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Population Bottleneck

Reconstructing Genealogies, Reconstructing Ancient Population Sizes, Technological And Social Influences On Past Population SizePopulation Bottlenecks and Expansions in Human Evolution

A population bottleneck is a significant reduction in the size of a population that causes the extinction of many genetic lineages within that population, thus decreasing genetic diversity. Population bottlenecks have occurred in the evolutionary history of many species, including humans. Present-day bottlenecks are seen in endangered species such as the Yangtze River dolphin, whose numbers have dwindled to less than 100. Endangered species that do not become extinct may expand their numbers later on, but with a limited amount of genetic diversity with which to adapt to changing conditions. The genomes of future populations will reflect the narrowing of genetic possibility for thousands of years.

The genetic structure of human populations suggests four bottlenecks in our lineage. Stanley Ambrose has proposed that two bottlenecks may be related to past environmental changes. Marta Lahr has attributed bottlenecks to migrations of small populations across geographic barriers, a phenomenon variously referred to as the founder effect or colonization bottlenecks.

Bottleneck 1.

When traced backward in time, all human lineages coalesce to an ancestral lineage that lived in Africa about 130 thousand years ago. This date coincides with the end of the penultimate glacial period (190 to 130 thousand years ago). Populations were probably very small during this ice age. Expansion (bottleneck release) occurred during the last interglacial (130 to 71 thousand years ago), when warm climates and higher rainfall returned. Other lineages probably existed at that time, but they left no modern descendants.

Bottleneck 2.

A severe bottleneck around 70,000 years ago may have reduced the effective population size in Africa to only 5,000 females. This date coincides with the super-eruption of Toba, a volcano located in northern Sumatra. Toba blasted over 800 cubic kilometers of volcanic ash and millions of tons of sulfur gas into the atmosphere. The volcanic ash settled relatively quickly, but the sulfur formed a long-lasting stratospheric haze that reflected sunlight and may have caused rapid global cooling. Annual layers of ice in the Greenland ice sheet suggest that this haze lasted six years, causing a "volcanic winter." This was followed by 1,000 years of the coldest temperatures of the last ice age. Analysis of air trapped in these ice layers suggests that temperatures dropped 16 °C over Greenland during this "instant ice age." Drought and famine during this cataclysmic event undoubtedly decimated populations in most parts of Africa.

Bottleneck 3.

Analysis of Y chromosomes shows that all modern populations in southern Australasia can trace their ancestry to a small founding population from the Horn of northeast Africa (Ethiopia and Somalia) around 60,000 to 70,000 years ago. Increases in windblown dust in Greenland ice indicate a rapid drop in sea level to more than 100 meters lower than at present. This would have greatly facilitated dispersal from Africa to the Arabian Peninsula. Expansion around the perimeter of the Indian Ocean culminated in the colonization of Australia about 60,000 years ago.

Bottleneck 4.

Analyses of gene sequences provide evidence of a possible second exodus from Africa by a small founding population that traveled overland Figure 2. Differentiation of human genetic lineages within a population through time, and the effects of environmental and colonization bottlenecks (founder events) on survival of human lineages. via the shoreline of the Red Sea. This colonization bottleneck occurred during a period of milder climate about 50,000 years ago, and also coincides with the appearance of advanced stone tool technologies. Expansion continued into Europe and northern Asia. All living humans outside of Africa can thus trace their ancestry to these colonizing populations.

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