In biological classification, a subfamily (Latin: ', plural ') is an auxiliary (intermediate) taxono ...
, 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 Genetic diversity is the total number of genetic characteristics in the genetic makeup of a species, it ranges widely from the number of species to differences within species In biology, a species is the basic unit of biological classificati ...
observed in both species. Other facts can sometimes be inferred from an observed population bottleneck. Among the
Galápagos Islands The Galápagos Islands (official name: , other Spanish language, Spanish name: , , ), part of the Republic of Ecuador, are an archipelago of High island, volcanic islands. They are distributed on either side of the equator in the Pacific Ocean ...
— themselves a prime example of a bottleneck — the comparatively large population on the slopes of the
Alcedo volcano Alcedo Volcano is one of the six coalescing shield volcanoes that make up Isabela Island (Galápagos), Isabela Island in the Galapagos. The remote location of the volcano has meant that even the most recent eruption in 1993 was not recorded until ...
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 Before Present (BP) years is a geologic time scale, time scale used mainly in archaeology, geology, and other science, scientific disciplines to specify when events occurred before the origin of practical radiocarbon dating in the 1950s. Because t ...
). About 100,000 YBP the
volcano A volcano is a rupture in the crust of a planetary-mass object A planet is an astronomical body orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet a ...

volcano
erupted violently, deeply burying much of the tortoise habitat in pumice and ash. Before Europeans arrived in
North America North America is a continent A continent is any of several large landmasses. Generally identified by convention (norm), convention rather than any strict criteria, up to seven geographical regions are commonly regarded as continen ...

North America
,
prairie Wheatfield intersection in the Southern Saskatchewan prairies, Canada. Prairies are ecosystem An ecosystem is a community (ecology), community of living organisms in conjunction with the nonliving components of their environment, interact ...
s served as habitats to
greater prairie chicken The greater prairie chicken or pinnated grouse (''Tympanuchus cupido''), sometimes called a boomer,Friederici, Peter (July 20, 1989)"The Last Prairie Chickens" ''Chicago Reader''. Retrieved August 27, 2014. is a large bird in the grouse Family (bi ...
s. In
Illinois Illinois ( ) is a state State may refer to: Arts, entertainment, and media Literature * ''State Magazine'', a monthly magazine published by the U.S. Department of State * The State (newspaper), ''The State'' (newspaper), a daily newspape ...

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 Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the rela ...
, 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 The greater prairie chicken or pinnated grouse (''Tympanuchus cupido''), sometimes called a boomer,Friederici, Peter (July 20, 1989)"The Last Prairie Chickens" ''Chicago Reader''. Retrieved August 27, 2014. is a large bird in the grouse Family (bi ...
is currently experiencing low reproductive success. Population bottlenecking poses a major threat to the stability of species populations as well. ''
Papilio homerus ''Papilio homerus'', the Homerus swallowtail or Jamaican swallowtail, is the largest butterfly in the Western Hemisphere. The species is endangered An endangered species is a species In biology, a species is the basic unit of biological ...

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
cheetah The cheetah (''Acinonyx jubatus'') is a large Felidae, cat native to Africa and central Iran. It is the Fastest animals, fastest land animal, estimated to be capable of running at with the fastest reliably recorded speeds being , and as suc ...

cheetah
s.


Selective breeding

Bottlenecks also exist among pure-bred animals (e.g.,
dog The dog or domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...

dog
s and
cat The cat (''Felis catus'') is a domestic Domestic may refer to: In the home * Anything relating to the human home A home, or domicile, is a space used as a permanent or semi-permanent residence for an individual, group or famil ...

cat
s:
pug The pug is a breed of dog originally from China, with physically distinctive features of a wrinkly, short-muzzled face and curled tail. The breed has a fine, glossy coat that comes in a variety of colors, most often light brown (Fawn (colour ...

pug
s,
Persian Persian may refer to: * People and things from Iran, historically called ''Persia'' in the English language ** Persians, Persian people, the majority ethnic group in Iran, not to be conflated with the Iranian peoples ** Persian language, an Iranian ...
) because breeders limit their
gene pool The gene pool is the set of all genes, or genetic information, in any population, usually of a particular species. Description A large gene pool indicates extensive genetic diversity, which is associated with robust populations that can surviv ...
s 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 Selective breeding (also called artificial selection) is the process by which humans use animal breeding Animal breeding is a branch of animal science Animal science (also bioscience) is described as "studying the biology Biology i ...
for
dog breeds The dog or domestic dog (''Canis familiaris'' or ''Canis lupus familiaris'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the ...
caused constricting breed-specific bottlenecks. These bottlenecks have led to dogs having an average of 2–3% more
genetic load Genetic load is the difference between the fitness of an average genotype Here the relation between genotype and phenotype is illustrated, using a Punnett square, for the character of petal colour in a pea plant. The letters B and b represent a ...
ing than . The strict breeding programs and population bottlenecks have led to the prevalence of diseases such as heart disease, blindness,
cancer Cancer is a group of diseases involving abnormal cell growth with the potential to invade or spread to other parts of the body. These contrast with benign tumor A benign tumor is a mass of cells Cell most often refers to: * Cell (biolo ...

cancer
s, hip dysplasia, cataracts, and more.
Selective breeding Selective breeding (also called artificial selection) is the process by which humans use animal breeding Animal breeding is a branch of animal science Animal science (also bioscience) is described as "studying the biology Biology i ...
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'' subsp. ''sandwicense'', the Mauna Kea silversword, is a highly endangered flowering plant endemism, endemic to the Hawaii (island), island of Hawaiʻi (''Big Island'') of Hawaii. It is the "crown jewel" of the volcani ...
(''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.


See also

*
Baby boom A baby boom is a period marked by a significant increase of birth rate The crude birth rate (CBR) in a period is the total number of live births per 1,000 population divided by the length of the period in years. The number of live births is n ...
*
Founder effect In population genetics Population genetics is a subfield of that deals with genetic differences within and between s, and is a part of . Studies in this branch of examine such phenomena as , , and . Population genetics was a vital ingredie ...

Founder effect
* Population boom *
Small population size Small may refer to: Science and technology * SMALL Small may refer to: Science and technology * SMALL, an ALGOL-like programming language * Small (anatomy), the lumbar region of the back * Small (journal), ''Small'' (journal), a nano-science pub ...


References


External links

*
Northern Elephant Seal History
* {{DEFAULTSORT:Population Bottleneck Population genetics Human evolution