What are human population bottlenecks, and why are they important?

Written with the assistance of Dual AI

Today we will be talking about a fascinating topic, one of which seems to go under the radar from time to time and yet has an extreme impact on the populations of modern humans (us)! We will discuss bottlenecks, what they are, and how they affect the populations they affect. This is not strictly based on what happens to humans, but that, of course, will be the main point of this article. Humans. 

A human population bottleneck is a sharp reduction in the size of a human population due to environmental events or human activities, such as famines, earthquakes, floods, fires, disease, droughts, wars, migrations, or cultural practices. Population bottlenecks can significantly affect the genetic diversity and health of human populations and their evolutionary history and adaptation. A bottleneck can occur when there is any reason or reasons for drastically reducing the genetic pool available to a population at any given point. By reducing the gene pool and minimizing admixture, we can begin to see the issues arise that small populations face. But of course, we survived and possibly even came out better and more adapted to our ecological niches at the end of each of these proposed bottlenecks. 

Genetic diversity is the variation in the genes of a population. It is essential for the survival and adaptation of a population, as it allows for natural selection to act on beneficial traits and eliminate harmful ones. Genetic diversity also provides the raw material for innovation and creativity in human culture and technology. If there is a small gene pool, we are going to see the same phenotypic features over and over, and if those features, for whatever reason, do not fit the new niche created by the bottle neck, much of the population will die off. For one reason to another, infighting, lack of breeding opportunities, or higher competition for resources such as food and water. Population genetics is very fragile, and changes in how things are done can have drastic impacts. 

Population bottlenecks can reduce genetic diversity by increasing the effects of genetic drift and inbreeding. Genetic drift is the random change in the frequency of alleles (different versions of a gene) in a population due to chance events. Inbreeding is the mating of closely related individuals, which increases the likelihood of inheriting identical copies of alleles from both parents. Both genetic drift and inbreeding can lead to the loss of alleles, the fixation of alleles (when one allele becomes the only one in a population), and the increase of harmful recessive traits that can cause diseases or impairments.

When we are dealing with smaller populations that are trying to survive, any evolutionary disadvantages they face are going to be increased. If a recessive trait is usually bred out of the population, allowing the population to flourish and grow, but only those individuals with this allele or trait remain, then whatever negative aspect about this feature will cause damage. Sometimes too much damage to the area’s ecology, and we have extinctions, or, as we are learning, near extinction events called bottlenecks. 

Population bottlenecks can also affect the evolutionary history and adaptation of human populations by altering their patterns of genetic variation and relationships with other populations. For example, population bottlenecks can create founder effects, which occur when a small group of individuals colonizes a new area or becomes isolated from the main population. Founder effects can result in genetic differences between the new population and the original one and among different populations that originated from the same source. Founder effects can also influence the cultural and linguistic diversity of human populations. At its most extreme, many anthropologists, when asked the question, “Where will humans be in a thousand years?” It is hard to answer, for if we do go into space for any extended time, and I do mean a long amount of time, those who went into space would be the “founders”; setting a new population base wherever they went, one which may not exactly represent the original gene pool back at home. The founder effect has had a great role in our development and evolution; it has allowed us to develop and to develop changes, changes which make is beautifully part of the same species. 

Examples of human population bottlenecks

Human populations have experienced many population bottlenecks throughout their history, some of which have left traces in their genomes that can be detected by genetic analysis. Some examples are:

– The Out-of-Africa bottleneck: This occurred when a small group of modern humans left Africa about 60,000 to 80,000 years ago and spread across the world, replacing other hominin species such as Neanderthals and Denisovans. This bottleneck reduced the genetic diversity of non-African populations compared to African ones, as well as introduced some genetic variants that were adaptive to different environments outside Africa.

– The Toba catastrophe bottleneck: This is a controversial hypothesis that proposes that a massive volcanic eruption at Lake Toba in Indonesia about 74,000 years ago caused a global cooling event that drastically reduced the human population to about 10,000 to 30,000 individuals. This bottleneck would have further decreased the genetic diversity of human populations and increased their genetic differentiation.

– The Ashkenazi Jewish bottleneck: This occurred when a small group of Jewish people migrated from Europe to Eastern Europe about 1,000 years ago and established a distinct cultural and religious identity. This bottleneck reduced the genetic diversity of Ashkenazi Jews compared to other Jewish groups and increased their susceptibility to certain genetic diseases such as Tay-Sachs disease and cystic fibrosis.

– The New Zealand black robin bottleneck: This occurred when only five individuals of this bird species survived after habitat destruction and predation by introduced mammals in the 1980s. This bottleneck severely reduced the genetic diversity and fitness of this species, which required intensive conservation efforts to prevent its extinction.

So there we have it! I hope this has answered some of your questions surrounding what bottlenecks are when we are talking about genetics, human genetics, what it means, and the effects it could have evolutionarily on a species. 

Remember, there is always more to learn! 


Bottlenecks that reduced genetic diversity were common throughout human history | Berkeley News. (2022). Retrieved October 30, 2022, from https://news.berkeley.edu/2022/06/23/bottlenecks-that-reduced-genetic-diversity-were-common-throughout-human-history/

What is a Population Bottleneck? – Genetic Genealogy. (2015). Retrieved October 30, 2022, from https://dna-explained.com/2015/07/09/what-is-a-population-bottleneck/

Published by sethchagi

I am a Paleoanthropology Student, so far with two degrees, in Anthropology and Human Behavioral Science, pursuing my B.A and then my PhD I love to read (like a lot) and write, I love my family, and I adore anthropology! Remember, never stop exploring and never stop learning! There is always more to learn!

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