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Population Bottleneck: Definition & Explanation

Instructor: Beth Skwarecki
Once upon a time, the earth was full of cheetahs - but then, thousands of years ago, most of them died off. (Zombie cheetah apocalypse? We don't know.) Discover what a population bottleneck is and what it means for the cheetahs that are left.

How a Bottleneck Might Happen

Picture this: disaster threatens your whole town - no - the whole world. Zombies, earthquakes, the works. So you grab a few friends and family members, lock yourselves in the basement, and wait for everything to blow over.

In the morning, it's quiet, and the disaster is over. All of humanity is gone, except for the few people in your little shelter. You look at their faces and realize: we are the human population now. If the earth is ever again full of people, they will all be our children.

Definition

In this scenario, humans have just undergone a population bottleneck: a once-large population was reduced to just a few members, who then become the ancestors of everyone to follow.

It's not such a far-fetched idea. Population bottlenecks have happened to cheetahs, bison, and every animal that's ever been an endangered species and then bounced back.

Loss of Genetic Diversity

Cheetahs once ranged over Africa, Europe, parts of Asia, and even North America. But sometime about 10,000 years ago, their population was reduced to about seven individuals from Africa.

Hey, have you seen my friends? (Photo: Pharaoh Hound)
Cheetah

This means that genes from all the other cheetahs were lost - for example, the 250-pound mega-cheetah that lived in what's now the Republic of Georgia.

Think about your friends and family in the anti-zombie bunker. If none of you has freckles, then the gene for freckles might be lost forever. If you're all short, then post-apocalyptic humans may be missing some of the genes for being tall. (Maybe when you re-build basketball courts, the hoops should be lower.)

Cheetahs lost a lot of genetic diversity during their bottleneck, which means that their DNA is all very, very similar. In fact, cheetahs can accept skin grafts from other cheetahs! That's not possible in humans because your body's immune system would recognize that the skin is not yours, and would attack it. But cheetahs are so closely related that even distant relatives don't look like strangers to their immune system.

Harmful Effects

The problem with losing genetic variation is that you have to make do with whatever genes you've got. Many of us carry harmful recessive mutations without knowing it, because those mutations are only a problem if somebody gets two copies. So if the harmful mutation is rare, it might never meet up with another copy.

On the other hand, if you have a harmful mutation, your children or grandchildren who have that mutation might someday (ew) mate with each other - and they may not have a choice if there's a population bottleneck. This is why inbreeding is a problem; offspring can end up with two copies of a harmful gene.

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