Population bottleneck and recovery or extinction
A
population bottleneck (or
genetic bottleneck) is an
evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing.
Population bottlenecks increase
genetic drift, as the rate of drift is inversely proportional to the population size. They also increase
inbreeding due to the reduced pool of possible mates (see
small population size).
A slightly different sort of genetic bottleneck can occur if a small group becomes reproductively separated from the main population. This is called a
founder effect.
Examples
Humans
Evolutionary biologist
Richard Dawkins has postulated that human
mitochondrial DNA (inherited only from one's mother) and
Y chromosome DNA (from one's father) show
coalescence at around 140,000 and 60,000 years ago respectively. In other words, all living humans'
female line ancestry trace back to a single female (
Mitochondrial Eve) at around 140,000 years ago. Via the
male line, all humans can trace their ancestry back to a single male (
Y-chromosomal Adam) at around 60,000 to 90,000 years ago.
This is consistent with the
Toba catastrophe theory which suggests that a bottleneck of the human population occurred c. 70,000 years ago, proposing that the human population was reduced to c.15,000 individuals
when the
Toba supervolcano in
Indonesia erupted and triggered a major
environmental change. The theory is based on geological evidences of sudden climate change, and on coalescence evidences of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes) and the relatively low level of genetic variation with humans.
However, such coalescence is genetically expected and does 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. Most genes in the genome are inherited from either father or mother, thus can be traced back in time via either matrilineal or patrilineal ancestry.
[See the chapter All Africa and her progenies in ] Research on many (but not necessarily most) genes find different coalescence points from 2 million years ago to 60,000 years ago when different genes are considered, thus disproving the existence of more recent extreme bottlenecks (i.e. a
single breeding pair).
On the other hand, 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 AgeOther animals
Wisent, also called European bison, 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 (Luenser et al., 2005). The population of
American Bison fell due to overhunting, nearly leading to extinction around the year 1890 and has since begun to recover (see table).
A classic example of a population bottleneck is that of the
Northern Elephant Seals, whose population fell to about 30 in the 1890s although it now numbers in the hundreds of thousands. Another example are
Cheetahs, which are so closely related to each other that
skin grafts from one cheetah to another do not provoke
immune responses, thus suggesting an extreme population bottleneck in the past. Another largely bottlenecked species is the
Golden Hamster, of which the vast majority are descended from a single litter found in the
Syrian desert around 1930.
Saiga Antelope numbers have plummeted more than 95% from about 1 million in 1990 to less than 30,000 in 2004, mainly due to poaching for traditional Chinese medicine.
According to a paper published in 2002, the
genome of the
Giant Panda shows evidence of a severe bottleneck that took place 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 scale.
Sometimes further deductions can 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 Alcedo volcano is significantly less diverse than four other tortoise populations on the same island. Researchers' DNA analysis dates the bottleneck around 88,000 years before present (
YBP), according to a notice in
Science, October 3, 2003. About 100,000 YBP the volcano erupted violently, burying much of the tortoise habitat deep in pumice and ash.
Bottlenecks also exist among purebred animals like
pugs or
Persians because breeders are limiting the gene pools of the animals for their looks and behaviors by
breeding with close relatives.
Plants
Research showed that there is no genetic variability in the genome of the
Wollemi Pine (
Wollemia nobilis), indicating that the species (of which there are only around 100 specimens in the wild and tens of thousands cultivated) went through a severe population bottleneck.
Population bottlenecks in evolutionary theory
As a population becomes smaller,
genetic drift plays a bigger role in
speciation. A land animal like a
brown bear might find itself locally reduced to a few dozen pairs on an Arctic island. That likely happened as the last
Ice Age came to an end, and the
Bering land bridge receded into the sea. In that circumstance, a beneficial trait appearing in an
alpha male or two may change the color, size, swimming ability, cold resistance, or aggressiveness of the group in just a few
generations.
Minimum viable population size
In
conservation biology,
minimum viable population size (MVP) helps to determine the
effective population size when a population is at risk for
extinction (Gilpin and Soulé, 1986 and Soulé, 1987). There is considerable debate about the usefulness of the MVP.
See also
