# How to Calculate Genetic Drift

Updated on April 22, 2012

## What is Genetic Drift?

If you flipped a coin 1,000 times you would expect a roughly 50/50 split of heads to tails; if you ended up with 900 heads and 100 tails, you would think something was awry. What about if you flipped the coin 10 times and ended up with 9 heads? Would that make you suspicious? Of course not - you would know that this was just down to chance.

Genetic drift is the unpredictable fluctuation in allele frequencies from one generation to the next because of a population's finite size1. Genetic drift causes allele frequencies (versions of a gene) to fluctuate unpredictably, even if these alleles are 'neutral.' Genetic drift can result in a neutral allele becoming extinct in a population as other neutral alleles become fixed (ubiquitous in a population).

## Advantageous, Deleterious and Neutral Alleles

Alleles that confer a survival advantage are acted upon by natural selection until they become widely spread, or even fixed, in a population. Similarly, alleles that confer a survival disadvantage (so-called deleterious), are acted upon by natural selection until they become rare, or even extinct, in a population. Neutral alleles offer neither an advantage nor a disadvantage - they are essentially 'ignored' by natural selection.

The frequencies of these neutral alleles still change due to chance. It is important that these alleles are preserved as they may confer an advantage in the future for example. A neutral allele (say for resistance to a disease that does not exist in the area) might suddenly become an advantageous allele if that disease suddenly arrived. These genetic polymorphisms comprise a species' ability to adapt to future change.

## Keywords

Allele: a version of a gene.

Genetic Polymorphism: the existence of two or more distinct alleles at a given point in a population's gene pool

Gene Pool: the total collection of all the genes (and their alleles) in a population at any one time

Genetic Drift: Unpredictable fluctuations in allele frequencies from one generation to the next due to a population's finite size

Population: a group of individuals of the same species in a given area

## What is the Founder Effect?

Genetic drift has a greater effect the smaller the population. The founder effect is one of two situations that can dramatically increase the likelihood that genetic drift will have a significant impact on a population. When a few individuals become isolated, either genetically or geographically, from a large population, the smaller group may have a gene pool that is not reflective of the source population.

## What is the Bottleneck Effect?

The bottleneck effect is the second situation that increases the impact of genetic drift. A natural disaster such as a volcanic eruption or a flood can drastically reduce the size of a population - the gene pool of the survivors may not be representative of the original population. The survivors are said to have passed through a bottleneck event. With allele frequencies having been disturbed, genetic drift can continue to have huge effects on the gene pool until the population becomes large enough to mitigate the effects of genetic drift.

It is important to note that bottlenecking does not necessarily spell doom for the species. If, by chance, individuals with few deleterious (disadvantageous) mutations are the ones that survive, and these individuals show high genetic diversity from one another, the species may survive. How easily that species survives can vary.

## Examples of 'Bottleneck Species'

Click thumbnail to view full-size

## Bottleneck Species

Species
Bottleneck/Date
Current Numbers
Notes
Mauritius Pink Pigeon
12/1986
~470 (2010)
Efforts are in place to raise population levels to around 600 individuals
Mauritius Kestrel
4/1974
~500 (2010)
Only a single breeding female remained in 1974. The IUCN now class the kestrel as 'vulnerable'
Northern Elephant Seal
20/1890s
~30,000
In analysis of 24 gene loci in a representative sample, no variation was found
Cheetah
Severe/100,000years ago
~12,000
Unfortunately, bottlenecking has resulted in low reproductive success. Individuals are so closely related that skin grafts do not trigger the immune response
American Bison
100/ 1890s
500,000 (2009)
The species was saved through careful hybridization promoted by ranchers at the end of the 19th Century
Giant Panda
Severe/ 43,000 years ago
1590 (2004)

Bottlenecking can drastically reduce the genetic variation in a species. This leaves the species vulnerable to extinction: it may no longer have the variation necessary to adapt to changes in environment or resist disease

## How to Calculate Genetic Drift

Genetic drift can be calculated by hand. After all, it is merely the outcomes of a series of random selections. However, for anything more than a couple of alleles or a few populations over 5 or 6 generations, it is much more efficient to use a computer program to calculate genetic drift. The box to the right highlights three different software solutions for calculating genetic drift, in order of increasing depth of simulation.

Remember, that although genetic drift has the greatest and most rapid impact on small populations, that is not to say that it cannot have an impact on larger populations. Larger groups can also experience a loss of diversity, but over a much longer period of time. It is also less likely that a given allele will become fixed or extinct within a population.

## References

1: Campbell, N.A and Reece, J.B.Glossary in Biology (7th Edition). San Francisco: Benjamin Cummings

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7 years ago from Oregon

What's wrong with being bald?

• Southernmapart

7 years ago

Is the craziness resulting from inbreeding more appropriately referred to as "genetic drift?"

• leros003

7 years ago from Orlando, FL

The videos were very informative!

• Ann Leung

7 years ago from San Jose, California

Love the videos. Thank you for teaching me something new today. Excellent hub! Voted up and useful.

• whonunuwho

7 years ago from United States

Thanks for the info.

• AUTHOR

Rhys Baker

7 years ago from Peterborough, UK

@alliemacb and Cre8tor: I'm glad the hub was easy to follow. This is a complex topic - weeks of my university course were dedicated to it! :)

• AUTHOR

Rhys Baker

7 years ago from Peterborough, UK

@whonunuwho: Much of what you have raised is all due to mutation and the standard laws of Mendelian genetics. The frequency of these events changing may be due to Genetic Drift, but this is not what causes them.

As to the frequency of twins etc, this is due to the energy cost to us in raising a child. We do not often have multiples because natural selection selects against something that reduces the survival of either the parent or the child. This is also why we have only 2 nipples - to cater for the maximum number of children we could successfully raise at one time.

• Dan Reed

7 years ago

You did a very good job explaining something complex and making it simple to follow. Very well written and laid out. Thanks for sharing and putting a new wrinkle in my brain. Voted up and interesting!

• whonunuwho

7 years ago from United States

I have often wondered about the occasional strange genetic "flaw" or significant birth anomaly, as when a child was born with a tail, or some who are covered over the body with thick hair, resembling what we have termed,"Wolfman" in appearance. Could this be a holdover from ancient man?The cause of albinism in animals has also been a mystery to me. The rate of occurrence of twins, triplets, quads, or more has also seen quite perplexing in human births. The occurrence is quite common in many mammals, and I wonder just what in the DNA creates the genetic propensity for this to take place.Eye and hair color in children has also been a challenge to understand, Why is one child of three siblings, red haired and blue eyed, while the others has brown hair and green eyes?A lot to consume at one time,is this, and a challenging profession.You have chosen quite a profession, or could it have actually chosen you?

• alliemacb

7 years ago from Scotland

Wow - really interesting and I actually understood what you were saying. Voted up and awesome

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