That said, the possibility of a severe recent species-wide bottleneck can be ruled out.A 2005 study from Rutgers University theorized that the pre-1492 native population of the Americas are the descendants of only 70 individuals who crossed the land bridge between Asia and North America.Further deductions can sometimes be inferred from an observed population bottleneck.
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.
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.
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.
European bison, also called wisent (Bison bonasus), faced extinction in the early 20th century.
Before Europeans arrived in North America, prairies served as habitats to greater prairie chickens.
In Illinois alone their numbers plummeted from over 100 million in 1900 to about 50 in 1990.
Conversely, depending upon the causes of the bottleneck, the survivors may have been the genetically fittest individuals, hence increasing the frequency of the fitter genes within the gene pool, while shrinking it.
This genetic drift can change the proportional distribution of an allele by chance and even lead to fixation or loss of alleles.
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.