Bottleneck Effect: Definition & Example

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Lesson Transcript
Instructor: Terry Dunn

Terry has a master's degree in environmental communications and has taught in a variety of settings.

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.

bottle of jellybeans
bottleneck jellybeans spill

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.

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