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In this lesson, you will learn what the bottleneck effect is and how it affects populations, read about some examples of the bottleneck effect, and even have a chance to ponder jelly beans.

Definition of the Bottleneck Effect

We all know what a bottle looks like, but what does that have to do with biology?Take a look at this bottle filled with jelly beans.

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bottle of jellybeans

As you can see, there are three colors: red, yellow, and green.

The bottle has no cap, and when you turn it over and give the bottle a good shake, ten jelly beans fall out before the flow stops.

bottleneck jellybeans spill

Just by chance, the ones that fall out are mostly red, but there are a couple green ones and a yellow one, too.

It’s not really a representative sample of what’s in the bottle, but that’s just how things shake out, so to speak.Your population of jelly beans just went through a bottleneck. Now, if those ten jelly beans could breed, you would rightly expect that the abundance of red jelly beans would skew the color array of jelly beans in the growing population for a good long time. That’s the bottleneck effect. The bottleneck effect is a sharp lowering of a population’s gene pool because of an environmental, or human-caused, change. It might turn out to be an advantage to the red jelly bean as the environment changes, but it also might not be.

Having a diversity of characteristics in a population’s genetic pool is almost always helpful when a disease or other calamity comes along.

What Causes a Bottleneck Effect?

Periodically, Mother Nature sends a population of animals or plants through a bottleneck. Earthquakes, floods, drought, a bad winter, fire, and disease can all abruptly reduce the numbers in a population and cause the bottleneck effect. Humans can also cause bottleneck effects with hunting, deforestation, developing infrastructure, and other abrupt environmental changes.In a closely-related phenomenon, sometimes part of a population is cut off from the rest or a small group of individuals venture out to a new area, and that small group of individuals isn’t fully representative of the genetic diversity of the larger population. Perhaps it’s an extended family group, or all the red jelly beans by themselves.

It’s called the founder’s effect, which is when a small part of a population is cut off from the larger population and the gene pool is sharply reduced.After a few generations, either effect causes genetic drift to, well, drift. Sometimes it drifts so far that a new species develops. If the first population of jelly beans you encountered was all red because of a past bottleneck effect or founder’s effect, you may not know that green or yellow jelly beans even existed!

Examples of the Bottleneck Effect

Let’s take a look at some examples of the bottleneck effect in nature. Northern elephant seals were hunted until their population was reduced to 20 individuals by the end of the 19th century.

They were saved from extinction, and their population is now around 30,000. However, there’s now less genetic diversity in their population than that of Southern elephant seals that weren’t hunted to the same degree. One population went through a bottleneck; the other didn’t.Cheetahs are another good example of a species that probably went through a bottleneck; however, we weren’t around to see it. The cheetah population has a lower genetic diversity than might be expected.

Scientists think there was a bottleneck about 12,000 years ago. This bottleneck is thought to have resulted in cheetahs being reproductively challenged, more likely to catch certain diseases, and kinked tails.Humans seem to have gone through major bottlenecks, as well.

One pretty likely bottleneck happened when the Bering Strait land bridge allowed people to cross from Eurasia to the Americas. It was perhaps a series of founder’s effects: groups arriving at different times before water and ice closed the bridge.Another time may have been around 70,000 and 75,000 years ago after the Toba Catastrophe.

The Toba volcano in Indonesia erupted with such great force that the atmospheric ash dropped temperatures globally. Scientists believe that before the eruption and bottleneck, there may have been a variety of hominids. Afterwards, one type of hominid was left, us, the modern humans.

Lesson Summary

If you still have beer bottles and jelly beans on your mind, then hopefully you’ll also remember the bottleneck effect has to do with a big decline of a population’s gene pool caused by an environmental change. A related effect is the founder’s effect, where a small part of a population is cut off from the larger population and the gene pool is sharply reduced.

Examples of populations that have gone through a bottleneck are Northern elephant seals, cheetahs, and even humans.

The Few Who Carry On

Bottleneck diversity

When an environment is greatly changed, either by nature or human interaction, there is a chance that a population can be adversely affected. Sometimes the change can come in a sudden loss of population through natural disaster or over-hunting. Other times, a small population can become separated from the larger population. In both instances, the new group is left with a smaller gene pool known as the bottleneck effect.

Learning Outcomes

With a clear understanding of this lesson, you can:

  • Verbalize the definition of the bottleneck effect
  • Understand its causes
  • Describe the ways in which the bottleneck effect has affected different species around the world

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