What is the Bottleneck Effect? — Definition & Examples

Causes of Bottlenecking

When disaster strikes, an ecosystem can change very quickly. When an event causes a drastic decrease in a population, it can cause a type of genetic drift called a bottleneck effect.

This can be caused by a natural disaster, like an earthquake or volcano eruption. Today, it is also often caused by humans due to over-hunting, deforestation, and pollution.

When most members of a population die suddenly, genetic variation goes down and the frequencies at which different alleles are found in the population can change in a big way. Remember that alleles are different forms of a specific gene located in the same place on a chromosome.

Let's see how the bottleneck illustration can be applied here.

In the original population above, there are green, orange, and red individuals living in a habitat. Orange makes up the majority of the population. The colors represent alleles for a color gene. When disaster strikes, only a few random individuals make it out alive. As you can see, by pure luck, most of the survivors are red, with a few green and just one orange. Over time, the population grows again, but now the majority of members are red, and there are just a few orange. This is very different from the original population before the disaster happened.

This illustrates that when a bottleneck event happens, the variety in a population can decrease and the proportions of different alleles can change completely.

(Video) Genetic Drift

by Amoeba Sisters

In this video by the Amoeba Sisters, you will learn about the different types of genetic drift. Genetic drift is another mechanism of evolution and is most common among small populations.

Populations that experience the bottleneck effect often suffer from the loss of certain alleles. This removes certain phenotypes from the population. As a result, even when these populations bounce back, they are often way less genetically diverse than they were before the bottleneck.

An Example of the Bottleneck Effect

The bottleneck effect, also known as a population bottleneck, is when a species goes through an event that suddenly and significantly reduces its population. Think about how only so much stuff can come out of the neck of a narrow bottle at a time. It's the same for this effect, where only a small proportion of the original population makes it through.

The bottleneck can be due to a catastrophic environmental event (like an earthquake, drought, fire, flood, etc.) or human activities (like habitat destruction, hunting, or genocide). The individuals that survive have greatly reduced genetic diversity compared to the original population since fewer individuals means there are fewer genotypes. The effect is a type of genetic drift, which is defined as a random change in allele frequencies.

Here's an example:

This is the New Zealand black robin (cute, right?). Due to human activities, by 1980 the entire black robin population was down to just five birds. Out of the remaining five, there was only one fertile female left, nicknamed "Old Blue" by scientists.

Remarkably, the species was able to recover from this almost certain extinction through intense conservation efforts. The black robin population is now back up to around 250.

But as a result of the severe species bottleneck, every single black robin is descended from Old Blue. Although this is an extreme example, it helps to illustrate how a population bottleneck could lead to inbreeding.

So for this situation, you can imagine something like this:

The original population of black robins on the left had genetic variation with the different "red" and "blue" genotypes. Humans caused a population bottleneck for these birds by introducing non-native predators and destroying their natural habitat. Over time, the black robin population shrank until there were only five birds left.

In the aftermath of the bottleneck event, and with the help of conservation scientists, the remaining few birds were able to reproduce and slowly increase their population size. But as you can see, because the new population is descended from just a few individuals, the genetic diversity of the species is greatly reduced.

The important detail that makes this an example of a bottleneck event and not just natural selection is that the birds survived at random. If there were some heritable traits that allowed some birds to survive better than others, then it would be an example of natural selection.

Be careful not to confuse the bottleneck effect with the founder effect, which is another type of genetic drift.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Genetic Drift}}$ : A random change of allele frequency in a gene pool

$\color{#8925AE}{\text{Allele Frequency}}$ : How often a certain allele is expressed by a population

$\color{#8925AE}{\text{Founder Effect}}$ : Happens when a part of a population moves to a new location. These individuals might carry alleles that don't really represent the alleles of the original population

$\color{#8925AE}{\text{Bottleneck Effect}}$ : Happens when the size of a population is dramatically reduced due to natural disasters

Types of Genetic Drift

The bottleneck effect is a type of genetic drift. Genetic drift is when the frequencies of alleles change in a population based on chance events. The change in frequency is totally random, and there is no pressure selecting for particular alleles.

Genetic drift is different from natural selection, which is a process in which a population slowly becomes better suited for its environment. Natural selection is a process that favors the fittest alleles, and it is not random like genetic drift is.

A population bottleneck can be split into three unique stages:

1. Before it occurs: Before a chance event occurs, there is genetic diversity in the population. Different members of the population have different alleles for different traits.

2. A chance event occurs: The effect is prompted by some kind of chance event. Usually, this is a natural disaster like a forest fire or an earthquake. The event severely decreases the size of the population. Which members survive is based on chance.

3. Decreased genetic diversity: Because only some members of the population survive, only their alleles survive as well. The chance event leads to a loss of genetic diversity in the population. While the population can grow back to its original size, it will not have the same genetic diversity that it did before the event.

The more genetically diverse a population is, the healthier it is considered to be. This is the main reason why bottleneck events are so detrimental to populations.

Using the image above, let's say that the purple, red, orange, blue, and yellow alleles are for immunity to the particular disease. The members with the green allele, however, are susceptible to the disease. After the random bottleneck event, only the individuals with green and yellow alleles survive. Again, it is totally random which individuals do and do not survive.

If the disease were to hit the population before the bottleneck event, the majority of the population would be immune and the population would overall remain healthy. But if the disease were to be introduced after the bottleneck, half of the population would be affected. This would be detrimental to the population and would decrease the genetic diversity from two different alleles to only one.

In a nutshell: a population bottleneck leads to a decrease in the genetic diversity of a population. This means the population is less able to handle pressures.

Another type of genetic drift is the founder effect. It does not involve a random disaster event. Rather, it is dependent on a subset of a population moving away.