Genetic drift is one of the forces that can lead to evolution. We define evolution as the change in allele frequencies over time and this includes small changes within species (microevolution) as well as speciation (macroevolution), which is the formation of new species. We typically examine genetic drift in the context of microevolution; focusing on changes in a single gene over a relatively short period of time. Genetic drift is non-selective, which sets it apart from natural selection and sexual selection.
Rather than resulting from differential survival or reproduction due to traits, drift results from randomness. Drift is more likely to occur in small populations than in large populations. It is important to remember that evolution is occurring in populations, not individuals.
Let's look at the general concept of drift using some examples. For these examples, we'll be looking at fur color in the fictional species used in many of the evolution simulation options here on Biology Simulations. In these cases, there are three phenotypes (red, purple, and blue) determined by one gene with two alleles (red and blue). The heterozygote is purple.
For the first example, we have a very small population. In this scenario, some event kills off several individuals for reasons unrelated to phenotype.
In the starting population shown above, there are 4 red individuals, 4 blue, and 2 purple. Each red individual has 2 red alleles and each purple individual has 1 red allele. This gives us a total of 10 red alleles in the population.
(4 red individuals x 2 red alleles) + (2 purple individuals x 1 red allele) =
8 + 2 = 10 red alleles
With 10 individuals, there are a total of 20 alleles (2 for each individual).
The frequency of the red allele is 10/20 = 0.5
Because there are only two alleles, the frequency of the blue allele is also 0.5.
Three individuals die in the "unfortunate event." The resulting population has 4 red individuals 2 blue, and 1 purple. This means that the red allele frequency for the new population is 0.64.
Calculations:
Red alleles = (4 x 2 red alleles) + (1 x 1 red allele) = 8 + 1 = 9
Total alleles = 7 x 2 = 14
Red allele frequency = 9/14 = 0.64
Due to random chance, the red allele increased. Now, let's look at how the same situation would impact the allele frequencies of a larger population. In this large population, let's assume there are 5,000 individuals (image shows less) and that the starting frequency of both the red and blue alleles is 0.5. This means there are a total of 10,000 alleles with 5,000 red alleles and 5,000 blue alleles.
The "unfortunate event" again results in the loss of 2 blue and 1 purple individual which means the population has lost 5 blue alleles and 1 red allele. The population now has 4,999 red alleles and 4,995 blue alleles with a total of 9,994 alleles. The red allele frequency = 0.5002.
Red allele frequency = 4,999/9,994 = 0.5002
In the larger population, the event barely changed the allele frequencies. While drift can occur in any population, this demonstrates why small populations are more likely to be impacted.
Drift is often associated with a loss of variation. Let's look at a small population in which the blue allele is rare. In this example, there are 9 red individuals and 1 purple individual. This means that the red allele frequency is 0.95.
Red allele frequency = 19/20 = 0.95
If an "unfortunate event" kills the one purple individual, the red allele frequency will become 1 and the blue frequency will be 0.
Red allele frequency = 14/14 = 1
There are two specific genetic drift situations to be aware of; bottleneck events and the founder effect. In a bottleneck event, a population experiences a decrease in size. As a result of this decrease, the allele frequencies may change, often with a loss of diversity. As the population grows again, the new allele frequencies will be represented, resulting in a population that is different than the population before the bottleneck.
In the founder effect, a relatively small number of individuals leave an established population and start a new population. The new population may have different allele frequencies than the original population. As it grows, the new population will continue to have different allele frequencies than the original population. This process may result in a loss of diversity. Alternatively, populations that have experienced the founder effect may be identifiable by an overrepresentation of an allele that was rare in the original population (one example is the occurrence of Ellis Van Creveld syndrome in Old Order Amish).
Biology Simulations has several simulations that can be used to test genetic drift. The founder effect and bottleneck event simulations each compare starting and ending populations for a representative scenario. The population genetics simulation allows the user to manipulate population size so that the impact of population size on allele frequencies over time can be examined.