How to Calculate Genetic Drift
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.
Lego Genetic Drift
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
Mauritius Pink Pigeon
Efforts are in place to raise population levels to around 600 individuals
Only a single breeding female remained in 1974. The IUCN now class the kestrel as 'vulnerable'
Northern Elephant Seal
In analysis of 24 gene loci in a representative sample, no variation was found
Unfortunately, bottlenecking has resulted in low reproductive success. Individuals are so closely related that skin grafts do not trigger the immune response
The species was saved through careful hybridization promoted by ranchers at the end of the 19th Century
Severe/ 43,000 years ago
Population Genetics - Free Simulation Software
- Genetic Drift Simulation
Simple Java simulations of basic population genetic processes including Genetic Drift
Populus is a popular piece of software used by Universities the world over.
- Vortex PVA Software
Vortex Population Viability Analysis software. Used by the IUCN to create management options for the endangered species list.
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.
Video Review of Genetic Drift
1: Campbell, N.A and Reece, J.B.Glossary in Biology (7th Edition). San Francisco: Benjamin Cummings